Isothermal oxidation of sintered Ru-doped and undoped Ni-alloy coupons in the range of 1173–1423 K was investigated. To assess the effect of Ruthenium doping, microstructural characterization was performed to compare the samples before and after oxidation treatment at 1173 K. Furthermore, cyclic oxidation tests on the Ru-doped and undoped coupons were carried out in a thermogravimetry apparatus at 1223 and 1323 K. The addition of 0.8 wt.% of Ru in NiCoCrAlYTa powders sintered coupons does not degrade the material’s resistance to oxidation conditions

Juárez López, Fernando

Vahlas, Constantin

Álvarez Chávez, José Alfredo

English

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 Any correspondence concerning this service should be sent to the repository administrator:staff-oatao@inp-toulouse.fr  Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author -deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID: 3810 To link to this article: DOI:10.1016/j.corsci.2009.06.001 URL: http://dx.doi.org/10.1016/j.corsci.2009.06.001 To cite this document : Juárez L, F. and Vahlas, Constantin and Alvarez C, J.A. ( 2009) Microstructural characterization of Ru-doped NiCoCrAlYTa coupons treated by thermal oxidation. Corrosion Science, vol. 51 (n° 9). pp. 2192-2196. ISSN 0010-938X Microstructural characterization of Ru-doped NiCoCrAlYTa coupons treatedby thermal oxidationF. Juárez L a,*, C. Vahlas b, J.A. Alvarez C aaCIITEC-IPN 02250, Cerrada de Cecati S/N, Santa Catarina Azcapotzalco, 002250 México DF, MexicobCIRIMAT UMR 5085, ENSIACET, 118 Route de Narbonne, 31077 Toulouse cedex 4, FranceKeywords:A. SuperalloysB. Thermal cycling, SEMC. Oxidationa b s t r a c tIsothermal oxidation of sintered Ru-doped and undoped Ni-alloy coupons in the range of 1173–1423 Kwas investigated. To assess the effect of Ruthenium doping, microstructural characterization was per-formed to compare the samples before and after oxidation treatment at 1173 K. Furthermore, cyclic oxi-dation tests on the Ru-doped and undoped coupons were carried out in a thermogravimetry apparatus at1223 and 1323 K. The addition of 0.8 wt.% of Ru in NiCoCrAlYTa powders sintered coupons does notdegrade the material’s resistance to oxidation conditions.1. IntroductionIt is important for novel aerospace applications, to improve theoxidation and corrosion resistance of the protective bond coat ofturbine blades operating at elevated temperature, and to increasethe adhesion between the bond coat and the adjacent materials,i.e., the substrate and the external thermal barrier. To meet theserequirements, doping of nickel aluminide and of MCrAlY (M = Fe,Ni and/or Co) bond coats by various metals or rare earth elementshas been studied. It has been found, for example, that addition ofrhenium [1] or platinum group metals, namely platinum itself, pal-ladium and ruthenium, improve the mechanical properties of thebond coat and increase the lifetime of appropriate-quality aluminascale that are in contact with the ceramic top coat [2–4]. Thus thepresence of these two elements facilitates the hardening of the cphase, and the coalescence of the precipitates slows down c [5].The effect of weak additions of Re on the MCrAlY powders is fur-ther documented [6,7]. It has been shown [1] that additions of Rediminish the activity of aluminum in the coating, which wouldsupport the formation of the a-Al2O3 variety. Specific results con-cerning the addition of Pt in MCrAlY powders indicate that this de-creases the thermal dilation coefficient, moderately increases theelastic limit and ductility and makes possible to increase the life-time of the scale of Al2O3 [8].The process being investigated here is the superficial doping ofcommercial powders with platinum group metals and other ele-ments using the spouted-bed metal–organic chemical vapor depo-sition (SB-MOCVD) technique [9]. Initial results introducing theprocess were presented in [10–12]. The present work is devotedto the thermal oxidation behavior of sintered ruthenium (Ru-doped) coupons obtained by spouted-bed MOCVD and hot uniax-ial pressure. The choice of Ru for this study is based on resultspreviously obtained, in which doping of monocrystalline nickelsuperalloys with this element reduces their high-temperaturecreep [13] and references therein. The question then arises as towhether such doping affects other important characteristics ofthese alloys, such as corrosion and resistance high-temperatureoxidation. In the following, the isothermal oxidation test is firstconducted. Microstructural analysis are performed on severallocations marked on a Ru-doped coupon surface to study the oxi-dation in details. Furthermore the cyclic oxidation process is per-formed. The kinetics of oxidation and the resultant scalemorphology of the corresponding coupons are discussed and abrief conclusion is made.2. ExperimentalNiCoCrAlYTa is a prealloyed material which is mainly composedof Ni with additions of Co (21 wt.%), Cr (19 wt.%), Al (8 wt.%), Ta(5 wt.%) and Y (1 wt.%). NiCoCrAlYTa coupons doped by 0.8% RuMOCVD and undoped were used in all experiments. Sintered Ru-doped and undoped cylinder were machined down from 10 to8 mm and then 1 mm thick coupons were sliced, polished with1000-grade SiC paper.Isothermal oxidation runs were carried out on sintered coupons(undoped and Ru-doped) for 24 h at 1173, 1223, 1323 and 1423 K,under dry air in a SETARAM_ TAG 24S thermobalance with massand temperature accuracy better than 1 mg and 1 °C.doi:10.1016/j.corsci.2009.06.001* Corresponding author. Tel.:+52 5949572191; fax: +52 57296000.E-mail addresses: fjuarezl@ipn.mx, fjuarezl681209@yahoo.com.mx (F. Juárez L).To examine oxidation in greater details, an experiment was car-ried on Ru-doped coupon surface. Several zones were marked byindentation using a Knoop microhardness test setup. The orienta-tion and relative positions of these zones were identified usingmicrometric displacements on the turntable of a Scanning ElectronMicroscopy (SEM). The coupon is oxidized at 1173 K for 1 h in dryair. Microstructural analysis was performed on the marked loca-tions using SEM, EDX, and X-ray diffraction.Furthermore, to observe the effect of doping on improving theresistance to oxidation at high-temperature, Ru-doped and un-doped sintered coupons were sliced and polished. Then, cyclic oxi-dation tests were carried at stage temperatures of 1223 and 1323 Kunder dry air of laboratory.Scanning electron microscopy (SEM), X-ray dispersive spectros-copy (EDS), and X-ray diffraction at grazing incidence (GIXRD),wereused formorphological analysis on themicrostructure and composi-tion of both sinteredRu-doped samples andoxide scales. Finally sec-ondary ion mass spectrometry (SIMS; CAMECA IMS4F/6F) inimaging mode with O2+ primary ions were used for morphologicand microstructural analysis of the sintered Ru-doped samples.3. Results and discussion3.1. Isothermal oxidation and microstructural analysisFig. 1 presents the mass gain per unit area curves at the fourtemperatures investigated for the undoped and doped samples.The mass gain per unit area after 20 h ranges between 0.07 and0.6 mg/cm2, corresponding to a pure, compact alumina thicknessof 0.4 and 3 lm, respectively. At all temperatures studied, a tran-sient stage of faster oxidation kinetics can be observed duringthe initial period of the heat treatment. Curves appear to be verysimilar for doped and undoped materials, especially at 1323 and1423 K, where a-alumina is expected to be predominant in theoxide scale. Scale growth kinetic was interpreted using the mostgeneral expression for parabolic kinetics:t ¼ aþ bDmþ cDm2 ð1Þwhere t is the time (s) and Dm is the mass gain per unit area(mg cmÿ2). Coefficient c is equal to the reciprocal of the parabolicrate constant kp (mg2 cmÿ4 sÿ1) independent of the initial condi-tions for integration of the rate equation. Eq. (1) was fitted to asmall first part (first hour) of the set of (t, Dm) data representativeof the initial formation of scale on the surface of the coupons todetermine the corresponding kp values. This equation was also ap-plied to the subsequent range (1–20 h) of the (t, Dm) data to obtainthe stationary state value of kp. The results are presented in anArrhenius plot in Fig. 2, where variations with temperature of thevalues of transient and stationary kp are reported. This diagram con-firms the previously reported kinetic behaviour. The transient oxi-dation kinetics is approximately one order of magnitude higherthan the stationary oxidation kinetics at 1223 K. The values re-ported for kp in the transient state are in very good agreement withthe results of Brumm and Grabke for NiAl oxidation in conditionswhere transient (theta) alumina is formed [14].Ru-doped sample behaviour at 1173 K has a slightly differenttrend as revealed in this (kp) figure, possibly due to some artefactconcerning the heat treatment of the coupon (During MOCVDdoped).Time (hr)0 5 10 15 20Mass gain (mg/cm2  )0.000.020.040.060.080.100.120.14Time (hr)0 5 10 15 20Mass gain (mg/cm2  )0.00.10.20.30.40.50.61423 K1323 K1223 K1173 KFig. 1. Mass gain versus oxidation time at 1173 and 1223 (lower part), and 1323,1423 K (upper part); Ru-doped (black lines) and undoped coupons (gray lines).Fig. 2. Arrhenius plots of the parabolic rate constants for the oxidation of theundoped (circles) and Ru-doped (squares) samples, for oxidation times of 1 h (graysymbols) and 20 h (black symbols). Long dash lines mains area from grown ofalumina.Several zones were delimited by indentation using a Knoopmicrohardness indenter. Fig. 3, illustrates the experimental proto-col followed for the study on surface of solid phases alloy and theoxides formed.It was expected to observe that different solid phase induceddifferent behaviors in oxidation [15]. Fig. 4 shows an ensemblefrom several zones before oxidation (marks) and after oxidation,measured on same points on the Ru-doped sample.EDS Microanalysis is presented (Fig. 4). According to the lec-tures, Al concentration is present in the b phase, which is in agree-ment with dark areas in the backscattered electron SEMmicrograph. However, Ru signal was to weak for EDS to establisha elemental map.It appears as well that the clearest oxide is formed at the equi-librium areas or at inter-phases where they are already character-ized by clear contrast. The darkest areas correspond to oxideformed from b phase and light areas correspond to oxides formedover c phase. By comparing several zones we found interest behav-iour and clear relationship between the nature of the solids phasesand the scale oxides. Clear areas correspond to the formation ofnickel-rich oxide, which can be attributed to NiO oxide detectedby X-ray diffraction shown in Fig. 5.X-ray diffraction analysis shows that the two major oxides are aalumina oxide layer and probably other type of spinel oxides struc-ture of solid solution formed by (Ni, Co)(Al, Cr)2O4. Thickness ofoxide is very thin, according to observations of the very intense dif-fraction peaks of b and c phases alloy matrix.3.2. Cyclic oxidation and microstructural analysis of the products ofoxidationCyclic oxidation tests were carried out at temperatures 1223and 1323 K under dry air for Ru-doped and undoped samples. InFig. 6, the results were plotted in terms of mass gain versus num-bers of cycles. As a comparison, a sample of superalloy MCrAlY pro-duced by VPS (vacuum plasma spray) was treated under the sameconditions of temperature and dry air. Parabolic behavior was ob-tained of the mass gain in function of time during cyclic oxidation.Due to inherent error by successive weighting, these curves aremore irregular than those obtained by isothermal oxidation.That behavior at the end of plots curves in undoped couponscould be result of the damage of the coat oxide, which wouldPrint marked on Ru doped samples, before oxidationAfter oxidation100 µmFig. 3. Experimental protocol followed for the study of the Ru-doped surface(relationship of solids phases alloy and oxides formed).10 µmγNickel EDS Analysis AluminiumEDS Analysis Aluminium EDS analysisABefore oxidation (A)After oxidation ( B)BβFig. 4. Assemblage illustrating the state of the surface of Ru-doped sintered sample, polished to 3 lm, before and after isothermal oxidation at 1173 K. Before oxidation (A):SEM Backscattered electron micrograph (high), Al EDS map (high left). After oxidation (B): SEM backscattered electron image (low). Ni and Al EDS map (low left).become be less protective. In contrast, Ru-doped samples showsnegligible trend to form oxide layer, which could be attributed tothe presence of Ru.The comparison between Ru-doped sample and that producedby VPS revealed similar behavior. VPS sample shows mass loss be-yond 550 cycles, probably caused by a chipping and followed byseparation of the oxide formed. Ru-doped sample shows compara-ble behavior beyond 650 cycles. Loss mass was calculated on bothRu-doped and undoped samples of 0.4 mg/cm2 and 0.6 mg/cm2,respectively. X-ray diffraction of Ru-doped sample after cyclic oxi-dation at 1323 K shows scale oxide formed to be mainly a aluminaand a mix spinel (Ni, Co)(Al, Cr)O4.Fig. 7a illustrates the morphological aspect of the oxide formedfrom undoped sample after cyclic oxidation at 1323 K. It is ob-served, a negligible chipping developed at sample edge, approxi-mately at the suspension hole, and the general aspect of scale isuniform. In contrast to the tests of isothermal oxidation (shortduration), non-influence of the microstructure alloy was observed.This is probably due to the consumption of aluminum from cphase. Adhered scale oxide is majorly composed of two types ofdifferent oxides; one oxide consists of morphology seems to ‘‘pinkof sands”, this is characteristic of transition alumina, which is sim-ilarly observed during isothermal oxidation. We expect largeramount d transient alumina to be present; however, X-ray diffrac-tion does not reveal the presence of d alumina. We hypothesizethat these transient alumina are formed during of first’s stages ofcyclic oxidation, and subsequently transformed into a stable alu-mina while preserving their morphology. One other oxide is char-acterized by regular and smooth, probably a alumina which wasformed at the start of oxidation.Micrograph from Ru-doped and undoped, Fig. 7b shows that theoxide scale formed at 1323 K has a quite different morphology.First of all, spallation was not observed on the Ru-doped surfacesample. Morphology of the scale oxide has aspect similar to aand c phases corresponding to the matrix alloy (principal). Massgain measured corresponds to a 3 lm scale oxide thickness. Thepersistence of this microstructure can be the result of the transi-tory stage, with the formation of an a alumina scale, this formationraise up controlled by anionic diffusion, which would make it pos-sible to preserve the original oxide morphology.In addition, Fig. 8 illustrates Scanning SIMS maps Al27 and Ru 99of the same 50  50 mm2 area of a surface polished from Ru-dopedsample. Two phases were identified dark and gray on the surfaces.These correspond well to b-NiAl and c-Ni solid phases, respec-tively. From a comparison of the two maps, it appears that Ru ispresent in b-NiAl. This observation could be result of the enhancedplasticity of b-NiAl during the sintering conditions, and agrees withliterature reporting that platinum group metals show a remarkableaffinity for this phase [3].4. ConclusionsIsothermal oxidation of undoped and Ru-doped samples al-lowed us to investigate the transient and stationary oxidationkinetics in the range 1173–1473 K. Ru doping by MOCVD doesnot significantly degrade the oxidation behaviour of the NiCoCrA-lYTa sintered samples.SEM characterization of the sintered Ru-doped coupons andoxidized at 1173 k was investigated. This characterization revealedexact relationship between the scale oxides and the matrix alloymicrostructure. A thin film of a alumina oxide was formed fromb-NiAl, Al-rich zones, and a spinel mixed (Ni, Co)(Al, Cr)2O4 fromc Ni-rich zones.Finally, the addition of Ruthenium (0.8% mass) in NiCoCrAlYTasamples reveal a superior behavior under conditions of cyclic oxi-dation to that of the bond coat produced by VPS. This approachcould be extended to study other elements such as Ir, Re, etc. ap-plied to the vanes or turbine.Fig. 5. X-ray diffraction analysis of the Ru-doped samples treatment at 1173 K and 1 h.Fig. 6. Mass gain versus time duration cyclic oxidation: cyclic oxidation at 1223and 1323 K Ru of both Ru-doped and undoped samples. Further VPS sample.AcknowledgementsWe are grateful for the support of CIITEC-IPN, México ac-corded to Fernando Juarez L., with the administrative supportof both SIP-COFAA-IPN-México and SNI-CONACyT. The supportof the Institute National Polytechnic de Toulouse initiative is alsoacknowledged.References[1] N. Czech, F. Schmitz, W. Stamm, Surf. Coat. Technol. 17 (1994) 68–69.[2] I.M. Wolff, L.E. Iorio, T. Rumpf, P.V.T. Scheers, J.H. Potgieter, Mater. Sci. Eng. A241 (1998) 264–276.[3] S.A. Alperine, P. Josso, J.P. Fournes, J.L. Leger, A.H.L. Malie, D.G. Manesse, EPPatent 0792948 A1, 1997.[4] T.A. Taylor, D.E. Bettridge. R.C. Tucker, US Patent 5455119, 1995.[5] R.W. Smashey, C.S. Wukusick, USA, 2 254 651 Accession (2 254 651), 1975.[6] N. Czech, F. Schmitz, USA, 5 154 885 Accession (5 154 885), 1992.[7] N. Czech, F. Schmtiz, Patent, USA, 5 268 238 Accession (5 268 238), 1993.[8] T.A. Taylor, D.E. Bettridge, Surf. Coat. Technol. 86–87 (1996) 9–14.[9] F. Juarez, A. Castillo, B. Pieraggi, C. Vahlas, J. Phys. IV 11 (2001) 1117–1123.[10] C. Vahlas, F. Juarez, R. Feurer, P. Serp, B. Caussat, Adv. Mater. CVD 8 (4) (2002)127–143.[11] F. Juarez, D. Monceau, D. Tetard, B. Pieraggi, C. Vahlas, Surf. Coat. Technol. 163–164 (2003) 44–49.[12] M.-C. Lafont, L. Fernando Juáre, C. Vahlas, Scripta Mater. (51) (2004) 699–703.[13] P. Caron, S. Naveos, J.-L. Raffestin, EP Patent 0971041, 2000.[14] M.W. Brumm, H.J. Grabke, Corros. Sci. 33 (1992) 1677–1690.[15] U. Hemmersmeier, M. Feller-Kniepmeier, Mater. Sci. Eng. A248 (1998) 87–97.Fig. 7. Samples after cyclic oxidation at 1323 K: undoped; (a) Zone spallation (high left), and detail of oxide layer. Ru-doped; (b) Detail of oxide layer (low).Fig. 8. 27Al and Ru SIMS maps of the same 50  50 lm2 area from Ru-doped sample.

Microstructural characterization of Ru-doped NiCoCrAlYTa coupons treated by thermal oxidation

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Microstructural characterization of Ru-doped NiCoCrAlYTa coupons treated by thermal oxidation

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