Silicon nitride (Si3N4) ceramics were selected as substrates due to their thermal and chemical compatibility to diamond that ensure the adequate NCD adhesion for mechanical purposes. NCD deposition was performed by hot-filament chemical vapour method (HFCVD) using Ar/H2/CH4 gas mixtures. The tribological assessment of homologous pairs of NCD films was accomplished using reciprocating ball-on-flat tests using NCD coated Si3N4 plates and balls. The friction evolution is characterized by an initial running-in regime with a sharp peak up to 0.70, shortly followed by a steady-state regime identified by very low friction coefficients values (0.02–0.03). The threshold load prior to delamination depends on the starting surface roughness of the substrates and attains a value of ¨40 N. In terms of wear performance, the NCD films reveal a mild wear regime (K~10-7 mm3 N-1 m-1) for self-mated dry sliding conditions

Abreu, C. S.

Amaral, M.

Fernandes, A. J. S.

Gomes, J. R.

Oliveira, F. J.

Silva, R. F.

Diamond and Related Materials

Universidade do Minho: RepositoriUM

evier.com/locate/diamondDiamond & Related MaterialFriction and wear performance of HFCVD nanocrystalline diamond coatedsilicon nitride ceramicsC.S. Abreu a, M. Amaral b, A.J.S. Fernandes c, F.J. Oliveira b, R.F. Silva b,*, J.R. Gomes da Physics Department, ISEP, Portugalb CICECO, University of Aveiro, Portugalc Physics Department, University of Aveiro, Portugald Mech. Eng. Department, CIICS, Univ. of Minho, PortugalAvailable online 1 December 2005AbstractSilicon nitride (Si3N4) ceramics were selected as substrates due to their thermal and chemical compatibility to diamond that ensure the adequateNCD adhesion for mechanical purposes. NCD deposition was performed by hot-filament chemical vapour method (HFCVD) using Ar/H2/CH4gas mixtures. The tribological assessment of homologous pairs of NCD films was accomplished using reciprocating ball-on-flat tests using NCDcoated Si3N4 plates and balls. The friction evolution is characterized by an initial running-in regime with a sharp peak up to 0.7, shortly followedby a steady-state regime identified by very low friction coefficients values (0.02–0.03). The threshold load prior to delamination depends on thestarting surface roughness of the substrates and attains a value of ¨40 N. In terms of wear performance, the NCD films reveal a mild wear regime(K¨107 mm3 N1 m1) for self-mated dry sliding conditions.D 2005 Elsevier B.V. All rights reserved.Keywords: Nanocrystalline; Hot-filament CVD; Tribology1. IntroductionNanocrystalline diamond coatings (NCD) are gainingincreasing interest as serious candidate materials for highperformance engineering surfaces in mechanical and tribolog-ical applications, due to their intrinsic smoothness combinedwith most of the outstanding properties of natural single-crystaldiamond [1]. Highly demanding tribological applicationswhere NCD coatings are already in use or thought of varyfrom the fluid handling and cutting tools industries to, morerecently, micro-electromechanical systems (MEMS) and bio-medical applications [2–4].The wear performance and, more extensively, the frictionproperties of NCD coatings sliding against metallic or ceramicdissimilar counterpairs have been studied [5,6]. However, to ourknowledge, no reports are found in the literature concerning the0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.diamond.2005.10.042* Corresponding author. Tel.: +351 234370243; fax: +351 234425300.E-mail address: rsilva@cv.ua.pt (R.F. Silva).tribological performance of unlubricated homologous pairs ofNCD films under high applied stresses.Silicon nitride (Si3N4) ceramics constitute a novel andprospective substrate material for the deposition of NCD filmsdue to their chemical compatibility and low thermal mismatchwith diamond, which translates as an improved adhesion [7,8].Being one of the toughest ceramic materials, it possesses anadvantageous combination of intrinsic mechanical, thermal andchemical properties, as well as good wear resistance, impera-tive for demanding tribomechanical applications.The hot-filament chemical vapour deposition (HFCVD)technique is one of the most common available methods ofproducing polycrystalline diamond films and, more recently,NCD films. This versatile CVD based deposition techniqueallows the diamond deposition in a simple and cost-effectiveway, overcoming some of the limitations of other moresophisticated CVD techniques such as microwave plasmaCVD (MPCVD), namely in the ability of coating complexgeometries.Reciprocating ball-on-flat experiments were undertaken toassess the tribological behaviour of NCD homologous pairs.s 15 (2006) 739 – 744www.elsC.S. Abreu et al. / Diamond & Related Materials 15 (2006) 739–744740Three different surface finishing states were employed on theSi3N4 substrates. The applied load threshold at whichdelamination of the films occurs under tribological stresswas determined for all tested conditions. Different charac-terization techniques were used in order to examine thequality, morphology and topography of the deposited andworn NCD films, respectively, A-Raman spectroscopy,scanning electron microscopy (SEM) and atomic force micro-scopy (AFM).2. Experimental detailsDetails on powder preparation and sintering of Si3N4substrates can be found elsewhere [9]. Square substrates(10103 mm3) were cut from dense ceramics, ground andsubmitted to three different surface finish conditions: (i) flatlapping with 15 Am diamond slurry (FL samples); (ii) hardcloth polishing with 15 Am diamond slurry (PP); and (iii)polishing to a mirror-like finish with colloidal silica (P).Commercial silicon nitride balls (<5 mm, Kema Nord) werealso used for diamond deposition, in order to act as counter-bodies in the sliding experiments. Silicon nitride substrates1100 1200 1300 1400 1500 1600 1700 1800λ-1 (cm-1)Intensity (a.u.)(b) 10 μm  (a) Fig. 1. (a) Morphology of a representative NCD film on a Si3N4 ball substrate.The inset shows a detail five times magnified relative to the main micrograph;(b) Raman spectrum of a typical as-deposited NCD film.were then scratched for 1 h in an ultrasonic suspension with 1Am diamond powder in n-hexane (1 g/100 ml). Nanocrystallinediamond films were grown by the HFCVD technique using anAr/CH4/H2 gas mixture where the Ar/H2 and CH4/H2 ratioswere fixed at 0.1 and 0.04, respectively. The other depositionconditions were the following: W filament temperature—2300-C, total gas flow—50 mlImin1, total pressure—50 mbar,substrate temperature—750 -C. The nanodiamond growth rateswere, respectively, 1.6 AmIh1 on the plates and 2.2 AmIh1 onthe balls.All the substrates were characterized by AFM (DigitalInstruments, IIIa, USA) before and after deposition in order todistinguish their topographic features. The morphology andthickness of the NCD films were assessed using SEM (HitachiS4100, J). The NCD quality of the as-deposited films wascharacterized by a A-Raman spectrometer (Jobin Yvon T64000,F) with the 514.5 nm line.The tribological testing of self-mated NCD films wasperformed on a ball-on-flat reciprocating sliding tribometer(PLINT TE67/R, UK). Tests were conducted in ambientatmosphere (¨50–60% RH), under unlubricated conditions,with a constant stroke of 6 mm and frequency of 1 Hz. Slidingdistances were chosen to be of ¨86 m (regular tests) and forthe lengthier ones ¨690 m (endurance tests), which correspondto 2 h and 16 h of in-interrupt sliding, respectively. Normalloads in the range of 10 to 40 N were applied by means of deadweights and were kept constant for each run. Considering apurely elastic contact of the sphere/plane surface, theseconditions represent a mean Hertzian pressure (normal stress)in the range of 4.4–7.1 GPa [10,11].Following the sliding tests, the worn NCD surfaces wereexamined by SEM and AFM techniques. The wear coefficientof the balls was assessed by measuring the corresponding wearscar diameter in SEM.3. Results and discussion3.1. Morphology of substrates and as-grown NCD filmsThe morphology of the as-deposited NCD films on siliconnitride substrates reveals a good uniformity, as shown inFig. 1a, for a ¨25 Am thick film grown on a ball. The includedinset exhibits the presence of a pattern of smooth roundelevations on the top of which the nanosized diamond grainsare perceptible. Fig. 1b depicts a representative A-Ramanspectrum of an as-deposited NCD film, containing the diamondcharacteristic peak at ¨1332 cm1 and the features usuallyassigned to transpolyacetylene (at 1140 and 1480 cm1),typical of NCD films [12,13]. The D and G bands (at 1350cm1 and 1580 cm1, respectively) associated with the sp2coordinated material are also present.AFM scans of the Si3N4 substrates with the distinct surfacefinishing conditions analyzed in the present study, along-sidewith the corresponding deposited NCD films, are presented inFig. 2. Each 3D plot is accompanied by its respective surfaceroughness value, Rq (rms). The different uncoated substratesurface finishing quality is clearly evidenced: for the FL (d) (a) (b) (e)(c) (f)Rq = 169 nm R q = 206 nmR q = 118 nmR q = 15 nmR q = 92 nmR q = 41 nmFLPPP5000.0 nM4400.0 nM500.0 nm6000.0 nM4000.0 nM2000.0 nM20406080μm20406080μm20406080μm20406080μm20406080μm20406080μmFig. 2. 3D AFM images of uncoated (left column) and NCD coated (right column) substrates. FL—flat lapped substrate; PP—pre-polished; P—polished.C.S. Abreu et al. / Diamond & Related Materials 15 (2006) 739–744 741sample, deep grooves are visible as a result of the roughingoperation; for the PP sample, a somewhat irregular surfacetopography is still patent, however without the presence ofgrooves; finally, for the P sample, a high smoothness wasachieved. The above described surface distinct conditions arecorroborated by their relative Rq values, in the range of 169 nmfor the roughest and of 15 nm for the smoothest one. Aftercoating, the roughness rank is kept. However, particularphenomena must be taken into account. In fact, for thedeposition time used, the film tends to magnify the roughnessfor rougher starting surfaces, such as in the case of FL sample(Fig. 2a,d). As it can be seen, the NCD film growth follows thegrooves of this sample, while prominent conglomerates formedin the vicinity of local asperities, resulting on increased Rq upC.S. Abreu et al. / Diamond & Related Materials 15 (2006) 739–744742to 206 nm. On the other hand, since the PP sample presentslower starting roughness, the magnifying effect does not takeplace, a smoothing occurs instead (Fig. 2b,e). For this sample,the NCD smooth deposit actually improves the surface finish,whose initial roughness was not enough to promote the abovedescribed magnifying effect. The intrinsic smoothness of theNCD deposits is put in evidence for the case of the P sample,whose starting Rq roughness of 15 nm augmented to only 41nm for a ¨20 Am thick film. Concerning the NCD coated ball,a nominal roughness of 87 nm was obtained.3.2. Morphology of worn NCD films and tribologicalbehaviourAs can be seen from the low magnification SEM micro-graph of the FL wear track (Fig. 3a), material loss by polishingtakes place under an applied load of 20 N for shorter runs (¨86m), without completely masking the deep scratches inherent tothis surface finish condition. In the PP wear track, a similarmorphology of the worn surface indicates identical phenomena,although a more evenly distribution of polished sites can beobserved due to the lower initial roughness associated to thissurface finish condition.At high magnification (Fig. 3c), depressions can beobserved alongside with plateaus formed by a self-polishing(micro-abrasion) mechanism, which lead to the truncatingof NCD conglomerates and subsequent flattening. TheFig. 3. Aspect of the wear tracks for samples (a) FL and (b) PP under an applied loadfrom truncated NCD conglomerates; (d) lumps of cracked material at the edge of tdepressions between the plateaus correspond to undamagedNCD regions.Also noticeable (Fig. 3d) is the crack pattern in lumps ofmaterial localized at the edge of the wear tracks, originatedfrom the NCD wear debris smeared by the contact pressure andsliding interaction.Representative friction evolution curves for each surfacefinishing condition are depicted in the plots of Fig. 4a–c.Regardless the analyzed sample, each friction coefficient (l)curve is characterized by an intense narrow initial peak,originated from the mechanical interlocking of the filmasperities. Following this main feature, a relatively shorttransition stage sets-in associated to the flattening of asperities,until the accommodation of the antagonist surfaces is reached.Finally, due to widespread polishing, the friction coefficientlevels off, reaching extremely low values in the range of 0.02–0.03. The friction behaviour of FL (Fig. 4a) and PP (Fig. 4b)samples is similar but the initial peak of the FL sample reachesa higher intensity due to its rougher surface.On the contrary, the curve corresponding to the very smoothP sample reveals an enlarged friction peak (Fig. 4c) resultingfrom premature film detachment. In fact, the threshold load forthis tribosystem is below 10 N, much lower than the moreresistant FL (40 N) and PP ones (20 N), denoting pooradhesion. In the P system, the self-polishing transition regimeis absent. Nonetheless, very small values for the frictioncoefficient are observed after the occurrence of delamination asof 20 N and a sliding distance of 86 m; (c) detail of polished plateaus, originatedhe wear track.0 1 2 3 4 5 60.00.10.20.30.40.50.60.70.8μx (m)(a)0.0 0.4 0.8 1.2 1.60.00.10.20.30.40.50.60.70.8μx (m)(c)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00.00.10.20.30.40.50.60.70.8(b)μx (m)0 100 200 300 400 500 600 7000.00.10.20.30.40.50.60.70.8(d)μx (m)Fig. 4. Typical friction curves obtained for the various surface conditions: (a) FL under an applied load of 20 N, (b) PP under 10 N, (c) P under 10 N and (d) 690 mendurance test under 10 N for a PP sample.C.S. Abreu et al. / Diamond & Related Materials 15 (2006) 739–744 743a consequence of the tribological interaction between a smoothNCD film still present in the ball, sliding against a softerexposed Si3N4 flat substrate.For the endurance tests (Fig. 4d), the friction coefficientgradually increases with the sliding distance after remaining atits minimum for distances in the range of the regular tests. Thisrelates to a progressive surface degradation effect that isdocumented in Fig. 5 for the NCD coated balls, in whichcracking is discernible and a chipping wear mechanism isrevealed.The general trend for the NCD friction behaviour is alsocharacteristic of self-mated sliding systems of microcrystallinediamond coated Si3N4 ceramics [8]. However, a higher frictionpeak is obtained for NCD (l¨0.7) than for diamond(l¨0.25) for the same applied load (20 N). A possibleexplanation is related to the higher energy required for thetruncating of NCD conglomerates when compared to the easiercleavage of micropyramidal sharp crystals, the reported mainwear mechanism of diamond.The reported threshold load prior to delamination for theSi3N4 microcrystalline diamond coated tribosystem is 80 N, fora coating thickness close to that of the present work [8]. Thisvalue is twice the highest one obtained in the present work forthe FL specimen, what denotes lower adhesion levelscompared to the microcrystalline deposits. However, the samethreshold of 40 N is found for self-mated tribosystems of NCDgrown by MPCVD [14]. The wear performance of self-matedNCD films, in terms of the wear coefficient values for the ballspecimens are in the order of 107 mm3 N1 m1. Thesevalues point out to a mild wear regime. It is noteworthy that avery mild wear (5108 mm3 N1 m1) was measured forthe endurance tests, which represent a more accurate assess-ment of this tribological parameter than the shorter ones. Theseresults are similar to those obtained for self-mated microcrys-talline diamond films and NCD grown by MPCVD on thesame substrate material [8,14].4. ConclusionsNCD films were grown by HFCVD on Si3N4 ceramicsubstrates presenting surface roughness of 15 nm to 169 nm.NCD matches the starting surface during growth, mimicking itstopography.Fig. 5. SEM micrographs taken from a ball specimen, depicting the surfacedegradation after the endurance tests at different magnifications. Crack featuresand the chipping wear mechanism are clearly evidenced.C.S. Abreu et al. / Diamond & Related Materials 15 (2006) 739–744744The friction behaviour of the NCD self-mated system isaffected by the surface roughness, resulting in intense andephemeral friction coefficient peaks up to 0.7. After anaccommodation period also dependent on the surface rough-ness, very low l values of 0.02–0.03 are reached for drysliding conditions. Self-polishing (micro-abrasion) is thepredominant wear mechanism of the NCD films resulting inultra-smooth surfaces. The lengthier tests indicate an increaseof l up to 0.15 due to some chipping of the worn NCD film onthe ball.The threshold load prior to delamination attained a value of40 N, which denotes a good adhesion level. Wear coefficientvalues for the ball specimens are of the order of ¨107 mm3N1 m1, for the shorter tests, while the endurance tests resultin very mild wear regimes (108 mm3 N1 m1).AcknowledgementsThis work was supported by project NANODIAM(FCTPOCTI/CTM/59449/2004).References[1] A. Erdemir, G.R. Fenske, A.R. Krauss, D.M. Gruen, T. McCauley, R.T.Csencsits, Surf. Coat. Technol. 120–121 (1999) 565.[2] A. Olszyna, J. Smolik, Thin Solid Films 459 (2004) 224.[3] A.R. Krauss, O. Auciello, D.M. Gruen, A. Jayatissa, A. Sumant, J.Tucek, D. Mancini, N. Moldovan, A. Erdemir, D. Ersoy, M.N. Gardos,H.G. Busmann, E.M. Meyer, M.Q. Ding, Diamond Relat. Mater. 10(2001) 1952.[4] J.E. Gerbi, M. Sardela, J.A. Carlisle, Thin Solid Films 473 (2005) 41.[5] P. Hollman, O. Wanstrand, S. Hogmark, Diamond Relat. Mater. 7 (1998)1471.[6] C. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A.Erdemir, C. Bindal, G. Fenske, Thin Solid Films 270 (1995) 154.[7] M. Belmonte, A.J.S. Fernandes, F.M. Costa, F.J. Oliveira, R.F. Silva,Diamond Relat. Mater. 12 (2003) 733.[8] C.S. Abreu, F.J. Oliveira, M. Belmonte, A.J.S. Fernandes, R.F. Silva, J.R.Gomes, Wear 259 (2005) 771.[9] M. Amaral, F.J. Oliveira, M. Belmonte, A.J.S. Fernandes, F.M. Costa, R.F.Silva, Surf. Eng. 19 (6) (2003) 410.[10] I.M. Hutchings, Tribology—Friction and Wear of Engineering Materials,Ed. Butterworth-Heinmann, UK, 2003.[11] H.D. Espinosa, B. Peng, B.C. Prorok, N. Moldovan, O. Auciello, J.A.Carlisle, D.M. Gruen, D.C. Mancini, J. Appl. Phys. 94 (9) (2003) 6076.[12] A.C. Ferrari, J. Robertson, Phys. Rev .B 63 (2001) 121405.[13] R. Pfeiffer, H. Kuzmany, N. Salk, B. Gunter, Appl. Phys. Lett. 82 (2003)4149.[14] C.S. Abreu, M.S. Amaral, F.J. Oliveira, A. Tallaire, F. Be´ne´dic, O. Syll, G.Cicala, J.R. Gomes, R.F. Silva, Surf. Coat. Technol., in press.

Friction and wear performance of HFCVD nanocrystalline diamond coated silicon nitride ceramics

http://repositorium.sdum.uminho.pt/bitstream/1822/6129/1/DRM_15_2006_pp739.pdf

Friction and wear performance of HFCVD nanocrystalline diamond coated silicon nitride ceramics

Abstract

Similar works

Full text

Available Versions

Universidade do Minho: RepositoriUM