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Tribological characterization of diamond-like carbon (DLC) coatings sliding against DLC coatings
Diamond and Related Materials 12 (2003) 1389–1395
Tribological characterization of diamond-like carbon (DLC) coatings sliding against DLC coatings D. Sheejaa,*, B.K. Taya, S.M. Krishnana, L.N. Nungb a
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169608, Singapore Received 14 August 2002; received in revised form 17 March 2003; accepted 4 April 2003
Abstract Two different qualities (single-layer and multi-layer) of thick diamond-like carbon (DLC) coatings were prepared on both extremely polished (Ra ;0.01 mm) stainless steel disks and 6 mm diameter stainless steel balls. Morphological and microstructural evaluations reveal that multi-layer DLC coatings are superior in nature compared to the single-layer coatings. Tribological characterisations using a pin (ball)-on-disk tribometer shows that, even though the multi-layer DLC coating exhibits superior qualities, when it slides against the same displays relatively higher friction and wear compared to that of single-layer DLC sliding against the same. Multi-layer DLC coated steel ball sliding against the single-layer DLC coated disk displays the best tribological performance. However, stress cracking was observed very rarely at the sliding edge of the multi-layer film prepared on the ball and is believed to be due to the relatively higher compressive stress in the film combined with the applied stress. The tribological performance of the single-layer DLC sliding against the same is also found to be excellent, and is comparable to that of the multi-layer DLC coated ball against the single-layer DLC coated disk. No cracks were observed on the low stress single-layer DLC coated ball after the friction test. The study reveals that the single-layer DLC sliding against the same would provide the best tribological DLC pair for relatively higher load applications such as in orthopaedic implants. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: DLC coating; Friction and wear; Multi-layer; Raman spectra
1. Introduction Metallic or ceramic femoral head sliding against UHMWPE (ultra high molecular weight polyethylene) acetabular cup is quite common in artificial hip joints, even though metal–metal sliding pairs are getting popular in these days. The clinical performance of the components made of UHMWPE, is good except for the concern about their wear. Failure of the implant due to wear debris has been identified as the main limiting factor for the total joint replacement w1–3x. A new or improved (surface modification of the existing biomaterial) sliding pair, which produces least wear is one of the solutions for a long lasting orthopaedic implant. Some studies w4,5x have reported that diamond-like carbon (DLC) coatings can provide feasible solutions to the problem of wear and related failure of the total *Corresponding author. Tel.: q65-67906127; fax: q65-67933318. E-mail address: [email protected] (D. Sheeja).
joint replacements, due to high wear resistance, biocompatibility and chemical inertness of the coating. Firkins et al. w4x have investigated the wear of UHMWPE against DLC coated (by CVD) stainless steel and reported that the DLC coating reduces the wear of UHMWPE by about seven times. Xu and Pruitt w5x have also reported that, the DLC coating improved the frictional and wear characteristics of DLC coated Ti-6Al4V sliding against UHMWPE. This encouraged us to carry out studies on the tribological behaviour of DLC coated biomaterial against UHMWPE, where the DLC coatings were prepared by Filtered cathodic vacuum arc (FCVA) technique, which can produce high quality coatings w6–8x. However, the wear resistance of UHMWPE sliding against superior quality DLC coating prepared on Co–Cr–Mo material, in simulated body fluid, did not show any improvement over that of the un-coated w9x. The possible reason for the slightly higher wear rate of the DLCyUHMWPE sliding pair compared
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to the Co–Cr–MoyUHMWPE is that, as the polymer is being softer, sliding by a harder (good quality DLC) surface can remove more particle than that by a softer material (un-coated Co–Cr–Mo). This shows that coating the metal surface with DLC might not be a good solution to improve the wear resistance of metaly UHMWPE sliding pairs. The current trend in the total joint replacement is to substitute the metallicyceramic material sliding against UHMWPE (being a weak component in the total joint replacement) with metal sliding against metal, in which Co–Cr–Mo alloy is being one of the key materials. However, the dissolution of Cr ion and the uncertainty of that being a carcinogenic component is one of the main concerns in the case of Co–Cr–Mo sliding against the same, in addition to the tribological behaviour of the material pair. Surface modification of Co–Cr–Mo alloy material with an inert coating such as high quality DLC film may prolong the life of the component as well as reduce the release of Cr ions. This led us to investigate a preliminary study on the tribological behaviour of DLC coating sliding against the DLC coating of different qualities, to evaluate and optimise for the best DLC sliding pair. The difficulty in obtaining Co–Cr– Mo alloy balls, force us to deposit DLC films on steel substrates. The custom-made Co–Cr–Mo pins will have geometrical irregularities and that may lead to wrong conclusions. However, the study will be carried out using the custom-made Co–Cr–Mo pins to verify the results. As we know, the quality of the film varies greatly on the deposition condition. In this article we discuss two different qualities (single-layer and multilayer) of thick DLC coatings. The films were prepared using filtered cathodic vacuum arc (FCVA) system in conjunction with high substrate pulse biasing, which is found to be one of the best methods to prepare low stress, thick films w10–14x. 2. Experimental details 2.1. Materials and methods Stainless steel (316L) disks of 25 mm diameter and 3 mm thickness were ground and polished to get mirror like smoothness. The grinding and polishing were done in many stages. First step of grinding was done with 240 grit silicon carbide abrasive paper followed by 400, 800 and 1200 grit abrasive papers. Subsequently the grounded surfaces were polished with coarse and fine alumina solution to get mirror-like smoothness. The polished substrates were cleaned with soap solution, followed by water, acetone, isopropyl alcohol and DI water in an ultrasonic bath tank and dried with nitrogen blow off gun. The samples were then analysed for its roughness using a TENCOR P-10 Surface Profiler. The average surface roughness values of the samples were
maintained at approximately 0.01 mm, in order to achieve the roughness comparable to that of orthopaedic surface roughness. The 6 mm diameter stainless steel balls were obtained from a commercial supplier (CSEM). 2.2. Deposition conditions The films were deposited using the FCVA system, which is described in detail elsewhere w4,9x. The carbon plasma for the DLC films was obtained from a graphite target of 99.999% purity. The steel substrates were precleaned with acetone, followed by alcohol and deionised water, and blown dry using nitrogen gas. The cleaned substrates were placed in the deposition chamber. The chamber was then evacuated to a system base pressure below 3=10y4 Pa, but increased to 2=10y3 Pa during deposition. Before deposition, the native oxide layer on the stainless steel substrates was cleaned using Ar plasma for approximately 20 min. Two different qualities (multi-layer and single-layer) of DLC films were prepared, on both steel disks as well as steel balls. In both cases, an adhesive layer of approximately 20–25 nm was prepared under pulse parameters of 9 kV, 600 Hz and 25 ms. In the case of multi-layer films, the adhesive layer was followed by a relatively thick (80–100 nm) hard layer that was prepared without any substrate bias, subsequently soft layer of approximately 20–25 nm was prepared with pulse parameters of 3 kV, 600 Hz and 25 ms. The repetitive deposition of hard and soft layers of DLC film were continued until the desired thickness of approximately 1 mm was reached. In the case of single-layer films, the adhesive layer was followed by soft films prepared with pulse parameters of 3 kV, 600 Hz and 25 ms. 2.3. Characterisation of the films 2.3.1. Thickness and stress measurement In order to obtain the thickness of the film, a piece of silicon (Si) wafer was placed next to the steel sample. A line was marked on the Si wafer with a marker before it was placed in the deposition chamber. During the sputter cleaning, the Si wafer was covered with a metal plate. Before deposition (after etching), the metal plate was shifted aside and the Si wafer was deposited along with the stainless steel sample. Subsequently, the marker line on the Si was erased with alcohol and a surface profilometer was used to scan across the boundary of the coated and uncoated surfaces of the un-etched Si sample. As it is difficult to measure the intrinsic stress on the metal substrate by radius of curvature technique, a rough estimation of the intrinsic compressive stress in the film was obtained by measuring the stress in the films prepared on Si substrates, which were deposited along
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with the metal substrates. The intrinsic compressive stress in the film prepared on the Si (ss) is calculated using Stoney’s Equation, given by: sss
Es ts2 B 1 1E C y F 6(1yys) tc D R Ro G
Where Es, ys, and ts are the Young’s modulus, Poisson ratio and thickness of the substrate, and tc is the thickness of the film. Ro and R are the radius of curvatures of the bare and the coated substrate, respectively, and are measured using a surface profilometer. 2.3.2. Morphological and structural characterisation The surface morphology of the films was studied by using an Atomic Force Microscopy (AFM) in tapping mode. The morphological evaluations were carried out on the films prepared on the steel as well as the Si substrates, which were prepared along with the steel substrates. The RMS (Root Mean Square) roughness of the films was obtained from the digital image processing software, which comes with the AFM system over an area of 1=1 mm. A visible Raman spectroscopy, with 514 nm lines of Arq laser as the excitation source, was used to study the microstructure of the films that were prepared on the steel substrates. The spectra were acquired over a range 600–2000 cmy1.
Fig. 1. Geometry of the DLC coated stainless steel ball after friction test, for the purpose of the calculation of the wear volume.
of the film, which is around ;3q0.001 mm; t is unknown and it is possible to calculate the value of t from the trigonometric relations and is: tsRy yR2y(dy2)2. Once we know the wear volume, wear rate can be calculated using the same relation as mentioned above. 3. Results and discussion 3.1. Film thickness and stress
2.3.3. Tribological characterisation using pin (ball)-ondisk tribometer A commercially available unidirectional pin (ball)on-disk tribometer has been used to study the frictional and wear characteristics. The tests were carried out at a constant load of 5 N, with a linear speed of 3 cmys in ambient air (RH: 50%, temperature 23 8C). The software comes with the system records the dynamic friction spectra. The wear rates of the DLC films prepared on the disk surfaces were calculated from the residual wear tracks, obtained after the friction tests. The cross-sectional area of the wear track was measured using a stylus profilometer; and the wear volume was calculated from the track cross-sectional area. The wear rates were then calculated using the relation; wear rate (k)sVy(WS), where V is the wear volume in mm3, W is the normal applied load in Newton and S is the sliding distance in meters. Four measurements were taken on each sample at 908 apart. The wear volumes of the DLC film, prepared on the stainless steel balls were calculated from the change in DLC-coated stainless steel ball geometry. The wear scar diameter, d (see Fig. 1) was measured using an optical microscope. The wear volume, the shaded part shown in Fig. 1, is then calculated using the relation: Vs pt2(3Ryt) ; where R is radius of the ballqthe thickness 3
The thickness of the multi-layer and single-layer films were prepared on the steel disks and are found to be approximately 620–770 nm, respectively; and that prepared on the steel balls are found to be approximately 1.21–1.32 mm, respectively. As the ball being the static friction partner, the film on it wears off fast (the contact point is the same unlike the disk). Hence, thicker films were prepared on the balls. The intrinsic stress in the multi-layer film prepared on the Si substrate (deposited along with the steel substrate) is measured to be approximately 4.2 GPa and that in the single-layer film is approximately 0.59 GPa, which is relatively low. The actual intrinsic stress in the film prepared on the steel substrates would be much lower than that on the Si substrate, as it is possible to grow thicker films on steel substrate under floating condition. The above stress values give us a rough estimation of the intrinsic stress in the films. 3.2. Morphological and structural characterization The morphology of the multi-layer as well as the single-layer films prepared on the steel and the Si substrates is shown in Fig. 2. It is obvious that the multi-layer film exhibit smooth morphology compared to that of the single-layer film prepared on the Si substrates. The growth mechanism is observed to be
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Fig. 2. Morphology of the 1 mm thick multi-layer and single-layer DLC films prepared on Si and steel substrates (scan area: 1=1 mm).
almost same regardless of the substrate on which it has prepared. However, the RMS roughness (Rrms) values of the films prepared on the steel substrates are found to be slightly higher than that prepared on the Si substrates. Moreover, the single-layer film prepared on the steel substrate displays a lower Rrms than that of the multi-layer film. The probable reason for these differences in roughness could be due to the substrate surface roughness. Even though the single-layer DLC film is observed to be slightly rougher, the engineering surface roughness (Rrms) measured over larger length of the order of mm, by a surface profiler, is found to be comparable (approx. 0.012 mm) with that of the well polished steel substrates. The Raman spectra of the films prepared on the disks as well as balls are shown in Fig. 3. There is no obvious difference in the spectra of the single-layer film prepared on the disks as well as balls, and the same goes with the multi-layer films. Raman spectra were captured on different locations of the ball (only half of the ball is coated). However, we did not observe any significant changes in the spectra. The spectra were then de-
convoluted into ‘D’ and ‘G’ peaks to see the change in ID yIG ratio. The multi-layer films prepared on the disks and balls display ID yIG values of 0.35"0.003 and 0.43"0.08, respectively. The single-layer films prepared on the disks and balls display ID yIG values of 0.85"0.09, 0.88"0.17, respectively. The multi-layer film exhibits lower ID yIG ratio compared to the singlelayer films. This undoubtedly shows that multi-layer films are superior in nature compared to the single-layer films. In addition, the films prepared on the ball display slightly higher ID yIG ratio compared to that prepared on the disks, which suggests that the quality of film may vary slightly depending on the curvature of the substrate. However, in the present study, the values are quite comparable. 3.3. Friction and wear Tribological tests were carried out on the two different qualities of DLC coated steel disks sliding against the un-coated as well as two different qualities of DLC coated steel balls. Fig. 4 shows the dynamic friction
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Fig. 3. Raman spectra of (a) Multi-layer and single-layer DLC films prepared on the disks; (b) Multi-layer and single-layer DLC films prepared on the balls.
spectra of the single-layer and multi-layer DLC coated steel disks against the same kind of coating as well as the single-layer against the multi-layer DLC film; and Fig. 5 shows the typical wear scars of the single-layer and multi-layer coated steel balls after friction test. It is obvious from Fig. 4 that the single-layer DLC sliding against the same as well as the single-layer DLC coated disk sliding against the multi-layer DLC coated ball display low coefficient of friction (-0.05). However, the multi-layer DLC film sliding against the multi-layer DLC film displays slightly higher friction coefficient of approximately 0.09. It is observed from the friction tests carried out with different combinations of sliding pairs that, sliding of single-layer DLC coated disk sliding against the same or multi-layer DLC coated static counter-face would make the best tribological pair, with friction coefficient of 0.05 or lower. The wear rates of the coating on the disk and the ball (after the friction tests) were calculated and are given in Table 1a and b. It is obvious from the single-layer DLC coated disk sliding against the three different static friction partners (multi-layer DLC, single-layer DLC and un-coated balls) that DLC on both the sliding surfaces reduces the wear of both the disk and the ball. Between the first two sliding pairs shown in Table 1, with the best tribological behaviour, the single-layer DLC coated disk sliding against the multi-layer DLC coated ball gives the best properties. As the ball being a static friction partner (the same point will be sliding throughout the test), a harder (multi-layer) coating on the ball displays the lowest ball wear rate. As we can see from Table 1(b) that the multi-layer DLC on the disk sliding against the single-layer DLC
on the ball, reduces the disk wear rate but increases the ball wear rate significantly and is expected. Multi-layer DLC film demonstrates a longer lifetime when it slides against single-layer film. However, the multi-layer against multi-layer demonstrates a relatively higher wear rate, which is un-expected. From the friction spectra, one can see that in addition to the relatively higher friction, the noise is also slightly higher for this sliding pair (multi-layerymulti-layer). The results suggest that even though multi-layer DLC coating has higher wear resistance, sliding of it against the same may not be a good choice. In order to get some clues or explanations for the slight difference in their tribological behaviour, the worn surfaces were analysed using the Raman spectroscopy. However, the spectra were observed to be similar to that of the respective as-deposited films. We have also tried to transfer the wear particles to a bare Si substrate to study the Raman spectra of the wear debris. However, it was found quite difficult to get any reasonable amount of wear debris to capture the spectra. In addition, very rarely, we have observed stress crackingybulging on the multi-layer film prepared on the balls after the friction test (see Fig. 6). This might probably be due to the relatively high compressive stress in the multi-layer DLC film compared to that in the single-layer DLC film. However, we have never observed any such stress-crackingybulging of the multilayer film prepared on the flat surface (disk), which reveals that the curvature (ball) of the surface may also play a major role on the stress. Consider the wear rates of the single-layeryun-coated steel and multi-layeryun-coated steel sliding pairs. The wear rate of un-coated steel ball sliding against multi-
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Fig. 4. Dynamic friction spectra of different types DLC sliding pairs (5 N, 3 cmysec and atmospheric environment).
Fig. 5. Worn surface of (a) single-layer DLC coating on the ball (b) multi-layer DLC coating on the ball.
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Table 1 Wear rates of disk and ball in two-different qualities of DLC coated disk sliding against different static friction partners Sliding pair
Disk wear rate mm3 yNm
Ball wear rate mm3 yNm
Rotating (disk)
Static (ball)
(a) Single-layer Single-layer DLC coating Single-layer DLC coating Single-layer DLC coating
Multi-layer DLC coating Single-layer DLC coating Uncoated steel
2.42=10y7 2.54=10y7 4.32=10y7
5.0=10y9 6.36=10y9 4.06=10y8
(b) Multi-layer Multi-layer DLC coating Multi-layer DLC coating Multi-layer DLC coating
Multi-layer DLC coating Single-layer DLC coating Uncoated steel
3.0=10y7 1.83=10y7 1.22=10y7
1.4=10y8 1.32=10y8 3.43=10y7
DLC coated static friction partner against the singlelayer DLC coating displays the best performance. However, friction test under relatively higher loads cause cracking of the multi-layer DLC prepared on the curved surface, due to the combined effect of the intrinsic stress and the applied stress. Therefore, for relatively higher load applications such as orthopaedic implants, the low stress single-layer DLC sliding against the same would make a prefect sliding pair with low friction and wear. References
Fig. 6. Stress cracking of a multi-layer DLC coated steel ball tested with 10 N load.
layer film is about one order of magnitude higher than that against single-layer film. The probable reason for the higher wear rate of the un-coated steel ball with multi-layer coating might be the harder DLC film that removes more material from a relatively softer steel substrate. Comparing the disks wear rates, multi-layer DLC exhibits lower wear rate compared to the singlelayer DLC, and is expected due to the higher hardness of the multi-layer DLC coating. 4. Conclusion The tribological behaviour of a single-layer and multilayer DLC films, deposited on steel substrates, sliding against the same or the multi-layer DLC sliding against the single-layer DLC films reveal that the multi-layer
w1x A. Kobayashi, W. Bonfield, Y. Kadoya, T. Yamac, M.A.R. Freeman G. Scott, P.A. Revell. Proc. Instn. Mech. Engrs. 211, part H, H00596 IMechE, (1997). w2x M. Jasty, C.R. Braddon, K. Lee, A. Hanson, W.H. Harris, J. Bone Joint Surg. 76B (1994) 73. w3x G. Lewis, J. Biomed. Mater. Res. (Appl. Biomater.) 38 (1997) 55–75. w4x P. Firkins, J.S. Hailey, J. Fisher, A.H. Lettington, R. Butter, J. Mater. Sci. Mater. Med. 9 (1998) 597–601. w5x T. Xu, L. Pruitt, J. Mater. Sci. Mater. Med. 10 (1999) 83–90. w6x X. Shi, B.K. Tay, H.S. Tan, L. Zhong, Y.Q. Tu, S.R.P. Silva, et al., J. Appl. Phys. 79 (1996) 7234–7239. w7x S. Anders, A. Anders, I.G. Brown, B. Wei, K. Komvopoulos, J.W. Ager III, et al., Surf. Coat. Technol. 68y69 (1994) 388. w8x P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amarathunga, W.I. Milne, et al., Phys. Rev. B. 48 (1993) 4777. w9x D. Sheeja, B.K. Tay, S.P. Lau, L.N. Nung, Surf. Coat. Technol. 146–147 (2001) 410–416. w10x G.M. Pharr, D.L. Callahan, S.D. McAdams, T.Y. Tsui, S. Anders, A. Anders, et al., Appl. Phys. Lett. 68 (1996) 779–782. w11x S. Kumar, P.N. Dixit, D. Sarangi, R. Bhattacharyya, J. Appl. Phys. 85 (7) (1999) 3866–3877. w12x S. Anders, D.L. Callahan, G.M. Pharr, T.Y. Tsui, C.S. Bhatia, Surf. Coat. Technol. 94–95 (1997) 189–194. w13x M. Chhowalla, G.A. Amaratunga, J. Mater. Res. 16 (1) (2001) 5–8. w14x J.W. Ager. III, S. Anders, I.G. Brown, M. Nastasi, K.C. Walter, Surf. Coat. Technol. 91 (1997) 91–94.
Tribological characterization of diamond-like carbon (DLC) coatings sliding against DLC coatings D. Sheejaa,*, B.K. Taya, S.M. Krishnana, L.N. Nungb a
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169608, Singapore Received 14 August 2002; received in revised form 17 March 2003; accepted 4 April 2003
Abstract Two different qualities (single-layer and multi-layer) of thick diamond-like carbon (DLC) coatings were prepared on both extremely polished (Ra ;0.01 mm) stainless steel disks and 6 mm diameter stainless steel balls. Morphological and microstructural evaluations reveal that multi-layer DLC coatings are superior in nature compared to the single-layer coatings. Tribological characterisations using a pin (ball)-on-disk tribometer shows that, even though the multi-layer DLC coating exhibits superior qualities, when it slides against the same displays relatively higher friction and wear compared to that of single-layer DLC sliding against the same. Multi-layer DLC coated steel ball sliding against the single-layer DLC coated disk displays the best tribological performance. However, stress cracking was observed very rarely at the sliding edge of the multi-layer film prepared on the ball and is believed to be due to the relatively higher compressive stress in the film combined with the applied stress. The tribological performance of the single-layer DLC sliding against the same is also found to be excellent, and is comparable to that of the multi-layer DLC coated ball against the single-layer DLC coated disk. No cracks were observed on the low stress single-layer DLC coated ball after the friction test. The study reveals that the single-layer DLC sliding against the same would provide the best tribological DLC pair for relatively higher load applications such as in orthopaedic implants. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: DLC coating; Friction and wear; Multi-layer; Raman spectra
1. Introduction Metallic or ceramic femoral head sliding against UHMWPE (ultra high molecular weight polyethylene) acetabular cup is quite common in artificial hip joints, even though metal–metal sliding pairs are getting popular in these days. The clinical performance of the components made of UHMWPE, is good except for the concern about their wear. Failure of the implant due to wear debris has been identified as the main limiting factor for the total joint replacement w1–3x. A new or improved (surface modification of the existing biomaterial) sliding pair, which produces least wear is one of the solutions for a long lasting orthopaedic implant. Some studies w4,5x have reported that diamond-like carbon (DLC) coatings can provide feasible solutions to the problem of wear and related failure of the total *Corresponding author. Tel.: q65-67906127; fax: q65-67933318. E-mail address: [email protected] (D. Sheeja).
joint replacements, due to high wear resistance, biocompatibility and chemical inertness of the coating. Firkins et al. w4x have investigated the wear of UHMWPE against DLC coated (by CVD) stainless steel and reported that the DLC coating reduces the wear of UHMWPE by about seven times. Xu and Pruitt w5x have also reported that, the DLC coating improved the frictional and wear characteristics of DLC coated Ti-6Al4V sliding against UHMWPE. This encouraged us to carry out studies on the tribological behaviour of DLC coated biomaterial against UHMWPE, where the DLC coatings were prepared by Filtered cathodic vacuum arc (FCVA) technique, which can produce high quality coatings w6–8x. However, the wear resistance of UHMWPE sliding against superior quality DLC coating prepared on Co–Cr–Mo material, in simulated body fluid, did not show any improvement over that of the un-coated w9x. The possible reason for the slightly higher wear rate of the DLCyUHMWPE sliding pair compared
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to the Co–Cr–MoyUHMWPE is that, as the polymer is being softer, sliding by a harder (good quality DLC) surface can remove more particle than that by a softer material (un-coated Co–Cr–Mo). This shows that coating the metal surface with DLC might not be a good solution to improve the wear resistance of metaly UHMWPE sliding pairs. The current trend in the total joint replacement is to substitute the metallicyceramic material sliding against UHMWPE (being a weak component in the total joint replacement) with metal sliding against metal, in which Co–Cr–Mo alloy is being one of the key materials. However, the dissolution of Cr ion and the uncertainty of that being a carcinogenic component is one of the main concerns in the case of Co–Cr–Mo sliding against the same, in addition to the tribological behaviour of the material pair. Surface modification of Co–Cr–Mo alloy material with an inert coating such as high quality DLC film may prolong the life of the component as well as reduce the release of Cr ions. This led us to investigate a preliminary study on the tribological behaviour of DLC coating sliding against the DLC coating of different qualities, to evaluate and optimise for the best DLC sliding pair. The difficulty in obtaining Co–Cr– Mo alloy balls, force us to deposit DLC films on steel substrates. The custom-made Co–Cr–Mo pins will have geometrical irregularities and that may lead to wrong conclusions. However, the study will be carried out using the custom-made Co–Cr–Mo pins to verify the results. As we know, the quality of the film varies greatly on the deposition condition. In this article we discuss two different qualities (single-layer and multilayer) of thick DLC coatings. The films were prepared using filtered cathodic vacuum arc (FCVA) system in conjunction with high substrate pulse biasing, which is found to be one of the best methods to prepare low stress, thick films w10–14x. 2. Experimental details 2.1. Materials and methods Stainless steel (316L) disks of 25 mm diameter and 3 mm thickness were ground and polished to get mirror like smoothness. The grinding and polishing were done in many stages. First step of grinding was done with 240 grit silicon carbide abrasive paper followed by 400, 800 and 1200 grit abrasive papers. Subsequently the grounded surfaces were polished with coarse and fine alumina solution to get mirror-like smoothness. The polished substrates were cleaned with soap solution, followed by water, acetone, isopropyl alcohol and DI water in an ultrasonic bath tank and dried with nitrogen blow off gun. The samples were then analysed for its roughness using a TENCOR P-10 Surface Profiler. The average surface roughness values of the samples were
maintained at approximately 0.01 mm, in order to achieve the roughness comparable to that of orthopaedic surface roughness. The 6 mm diameter stainless steel balls were obtained from a commercial supplier (CSEM). 2.2. Deposition conditions The films were deposited using the FCVA system, which is described in detail elsewhere w4,9x. The carbon plasma for the DLC films was obtained from a graphite target of 99.999% purity. The steel substrates were precleaned with acetone, followed by alcohol and deionised water, and blown dry using nitrogen gas. The cleaned substrates were placed in the deposition chamber. The chamber was then evacuated to a system base pressure below 3=10y4 Pa, but increased to 2=10y3 Pa during deposition. Before deposition, the native oxide layer on the stainless steel substrates was cleaned using Ar plasma for approximately 20 min. Two different qualities (multi-layer and single-layer) of DLC films were prepared, on both steel disks as well as steel balls. In both cases, an adhesive layer of approximately 20–25 nm was prepared under pulse parameters of 9 kV, 600 Hz and 25 ms. In the case of multi-layer films, the adhesive layer was followed by a relatively thick (80–100 nm) hard layer that was prepared without any substrate bias, subsequently soft layer of approximately 20–25 nm was prepared with pulse parameters of 3 kV, 600 Hz and 25 ms. The repetitive deposition of hard and soft layers of DLC film were continued until the desired thickness of approximately 1 mm was reached. In the case of single-layer films, the adhesive layer was followed by soft films prepared with pulse parameters of 3 kV, 600 Hz and 25 ms. 2.3. Characterisation of the films 2.3.1. Thickness and stress measurement In order to obtain the thickness of the film, a piece of silicon (Si) wafer was placed next to the steel sample. A line was marked on the Si wafer with a marker before it was placed in the deposition chamber. During the sputter cleaning, the Si wafer was covered with a metal plate. Before deposition (after etching), the metal plate was shifted aside and the Si wafer was deposited along with the stainless steel sample. Subsequently, the marker line on the Si was erased with alcohol and a surface profilometer was used to scan across the boundary of the coated and uncoated surfaces of the un-etched Si sample. As it is difficult to measure the intrinsic stress on the metal substrate by radius of curvature technique, a rough estimation of the intrinsic compressive stress in the film was obtained by measuring the stress in the films prepared on Si substrates, which were deposited along
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with the metal substrates. The intrinsic compressive stress in the film prepared on the Si (ss) is calculated using Stoney’s Equation, given by: sss
Es ts2 B 1 1E C y F 6(1yys) tc D R Ro G
Where Es, ys, and ts are the Young’s modulus, Poisson ratio and thickness of the substrate, and tc is the thickness of the film. Ro and R are the radius of curvatures of the bare and the coated substrate, respectively, and are measured using a surface profilometer. 2.3.2. Morphological and structural characterisation The surface morphology of the films was studied by using an Atomic Force Microscopy (AFM) in tapping mode. The morphological evaluations were carried out on the films prepared on the steel as well as the Si substrates, which were prepared along with the steel substrates. The RMS (Root Mean Square) roughness of the films was obtained from the digital image processing software, which comes with the AFM system over an area of 1=1 mm. A visible Raman spectroscopy, with 514 nm lines of Arq laser as the excitation source, was used to study the microstructure of the films that were prepared on the steel substrates. The spectra were acquired over a range 600–2000 cmy1.
Fig. 1. Geometry of the DLC coated stainless steel ball after friction test, for the purpose of the calculation of the wear volume.
of the film, which is around ;3q0.001 mm; t is unknown and it is possible to calculate the value of t from the trigonometric relations and is: tsRy yR2y(dy2)2. Once we know the wear volume, wear rate can be calculated using the same relation as mentioned above. 3. Results and discussion 3.1. Film thickness and stress
2.3.3. Tribological characterisation using pin (ball)-ondisk tribometer A commercially available unidirectional pin (ball)on-disk tribometer has been used to study the frictional and wear characteristics. The tests were carried out at a constant load of 5 N, with a linear speed of 3 cmys in ambient air (RH: 50%, temperature 23 8C). The software comes with the system records the dynamic friction spectra. The wear rates of the DLC films prepared on the disk surfaces were calculated from the residual wear tracks, obtained after the friction tests. The cross-sectional area of the wear track was measured using a stylus profilometer; and the wear volume was calculated from the track cross-sectional area. The wear rates were then calculated using the relation; wear rate (k)sVy(WS), where V is the wear volume in mm3, W is the normal applied load in Newton and S is the sliding distance in meters. Four measurements were taken on each sample at 908 apart. The wear volumes of the DLC film, prepared on the stainless steel balls were calculated from the change in DLC-coated stainless steel ball geometry. The wear scar diameter, d (see Fig. 1) was measured using an optical microscope. The wear volume, the shaded part shown in Fig. 1, is then calculated using the relation: Vs pt2(3Ryt) ; where R is radius of the ballqthe thickness 3
The thickness of the multi-layer and single-layer films were prepared on the steel disks and are found to be approximately 620–770 nm, respectively; and that prepared on the steel balls are found to be approximately 1.21–1.32 mm, respectively. As the ball being the static friction partner, the film on it wears off fast (the contact point is the same unlike the disk). Hence, thicker films were prepared on the balls. The intrinsic stress in the multi-layer film prepared on the Si substrate (deposited along with the steel substrate) is measured to be approximately 4.2 GPa and that in the single-layer film is approximately 0.59 GPa, which is relatively low. The actual intrinsic stress in the film prepared on the steel substrates would be much lower than that on the Si substrate, as it is possible to grow thicker films on steel substrate under floating condition. The above stress values give us a rough estimation of the intrinsic stress in the films. 3.2. Morphological and structural characterization The morphology of the multi-layer as well as the single-layer films prepared on the steel and the Si substrates is shown in Fig. 2. It is obvious that the multi-layer film exhibit smooth morphology compared to that of the single-layer film prepared on the Si substrates. The growth mechanism is observed to be
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Fig. 2. Morphology of the 1 mm thick multi-layer and single-layer DLC films prepared on Si and steel substrates (scan area: 1=1 mm).
almost same regardless of the substrate on which it has prepared. However, the RMS roughness (Rrms) values of the films prepared on the steel substrates are found to be slightly higher than that prepared on the Si substrates. Moreover, the single-layer film prepared on the steel substrate displays a lower Rrms than that of the multi-layer film. The probable reason for these differences in roughness could be due to the substrate surface roughness. Even though the single-layer DLC film is observed to be slightly rougher, the engineering surface roughness (Rrms) measured over larger length of the order of mm, by a surface profiler, is found to be comparable (approx. 0.012 mm) with that of the well polished steel substrates. The Raman spectra of the films prepared on the disks as well as balls are shown in Fig. 3. There is no obvious difference in the spectra of the single-layer film prepared on the disks as well as balls, and the same goes with the multi-layer films. Raman spectra were captured on different locations of the ball (only half of the ball is coated). However, we did not observe any significant changes in the spectra. The spectra were then de-
convoluted into ‘D’ and ‘G’ peaks to see the change in ID yIG ratio. The multi-layer films prepared on the disks and balls display ID yIG values of 0.35"0.003 and 0.43"0.08, respectively. The single-layer films prepared on the disks and balls display ID yIG values of 0.85"0.09, 0.88"0.17, respectively. The multi-layer film exhibits lower ID yIG ratio compared to the singlelayer films. This undoubtedly shows that multi-layer films are superior in nature compared to the single-layer films. In addition, the films prepared on the ball display slightly higher ID yIG ratio compared to that prepared on the disks, which suggests that the quality of film may vary slightly depending on the curvature of the substrate. However, in the present study, the values are quite comparable. 3.3. Friction and wear Tribological tests were carried out on the two different qualities of DLC coated steel disks sliding against the un-coated as well as two different qualities of DLC coated steel balls. Fig. 4 shows the dynamic friction
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Fig. 3. Raman spectra of (a) Multi-layer and single-layer DLC films prepared on the disks; (b) Multi-layer and single-layer DLC films prepared on the balls.
spectra of the single-layer and multi-layer DLC coated steel disks against the same kind of coating as well as the single-layer against the multi-layer DLC film; and Fig. 5 shows the typical wear scars of the single-layer and multi-layer coated steel balls after friction test. It is obvious from Fig. 4 that the single-layer DLC sliding against the same as well as the single-layer DLC coated disk sliding against the multi-layer DLC coated ball display low coefficient of friction (-0.05). However, the multi-layer DLC film sliding against the multi-layer DLC film displays slightly higher friction coefficient of approximately 0.09. It is observed from the friction tests carried out with different combinations of sliding pairs that, sliding of single-layer DLC coated disk sliding against the same or multi-layer DLC coated static counter-face would make the best tribological pair, with friction coefficient of 0.05 or lower. The wear rates of the coating on the disk and the ball (after the friction tests) were calculated and are given in Table 1a and b. It is obvious from the single-layer DLC coated disk sliding against the three different static friction partners (multi-layer DLC, single-layer DLC and un-coated balls) that DLC on both the sliding surfaces reduces the wear of both the disk and the ball. Between the first two sliding pairs shown in Table 1, with the best tribological behaviour, the single-layer DLC coated disk sliding against the multi-layer DLC coated ball gives the best properties. As the ball being a static friction partner (the same point will be sliding throughout the test), a harder (multi-layer) coating on the ball displays the lowest ball wear rate. As we can see from Table 1(b) that the multi-layer DLC on the disk sliding against the single-layer DLC
on the ball, reduces the disk wear rate but increases the ball wear rate significantly and is expected. Multi-layer DLC film demonstrates a longer lifetime when it slides against single-layer film. However, the multi-layer against multi-layer demonstrates a relatively higher wear rate, which is un-expected. From the friction spectra, one can see that in addition to the relatively higher friction, the noise is also slightly higher for this sliding pair (multi-layerymulti-layer). The results suggest that even though multi-layer DLC coating has higher wear resistance, sliding of it against the same may not be a good choice. In order to get some clues or explanations for the slight difference in their tribological behaviour, the worn surfaces were analysed using the Raman spectroscopy. However, the spectra were observed to be similar to that of the respective as-deposited films. We have also tried to transfer the wear particles to a bare Si substrate to study the Raman spectra of the wear debris. However, it was found quite difficult to get any reasonable amount of wear debris to capture the spectra. In addition, very rarely, we have observed stress crackingybulging on the multi-layer film prepared on the balls after the friction test (see Fig. 6). This might probably be due to the relatively high compressive stress in the multi-layer DLC film compared to that in the single-layer DLC film. However, we have never observed any such stress-crackingybulging of the multilayer film prepared on the flat surface (disk), which reveals that the curvature (ball) of the surface may also play a major role on the stress. Consider the wear rates of the single-layeryun-coated steel and multi-layeryun-coated steel sliding pairs. The wear rate of un-coated steel ball sliding against multi-
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Fig. 4. Dynamic friction spectra of different types DLC sliding pairs (5 N, 3 cmysec and atmospheric environment).
Fig. 5. Worn surface of (a) single-layer DLC coating on the ball (b) multi-layer DLC coating on the ball.
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Table 1 Wear rates of disk and ball in two-different qualities of DLC coated disk sliding against different static friction partners Sliding pair
Disk wear rate mm3 yNm
Ball wear rate mm3 yNm
Rotating (disk)
Static (ball)
(a) Single-layer Single-layer DLC coating Single-layer DLC coating Single-layer DLC coating
Multi-layer DLC coating Single-layer DLC coating Uncoated steel
2.42=10y7 2.54=10y7 4.32=10y7
5.0=10y9 6.36=10y9 4.06=10y8
(b) Multi-layer Multi-layer DLC coating Multi-layer DLC coating Multi-layer DLC coating
Multi-layer DLC coating Single-layer DLC coating Uncoated steel
3.0=10y7 1.83=10y7 1.22=10y7
1.4=10y8 1.32=10y8 3.43=10y7
DLC coated static friction partner against the singlelayer DLC coating displays the best performance. However, friction test under relatively higher loads cause cracking of the multi-layer DLC prepared on the curved surface, due to the combined effect of the intrinsic stress and the applied stress. Therefore, for relatively higher load applications such as orthopaedic implants, the low stress single-layer DLC sliding against the same would make a prefect sliding pair with low friction and wear. References
Fig. 6. Stress cracking of a multi-layer DLC coated steel ball tested with 10 N load.
layer film is about one order of magnitude higher than that against single-layer film. The probable reason for the higher wear rate of the un-coated steel ball with multi-layer coating might be the harder DLC film that removes more material from a relatively softer steel substrate. Comparing the disks wear rates, multi-layer DLC exhibits lower wear rate compared to the singlelayer DLC, and is expected due to the higher hardness of the multi-layer DLC coating. 4. Conclusion The tribological behaviour of a single-layer and multilayer DLC films, deposited on steel substrates, sliding against the same or the multi-layer DLC sliding against the single-layer DLC films reveal that the multi-layer
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