Hosson, J.Th.M. De,

Kloosterman, A.B.,

English

Kloosterman, A.B

de Hosson, J.T.M.

University of Groningen

University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-12-2025Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect to solid state diffusion is that the intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross sections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural analyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished in two different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate enough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph of cross section of laserback (P=1.5 kW, v=9 mG.u) resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always situated on thesame spot in different cross sections of the same laser track, the general observation is that TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places small cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of 300 f 20 Vickers. Probably, the “needle like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds to resolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic percent, a martensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further it is not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment of materials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these experiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular to the surface become significant and result in parallel crack formation. Cracks may be circumvented by preheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM

ARTS repository - University of Groningen

   University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 20-06-2022Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect to solid state diffusion is that the intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross sections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural analyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished in two different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate enough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph of cross section of laserback (P=1.5 kW, v=9 mG.u) resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always situated on thesame spot in different cross sections of the same laser track, the general observation is that TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places small cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of 300 f 20 Vickers. Probably, the “needle like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds to resolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic percent, a martensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further it is not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment of materials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these experiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular to the surface become significant and result in parallel crack formation. Cracks may be circumvented by preheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 23-12-2025Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect to solid state diffusion is that the intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross sections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural analyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished in two different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate enough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph of cross section of laserback (P=1.5 kW, v=9 mG.u) resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always situated on thesame spot in different cross sections of the same laser track, the general observation is that TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places small cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of 300 f 20 Vickers. Probably, the “needle like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds to resolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic percent, a martensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further it is not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment of materials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these experiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular to the surface become significant and result in parallel crack formation. Cracks may be circumvented by preheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

   University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess everal excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. Incontrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect o solid state diffusion is that he intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption f the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross ections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural nalyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost o the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished intwo different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular tothe scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough aflat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate nough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph ofcross ection of laserback (P=1.5 kW, v=9 mG.u) resolving power Better esults umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always ituated on thesame spot in different cross sections of the same laser track, the general observation isthat TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness ofthe TiN dendritic layer increases with increasing interaction times and even can be pre- sent hroughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places mall cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular lso cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular tothe surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness ofthe TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of300 f 20 Vickers. Probably, the “needle ike” shuctum isnitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds toresolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration i  Ti lies below 6.2 atomic percent, amartensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further itis not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination ofhard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment ofmaterials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these xperiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular c acking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular tothe surface become significant and result in parallel crack formation. Cracks may be circumvented bypreheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a uctile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

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Scripta  Metalluigica  et Matcrialia,  Vol. 33, No. 4. pp. 567~573,1595 
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MICROSTRUCTURAL 
CHARACTERIZATION  OF LASER 
NITRIDED  TITANIUM 
A.B. Kloosterman  and J.Th.M. De Hosson 
.Department of Applied Physics, University of Groningen, Zernike Complex, 
Nijenborgh 4,9747  AG Groningen, The Netherlands 
(Received February 2 1,1995) 
Introduction 
Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high 
strength  to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for 
aircratt applications. However  a disadvantage of titanitmr is its high friction and poor wear resistance. This 
problem  can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- 
sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- 
tion  (CVD)  and physical vapor  deposition  (PVD) have a limited surface layer of a few micrometers.  In 
contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 
For that purpose, laser gas alloying  experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen 
atmosphere  at  various  laser  scan  velocities.  This  paper  concentrates  on  the  characterization  of  the 
microstructural  features. The basic concepts are the following: during laser processing the Ti will melt and 
a di.&rence  in surface tension, as a consequence of temperature gradients, causes a convective flow in the 
liquid Ti.  This flow, which in fact is directly controlled  by the intensity distribution of the laser  beam, 
increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d  surface layer of 
a few hundred micrometers.  The advantage of melting the Ti with respect to solid state diffusion is that the 
intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: 
Ti + 1/2N, Q  TiN. 
The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake 
of 23 atomic percent  nitrogen. [  lo]  Furthermore there exist a T&N and a d’ phase with a specific nitrogen 
concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting 
point and is stable t+om 30 to 5.5  atomic percent nitrogen. 
If the  temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN 
will almost immediately nucleate. Ifthe  temperature is high enough, TiN can be present in the liquid phase. 
At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface 
morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will 
develop  leading to a dendritic microstructure.  Since these hard ceramic TiN dendrites are embedded in a 
ductile Ti matrix better wear properties  are expected. 
567 568  MICROSTRUCTURAL  CHARACTEXEATION 
Wright  l’crccnl  NI~~ORCII 
10  IS  20  25 
_,:.__,._:._,_.1”_.,._.~..,_..,._  .  . .  p...I.._,.__.._.J  .  .  . .._....._ I 
Vol.  33, No.  4 
Ti  Atomic  Perceni  Nitrogen 
Figure 1. Ti-N equilibrium phase  diagram.  [lo] 
The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates 
in a TEM,  mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out 
of focus with scan velocities in the range of 1.5-l 5 r&&The  laser output power varies between  300 and 
1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side 
flow. This was applied to have a small  drift in order to decrease the possibility of  plasma formation. The total 
gas flow amounts 10 l/mm. 
Commerciahy pure (grade 4) Ti sheets  with a thickness of 5 mm were laser treated. Before processing the 
samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples 
were attached with heat sink pasta on a water-cooled copper block. 
After laser pmceAng  cross sections  were made by embedding the material followed by several sandpaper 
and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, 
in water. lvIicrostructural analyses were carried out by optical microscopy,  scanning electron microscopy 
(SEM) and Vickers hardness measurements with 50 g load. 
Results 
Them appeared a transition from a partially covered surface with TiN to completely covered. This transition 
can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power 
densities  (LPD)  above -2.5  10’  W/mm’ (this is an average value for the Gaussian beam distribution) the 
surface  will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the 
complete width of the beam for higher LPD’s. 
Partially  covered  surfaces with TiN exhibited  a considerable roughness compared  with a completely 
covered one. In the case of  a complete covered surface, the morphology of the TiN layer can be characterized 
by a combination  of 1amelIae and ripples in the radial dir&ion  (Fig. 2 and 3). These ripples represent the 
solidification fi-ants  which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that 
surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4  MICROSTRUCTURAL  CHARACTERIZATION  569 
Figure  2. Optical  micrograph  of laser  niirided  surf&e  showing  lam&e  and  ripples. 
power  In the case of two successive laser treatments, both with a high scan velocity and a high laser power, 
there arose very big lamellae and there was nearly no rippling anymore. 
Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The 
cracks can be distinguished in two different types, namely tiny cracks with di&rent  orientations and cracks 
which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of 
these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the 
microcracks  penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the 
bottom  of the :melt pool. At the surface the cracks tend tlrst to propagate  along the border  between  two 
lamellae and then to cross the lamellae perpendicularly 
It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan 
velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. 
With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always 
cross the complete trackwidth anymom.  with increasing  laser power the number of cracks decrease. However 
after  studying  the  longitudinal  cross  sections  the  penetration  depth  changes  also  depending  on  laser 
parameters. 
The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity 
of the meltpooL(Fig.  4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3)  micrometers 
is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost 
perpemhcular to the surface.(Fig.  6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive 
X-ray  (EDS)  rneasurem  ents  appeared  to be not adequate enough to measure  the nitrogen  concentration 
because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570  MICROSTRUCTUML  CHARACTERIZATION  Vol.  33, No.  4 
Figure  4. Optical  micrograph  of cross  section  of laserback (P=1.5 kW, v=9 mG.u) 
resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro 
analysis  (EPMA).  [4,8] The structure of the surrounding Ti is needle like which locally seem to have two 
preferable,  almost  perpendicular  directions.  The  microstructure  below  the  TiN  dendritic  region  is  a 
composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are 
swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. 
AlthoughthedSrentml  ‘crostructures  below the TiN dendritic layer are not always situated on thesame  spot 
in different  cross sections of the same laser track, the general observation is that TiN dendrites are always 
surrounded  by  a “needle like” structure.  Further  the amount of a “needle like” structure  increase  with 
inueas&  duration of the convective flow. This can be explained by the fact that with increasing duration the 
amount of N in the meltpool increases which probably result in a needle like structure a&r  solidification. 
The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- 
sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- 
face roughness caused by the fast convective flow. Besides the position of the different microstructures, the 
shape of the melt pool also changes for d&rent  laser parameters. At higher interaction times the shape of the 
melted zone indicate the presence of capillary flows. 
At some places small cracks are observed which can be correlated to the microcracks observed at the sur- 
face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of 
the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface 
and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition 
where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. 
. 
NM?  se!&Qm 
A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When 
the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm.  Also in 
the  longitudinal  cross section there were no specific areas or periodicities  observed  that correlate  with a 
particular microstructure. 
The penetration  depth of the periodic cracks observed  at the surface were found to change upon laser 
power With increasmg  laser power the cracks penetrate deeper into the melt pool until they reach the bottom 
where they terminate. However at the same time the number of cracks per unit length becomes less. 
The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration 
of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the 
melt pool. The “needle like” structure has a hardness of 560 f  60 Vickers and the “coarse structure” of 300 
f  20 Vickers.  Probably,  the “needle  like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4  MICROSTRUCTURAL  CHARACTE~TION  571 
Figure  5.  (right) SEM  micrograph 
res  (A),  ‘tome”  st~ctures  (B) 
oross section  revealing  difkent  mioroskuctures. 
below the heat  affected zone. 
Tii  dendrites  on  top, “needle  iike” 
atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large 
star&d  deviation. The “coarse structure” corresponds to resolitied  nearly pure Ti because for Ti melted in 
Ar same hardnesses were found.  The heat afTected  zone is a little harder than the substrate material, nl. 230 
f  30 against 180 f  50 Vickers. 
Discussioq 
Due  to the continuous intake of nitrogen in Ti, even shortly af&er  the interaction with the laser beam, the 
soliditication temperature  will increase. First of all a thin TiN layer will solidifil out of which dendrites will 
grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At 
the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572  MICR0STRUC-lTJW.L  CHARACTERIZATION  Vol. 33, No. 4 
Figure 6. Optical micrograph  of dendritic Tii  region with on  Figure  7.  Optical  micrograph  showing  cracks  parallel  to the 
top a flat TIN  layer.  Surface. 
meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic 
percent, a martensitic transformation from the b-phase is possible during cooling down. [9] 
For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed 
depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence  of 
this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, 
which goes much slower.  Further it is not clear if this thin ceramic TiN layer could be of practical use because 
of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well 
embedded  in a ductile material is likely to  improve the surface properties.  Preliminary wear test indeed 
confirm this, the results of which will be reported separately. 
Generally,  laser treatment of materials induce tensile stresses in the material. In the case of a metal, these 
stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle 
behavior and laser treatments may cause cracking.  In these experiments cracks in the laser track indicate the 
build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds 
a critical  stress. For single lasertracks, crack formation in the scan direction only occur for very high laser 
energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can 
be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to 
a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite 
density  become  too high. In that case stress gradients perpendicular to the surface become  significant and 
result in parallel crack formation.  Cracks may be circumvented by preheating the sample or by using diluted 
nitrogen.[6]  The mechanism of prevent  cracking by preheating is that it increases the ductility of Ti and 
enhances the possibility of stress relieve  by plastic deformation. Diluted nitrogen by argon results in less 
cracking due to a decreased reaction rate and thus a decreased presence of TiN. However  a disadvantage is 
that at the same time the TiN dendrite layer also decreases. 
Conclusions 
Laser gas alloying of Ti in a nitrogen atmosphere  results in a modified surface layer consisting of  hard ceramic 
TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers 
has been measured. However  a&r  laser processing, as a consequence of residual stmsses, cracking occurs. 
These cracks can be prevented by preheating the sample or by using diluted nitrogen 
This work is part ofthe research program of IOP-Metalen,  The Hague, The Netherlands and of the Foundation 
for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from 
the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4  MICROSTRUCTURAL  CHARACTERIZATION  573 
1.  S. Katayama,  A  Matsuww,  A Morimoto,  S.  Ishomoto  and  Y. Arata,  Lasex  Prows@  of Mat.,  p.  159,  Conf.  Proc.,  Los  Angeles 
(1984). 
2.  B.L.  Mordike,  Laser  Surface  Treatment  of Metals,  Eds.  C.W.  Draper  and  P. Mazzoldi,  Kluwer  Do&e&t,  389  (1986). 
3.  T. Bell,  H.W.  Beqqum,  J. Laq+u,  P.H.  Morton  and  AM.  Stahes,  Surface  Ensineering  2(2),  133  (1986) 
4.  M.  Se&ten,  Pmwehga  Fii  Nordic  Laser  Mat.  Pmt.  Conf.,  ed Thorsknsen  B., 66  (1988). 
5.  P.H.  Morton,  T.Bell,  A  Weisheit,  J. Kroll,  B.L.  h&dike  and  K.  Sagoo,  Surhce  Mod&&m  Teclmologies  V,  553  (1992). 
6.  S. Mridha  and  T.N.  Baker,  Materials  Science  and  Engineer&  A188,229  (1994). 
7.  J.M.  Robiason,  B.A.  van  Bnwel  and  J.Th,.M.  De Hosson,  to be published  in J. Mat.  Research. 
8.  M.  Ignatiev,  11.  Kovalev,  I. Melekhin,  I. Yu.  Smurov,  S. St&se,  Wear  166,233  (1993). 
9.  J.FolkeqD.RF.  WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 
451(1986). 
10.  H.A  Wriedt  and  J.L.  Murray,  Bulletin  of Alloy  Phase  Diagrams  8(4),  378  (1987). 

Microstructural Characterization of Laser Nitrided Titanium

   University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 25-04-2023Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect to solid state diffusion is that the intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross sections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural analyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished in two different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate enough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph of cross section of laserback (P=1.5 kW, v=9 mG.u) resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always situated on thesame spot in different cross sections of the same laser track, the general observation is that TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places small cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of 300 f 20 Vickers. Probably, the “needle like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds to resolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic percent, a martensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further it is not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment of materials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these experiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular to the surface become significant and result in parallel crack formation. Cracks may be circumvented by preheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

   University of GroningenMICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUMKloosterman, A.B; de Hosson, J.T.M.Published in:Scripta Metallurgica et MaterialiaDOI:10.1016/0956-716X(95)00238-QIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OFLASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573.https://doi.org/10.1016/0956-716X(95)00238-QCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-09-2023Pergamon Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 ElsevierScienccLtd coprigbt01995ActaMctall~~Inc. FbtedintheUSAAllri$&- 0956-716x/95 $9.50 + .oo 0956-716x(95)00238-3 MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM A.B. Kloosterman and J.Th.M. De Hosson .Department of Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4,9747 AG Groningen, The Netherlands (Received February 2 1,1995) Introduction Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen atmosphere at various laser scan velocities. This paper concentrates on the characterization of the microstructural features. The basic concepts are the following: during laser processing the Ti will melt and a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of a few hundred micrometers. The advantage of melting the Ti with respect to solid state diffusion is that the intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: Ti + 1/2N, Q TiN. The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting point and is stable t+om 30 to 5.5 atomic percent nitrogen. If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a ductile Ti matrix better wear properties are expected. 567 568 MICROSTRUCTURAL CHARACTEXEATION Wright l’crccnl NI~~ORCII 10 IS 20 25 _,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I Vol. 33, No. 4 Ti Atomic Perceni Nitrogen Figure 1. Ti-N equilibrium phase diagram. [lo] The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total gas flow amounts 10 l/mm. Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the samples were sand blasted to incresse the absorption of the laser beam. During laser processing the Ti samples were attached with heat sink pasta on a water-cooled copper block. After laser pmceAng cross sections were made by embedding the material followed by several sandpaper and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, in water. lvIicrostructural analyses were carried out by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness measurements with 50 g load. Results Them appeared a transition from a partially covered surface with TiN to completely covered. This transition can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost to the complete width of the beam for higher LPD’s. Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, there arose very big lamellae and there was nearly no rippling anymore. Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The cracks can be distinguished in two different types, namely tiny cracks with di&rent orientations and cracks which partially or completely cross the laser track perpendicular to the scan direction. (Fig.3) The width of these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two lamellae and then to cross the lamellae perpendicularly It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However after studying the longitudinal cross sections the penetration depth changes also depending on laser parameters. The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough a flat thin TiN layer of a few (l-3) micrometers is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive X-ray (EDS) rneasurem ents appeared to be not adequate enough to measure the nitrogen concentration because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 Figure 4. Optical micrograph of cross section of laserback (P=1.5 kW, v=9 mG.u) resolving power Better results umceming the nitrogen concentration can be obtained by electron probe micro analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always situated on thesame spot in different cross sections of the same laser track, the general observation is that TiN dendrites are always surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. The thickness of the TiN dendritic layer increases with increasing interaction times and even can be pre- sent throughout the complete resolidified molten zone. However at the same time there is an increase in sur- face roughness caused by the fast convective flow. Besides the position of the different microstructures, the shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the melted zone indicate the presence of capillary flows. At some places small cracks are observed which can be correlated to the microcracks observed at the sur- face. Besides perpendicular also cracks are running parallel to the surface.(Fig. 7) At increasing thickness of the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition where besides lateral stresses also stress gradients perpendicular to the surface becomes predominant. . NM? se!&Qm A noticeable variation of the thickness of the TiN dendritic layer within the same laser track was found. When the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in the longitudinal cross section there were no specific areas or periodicities observed that correlate with a particular microstructure. The penetration depth of the periodic cracks observed at the surface were found to change upon laser power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom where they terminate. However at the same time the number of cracks per unit length becomes less. The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of 300 f 20 Vickers. Probably, the “needle like” shuctum is nitrogen enriched Ti because laser melting Ti in an argon Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 Figure 5. (right) SEM micrograph res (A), ‘tome” st~ctures (B) oross section revealing difkent mioroskuctures. below the heat affected zone. Tii dendrites on top, “needle iike” atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large star&d deviation. The “coarse structure” corresponds to resolitied nearly pure Ti because for Ti melted in Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 f 30 against 180 f 50 Vickers. Discussioq Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the top a flat TIN layer. Surface. meet each other. If the cooling rate is fast enough and the nitrogen concentration in Ti lies below 6.2 atomic percent, a martensitic transformation from the b-phase is possible during cooling down. [9] For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, which goes much slower. Further it is not clear if this thin ceramic TiN layer could be of practical use because of the height variation at the surface. Nevertheless the combination of hard TiN dendrites which are very well embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed confirm this, the results of which will be reported separately. Generally, laser treatment of materials induce tensile stresses in the material. In the case of a metal, these stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle behavior and laser treatments may cause cracking. In these experiments cracks in the laser track indicate the build up of a tensile stress in the scan direction which will be relieved by perpendicular cracking if it exceeds a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite density become too high. In that case stress gradients perpendicular to the surface become significant and result in parallel crack formation. Cracks may be circumvented by preheating the sample or by using diluted nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is that at the same time the TiN dendrite layer also decreases. Conclusions Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiNdelldtiitesembedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from the Netherlands Organization for Research @IWO-The Hague). Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles (1984). 2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 451(1986). 10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 

NARCIS 

University of Groningen Research Database

  
 University of Groningen
MICROSTRUCTURAL CHARACTERIZATION OF LASER NITRIDED TITANIUM
Kloosterman, A.B; De Hosson, J.T.M.
Published in:
Scripta Metallurgica et Materialia
DOI:
10.1016/0956-716X(95)00238-Q
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
1995
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Kloosterman, A. B., & de Hosson, J. T. M. (1995). MICROSTRUCTURAL CHARACTERIZATION OF
LASER NITRIDED TITANIUM. Scripta Metallurgica et Materialia, 33(4), 567-573. DOI: 10.1016/0956-
716X(95)00238-Q
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 10-02-2018
Pergamon 
Scripta Metalluigica et Matcrialia, Vol. 33, No. 4. pp. 567~573,1595 
ElsevierScienccLtd 
coprigbt01995ActaMctall~~Inc. 
FbtedintheUSAAllri$&- 
0956-716x/95 $9.50 + .oo 
0956-716x(95)00238-3 
MICROSTRUCTURAL 
CHARACTERIZATION OF LASER 
NITRIDED TITANIUM 
A.B. Kloosterman and J.Th.M. De Hosson 
.Department of Applied Physics, University of Groningen, Zernike Complex, 
Nijenborgh 4,9747 AG Groningen, The Netherlands 
(Received February 2 1,1995) 
Introduction 
Titanium and titanium alloys possess everal excellent properties like a good corrosion resistance and high 
strength to weight ratio. Its light weight and ability to withstand extreme temperatures make it suitable for 
aircratt applications. However a disadvantage of titanitmr is its high friction and poor wear resistance. This 
problem can be tackled by nitriding the surface which results in a hard ceramic surface layer and as a con- 
sequence enhance both wear and friction properties. [2-5,8] Nitriding techniques like chemical vapor deposi- 
tion (CVD) and physical vapor deposition (PVD) have a limited surface layer of a few micrometers. In
contrast, laser nitriding may result in a modified layer of several hundreds of micrometers. [ l-61 
For that purpose, laser gas alloying experiments were done on cp-Ti with a 1.5 kW CO, laser in a nitrogen 
atmosphere at various laser scan velocities. This paper concentrates on the characterization of the 
microstructural features. The basic concepts are the following: during laser processing the Ti will melt and 
a di.&rence in surface tension, as a consequence of temperature gradients, causes a convective flow in the 
liquid Ti. This flow, which in fact is directly controlled by the intensity distribution of the laser beam, 
increases the amount of Ti in which nitrogen diffusion takes place and results in a mod&d surface layer of 
a few hundred micrometers. The advantage of melting the Ti with respect o solid state diffusion is that he 
intake of nitrogen goes much faster. Besides nitrogen diffusion TiN will be formed by the exothermal reaction: 
Ti + 1/2N, Q TiN. 
The binary phase diagram (Fig. 1) shows that a-Ti is stable as an interstitial solid solution until the intake 
of 23 atomic percent nitrogen. [ lo] Furthermore there exist a T&N and a d’ phase with a specific nitrogen 
concentration but these are expected to be absent due to the high cooling rates. TiN possesses a high melting 
point and is stable t+om 30 to 5.5 atomic percent nitrogen. 
If the temperature of molten Ti is somewhat lower than the melting point of TiN, the reaction product TiN 
will almost immediately nucleate. Ifthe temperature is high enough, TiN can be present in the liquid phase. 
At the same time, the increase of TiN in the melt enhances its viscosity which may influence the final surface 
morphology. Because of constitutional supercooling there is no flat solidification front but protrusions will 
develop leading to a dendritic microstructure. Since these hard ceramic TiN dendrites are embedded in a 
ductile Ti matrix better wear properties are expected. 
567 
568 MICROSTRUCTURAL CHARACTEXEATION 
Wright l’crccnl NI~~ORCII 
10 IS 20 25 
_,:.__,._:._,_.1”_.,._.~..,_..,._ . . . p...I.._,.__.._.J . . . .._....._ I 
Vol. 33, No. 4 
Ti Atomic Perceni Nitrogen 
Figure 1. Ti-N equilibrium phase diagram. [lo] 
The experiments were carried out with the use of 1.5 kW Spectra Physics 820 CO2 laser. This laser operates 
in a TEM, mode which results in a Gaussian intensity distribution. The laser beam hits the surface 4 mm out 
of focus with scan velocities in the range of 1.5-l 5 r&&The laser output power varies between 300 and 
1500 W. The laser beam diameter amounts 0.6 mm.. Besides an axial Nz flow there was also a small side 
flow. This was applied to have a small drift in order to decrease the possibility of plasma formation. The total 
gas flow amounts 10 l/mm. 
Commerciahy pure (grade 4) Ti sheets with a thickness of 5 mm were laser treated. Before processing the 
samples were sand blasted to incresse the absorption f the laser beam. During laser processing the Ti samples 
were attached with heat sink pasta on a water-cooled copper block. 
After laser pmceAng cross ections were made by embedding the material followed by several sandpaper 
and polish steps. Finally the polished samples were chemically etched in a solution of 0.3 % HF and 1% HNO, 
in water. lvIicrostructural nalyses were carried out by optical microscopy, scanning electron microscopy 
(SEM) and Vickers hardness measurements with 50 g load. 
Results 
Them appeared a transition from a partially covered surface with TiN to completely covered. This transition 
can be d&inguished quite easily by a yellow and a shiny yellow golden colour, respectively. For laser power 
densities (LPD) above -2.5 10’ W/mm’ (this is an average value for the Gaussian beam distribution) the 
surface will be covered with a thin TiN layer starting f?om the middle of the track extending almost o the 
complete width of the beam for higher LPD’s. 
Partially covered surfaces with TiN exhibited a considerable roughness compared with a completely 
covered one. In the case of a complete covered surface, the morphology of the TiN layer can be characterized 
by a combination of 1amelIae and ripples in the radial dir&ion (Fig. 2 and 3). These ripples represent the 
solidification fi-ants which can be seen more clearly by laser melting Ti in an Ar atmosphere. It is found that 
surface rippling decreases with increasing scan velocities and the lamellae become larger at increasing laser 
Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 569 
Figure 2. Optical micrograph of laser niirided surf&e showing lam&e and ripples. 
power In the case of two successive laser treatments, both with a high scan velocity and a high laser power, 
there arose very big lamellae and there was nearly no rippling anymore. 
Besides lamellae and ripples, cracks are observed which depend on laser power and scan velocity. The 
cracks can be distinguished intwo different types, namely tiny cracks with di&rent orientations and cracks 
which partially or completely cross the laser track perpendicular tothe scan direction. (Fig.3) The width of 
these cracks is much larger thau the width of these microcracks. After careful examination it appeared that the 
microcracks penetrate only a few (-2- 10) micrometers whereas the larger cracks possibly penetrate to the 
bottom of the :melt pool. At the surface the cracks tend tlrst to propagate along the border between two 
lamellae and then to cross the lamellae perpendicularly 
It is found that there exists a correlation between the periodicity of the cracks, the laser power and scan 
velocity. The large cracks return periodically with a periodicity changing upon scan velocity and laser power. 
With increasing scan velocity the density of cracks increases, i.e. become densely spaced and do not always 
cross the complete trackwidth anymom. with increasing laser power the number of cracks decrease. However 
after studying the longitudinal cross sections the penetration depth changes also depending on laser 
parameters. 
The cross sections taken in the transverse direction with respect to the laser track reveal the inhomogeneity 
of the meltpooL(Fig. 4 and 5) Ifthe laser power is high enough aflat thin TiN layer of a few (l-3) micrometers 
is created on top of the melt pooL Below this thin layer TiN dendrites are observed which are oriented almost 
perpemhcular to the surface.(Fig. 6) The TiN dendrites are surrounded by nitrogen rich Ti. Energy dispersive 
X-ray (EDS) rneasurem ents appeared to be not adequate nough to measure the nitrogen concentration 
because of its low yield. In addition, the energy peaks of nitrogen and Ti (L-peak) do overlap due to its bad 
570 MICROSTRUCTUML CHARACTERIZATION Vol. 33, No. 4 
Figure 4. Optical micrograph ofcross ection of laserback (P=1.5 kW, v=9 mG.u) 
resolving power Better esults umceming the nitrogen concentration can be obtained by electron probe micro 
analysis (EPMA). [4,8] The structure of the surrounding Ti is needle like which locally seem to have two 
preferable, almost perpendicular directions. The microstructure below the TiN dendritic region is a 
composition of “needle like” structures, “coarse” structures and eventual groups of TiN dendrites, which are 
swept away by the convective flow from the edge of the melt pool where a pile up of TiN dendrites exists. 
AlthoughthedSrentml ‘crostructures below the TiN dendritic layer are not always ituated on thesame spot 
in different cross sections of the same laser track, the general observation isthat TiN dendrites are always 
surrounded by a “needle like” structure. Further the amount of a “needle like” structure increase with 
inueas& duration of the convective flow. This can be explained by the fact that with increasing duration the 
amount of N in the meltpool increases which probably result in a needle like structure a&r solidification. 
The thickness ofthe TiN dendritic layer increases with increasing interaction times and even can be pre- 
sent hroughout the complete resolidified molten zone. However at the same time there is an increase in sur- 
face roughness caused by the fast convective flow. Besides the position of the different microstructures, the 
shape of the melt pool also changes for d&rent laser parameters. At higher interaction times the shape of the 
melted zone indicate the presence of capillary flows. 
At some places mall cracks are observed which can be correlated to the microcracks observed at the sur- 
face. Besides perpendicular lso cracks are running parallel to the surface.(Fig. 7) At increasing thickness of 
the TiN dendritic layer and consequently increasing width of dendrites cracking occurs parallel to the surface 
and even parts of the layer were broken out atler preparing cross sectional samples. So there exists a transition 
where besides lateral stresses also stress gradients perpendicular tothe surface becomes predominant. 
. 
NM? se!&Qm 
A noticeable variation of the thickness ofthe TiN dendritic layer within the same laser track was found. When 
the power is 1.2 kW and laser beam scan velocity is 12 m/min, the thickness varies over lo-25mm. Also in 
the longitudinal cross section there were no specific areas or periodicities observed that correlate with a 
particular microstructure. 
The penetration depth of the periodic cracks observed at the surface were found to change upon laser 
power With increasmg laser power the cracks penetrate deeper into the melt pool until they reach the bottom 
where they terminate. However at the same time the number of cracks per unit length becomes less. 
The layer with TiN dendrites and N-rich a-Ti exhibits a hardness that depends basically on the concentration 
of the dendrites. It varies Tom 700 to 1800 Vickers for very closely compacted dendrites at the edge of the 
melt pool. The “needle like” structure has a hardness of 560 f 60 Vickers and the “coarse structure” of300 
f 20 Vickers. Probably, the “needle ike” shuctum isnitrogen enriched Ti because laser melting Ti in an argon 
Vol. 33, No. 4 MICROSTRUCTURAL CHARACTE~TION 571 
Figure 5. (right) SEM micrograph 
res (A), ‘tome” st~ctures (B) 
oross section revealing difkent mioroskuctures. 
below the heat affected zone. 
Tii dendrites on top, “needle iike” 
atmosphere does not increase the hardness that much. DiEerent nitrogen contents possibly explain the large 
star&d deviation. The “coarse structure” corresponds toresolitied nearly pure Ti because for Ti melted in 
Ar same hardnesses were found. The heat afTected zone is a little harder than the substrate material, nl. 230 
f 30 against 180 f 50 Vickers. 
Discussioq 
Due to the continuous intake of nitrogen in Ti, even shortly af&er the interaction with the laser beam, the 
soliditication temperature will increase. First of all a thin TiN layer will solidifil out of which dendrites will 
grow into the melt due to constitutional supercooling. These dendrites grow by rejecting Ti into the melt_ At 
the same time there will be a solidification front at the bottom of the melt pool and somewhere these tints 
572 MICR0STRUC-lTJW.L CHARACTERIZATION Vol. 33, No. 4 
Figure 6. Optical micrograph of dendritic Tii region with on Figure 7. Optical micrograph showing cracks parallel to the 
top a flat TIN layer. Surface. 
meet each other. If the cooling rate is fast enough and the nitrogen concentration i  Ti lies below 6.2 atomic 
percent, amartensitic transformation from the b-phase is possible during cooling down. [9] 
For the case that TiN has been present in the liquid phase, a smoothly varying surface layer will formed 
depending on the viscosity and velocity of the convective flow. However, a disadvantage of the presence of 
this top layer is that the nitrogen diffusion into the Ti is controlled by the diffusion through the TiN layer, 
which goes much slower. Further itis not clear if this thin ceramic TiN layer could be of practical use because 
of the height variation at the surface. Nevertheless the combination ofhard TiN dendrites which are very well 
embedded in a ductile material is likely to improve the surface properties. Preliminary wear test indeed 
confirm this, the results of which will be reported separately. 
Generally, laser treatment ofmaterials induce tensile stresses in the material. In the case of a metal, these 
stmsses can be relieved by elastic and plastic deformation. However a ceramic material shows a more brittle 
behavior and laser treatments may cause cracking. In these xperiments cracks in the laser track indicate the 
build up of a tensile stress in the scan direction which will be relieved by perpendicular c acking if it exceeds 
a critical stress. For single lasertracks, crack formation in the scan direction only occur for very high laser 
energy densities. However for overlapping tracks periodic cracking occurs in the scan direction, which can 
be explained by a stepwise increase in the residual stress with each successive, overlapping laser track up to 
a maximum value. [7] Parallel cracking probably takes place if the dendritic layer thickness and the dendrite 
density become too high. In that case stress gradients perpendicular tothe surface become significant and 
result in parallel crack formation. Cracks may be circumvented bypreheating the sample or by using diluted 
nitrogen.[6] The mechanism of prevent cracking by preheating is that it increases the ductility of Ti and 
enhances the possibility of stress relieve by plastic deformation. Diluted nitrogen by argon results in less 
cracking due to a decreased reaction rate and thus a decreased presence of TiN. However a disadvantage is 
that at the same time the TiN dendrite layer also decreases. 
Conclusions 
Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic 
TiNdelldtiitesembedded in a uctile substrate with on top a thin TiN layer. Hardness as high as 1800 vickers 
has been measured. However a&r laser processing, as a consequence of residual stmsses, cracking occurs. 
These cracks can be prevented by preheating the sample or by using diluted nitrogen 
This work is part ofthe research program of IOP-Metalen, The Hague, The Netherlands and of the Foundation 
for Fundamental Research on Matter (FOM-Utrecht) and has been made possible by financial support from 
the Netherlands Organization for Research @IWO-The Hague). 
Vol. 33, No. 4 MICROSTRUCTURAL CHARACTERIZATION 573 
1. S. Katayama, A Matsuww, A Morimoto, S. Ishomoto and Y. Arata, Lasex Prows@ of Mat., p. 159, Conf. Proc., Los Angeles 
(1984). 
2. B.L. Mordike, Laser Surface Treatment of Metals, Eds. C.W. Draper and P. Mazzoldi, Kluwer Do&e&t, 389 (1986). 
3. T. Bell, H.W. Beqqum, J. Laq+u, P.H. Morton and AM. Stahes, Surface Ensineering 2(2), 133 (1986) 
4. M. Se&ten, Pmwehga Fii Nordic Laser Mat. Pmt. Conf., ed Thorsknsen B., 66 (1988). 
5. P.H. Morton, T.Bell, A Weisheit, J. Kroll, B.L. h&dike and K. Sagoo, Surhce Mod&&m Teclmologies V, 553 (1992). 
6. S. Mridha and T.N. Baker, Materials Science and Engineer& A188,229 (1994). 
7. J.M. Robiason, B.A. van Bnwel and J.Th,.M. De Hosson, to be published in J. Mat. Research. 
8. M. Ignatiev, 11. Kovalev, I. Melekhin, I. Yu. Smurov, S. St&se, Wear 166,233 (1993). 
9. J.FolkeqD.RF. WestandW.MSteeqLasxLaserTTreahnentofMetals,EdrC.W.DraperaadP.Mazzoldi,Kluwer~ 
451(1986). 
10. H.A Wriedt and J.L. Murray, Bulletin of Alloy Phase Diagrams 8(4), 378 (1987). 


Microstructural Characterization of Laser Nitrided Titanium

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