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Surfaceand Coatings Technology76-77 (1995)572-578
Tribological behaviour of smooth diamond films S.M. Pimenov a, A.A. Smolin a, E.D. Obraztsova a, V.I. Konov a, U. Bhgli b, A. Blatter °, M. Maillat d, A. Leijala a, j. Burger e, H.E. Hintermann e, E.N. Loubnin r a General Physics Institute, Russian Academy of Sciences, 38 Vavilov str., Moscow II7942, Russia b Institute of Applied Physics, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland ° M + FT, AIlmendstr. 74, CH-3602 Thun, Switzerland a CSEM, Maladiere 71, CH-2007 Neuchatel, Switzerland e Faculty of Science, rdh~iversityofNeuchatel, CH-2000 Neuchatel, Switzerland f Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky prospekt 31, Moscow II7915, Russia
Abstract The tribological properties of smooth diamond coatings sIiding against ruby were studied using a pin-on-disk tribometer. Smooth diamond film surface was prepared by (i) deposition of thin nanocrystalline films in the thickness range from 0.2 to 2 grn and by (ii) post growth polishing. Exclmer laser surface ablation, microwave plasma etching and mechanical lapping with diamond grit were applied for post growth polishing. A correlation of fiLmsurface properties examined with atomic force microscopy, Auger electron spectroscopy, Raman spectroscopy and stylus profilometry has been established with the tribological performance of the tested diamond films. Keywords: Diamond film; TriboIogy; Laser poIishing
1. Introduction Surface smoothness of CVD diamond films is a requirement for the films to be used as low friction coatings. The effect of roughness and surface morphology on the tribological properties of diamond films was investigated for various friction pairs, including diamond sliding on diamond coatings [ 1-4] and softer materials sliding on diamond coatings [2,5-9]. It has been demonstrated [ 1,7,9] that polished diamond film surfaces were characterized by friction and wear properties very close to the properties of natural diamonds El0]. In the present work we studied the friction and wear properties of diamond films against a ruby ball using a pin-on-disk tribometer. Emphasis was put on the comparison of triboproperties of smooth diamond film surfaces prepared by (i) growth of thin nanocrystalline films in the thickness range from 0.2 btm to 2 btm and by (ii) post gro;~vth polishing. A correlation of surface roughness and morphology of thin films has been established with the friction and wear behaviour. For the smoothest films of submicron thickness, we found that an early stage of sliding with a high friction coefficient was followed by a wearless sliding with a low coefficient of friction. Polishing of the diamond films by laser, plasma etching 0257-8972/95/$09.50© 1995ElsevierScienceS.A. AUrights reserved ~T3T
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and conventional mechanical lapping resulted in improved tribological performance of the films. In the case of laser polishing, the influence of laser-induced surface graphitization on the friction coefficient was studied.
2. Experimental Diamond films were deposited onto Si (100) substrates from methane and hydrogen using activation of the gas phase by a DC arc discharge. Substrate pretreatment with ultrafine diamond powder to enhance nucleation density to as high as 10l° cm -2 and in situ laser interferometry monitoring of the film thickness were used [ 11]. As a result, continuous diamond films of thickness 0.2 btm, 0.5 gm, 1 btm, 1.5 btm and 2 btm were produced. The roughness, Ra, of the films was measured with a stylus profilometer with a diamond tip of radius 1 ~tm. The roughness values (scanning length is 400 gm) are presented in Table 1. Details of the surface microrelief were studied with an atomic force microscope (AFM) 'AST-CSEMEX". The AFM images of the smoothest 0.5 gm thick film (N2) and of the 2 btm thick film (Nh) are presented in Fig. 1.
S.M Pimenov et aL/Sufface and Coatings Technology 76-77 (1995) 572-578 Table 1 Thickness (h) and surface roughness (Ra) of the diamond films used in the friction tests Film number
h (gm)
Ra (rim)
Surface treatment
N1 N2 N3 N4 N5 AI A2 L1 M1
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10 7 i9 30 65 16 27 123 35
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Three methods for smoothing diamond film surface were employed: (i) excimer laser polishing, (ii) electron cyclotron resonance (ECR) microwave plasma etching in argon, and (iii) mechanical polishing with diamond grit. Laser polishing of 10-30 txm thick films was performed with an ArF excimer laser (2=193 nm) or KrF laser (2=248nm) [-12,13]. The film L1 (in Table 1) ~vas polished with the ArF laser at 73 ° angle of beam incidence, the R~ being reduced from 0.45 btm to 0.12 ~tm. For argon plasma etching of the films an ECR microwave plasma reactor was used. The Ar gas pressure was 2 vtbar and the bias voltage was - 5 0 0 V. Two diamond films Of 1 btm and 1.5 btm thickness were etched res'pectively to a depth of 0.1 I~m and 0.5 •m in order to compare
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S.M Pimenovet aL/Swface and Coat#zgs Technology76-77 (1995) 572-578
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the triboproperties of these films (A1 and A2 in Table 1) with those of the as grown 1 ~tm and 1.5 gm thick films (N3 and N4). As a result of the Ar ion processing of the films, the roughness Ra was reduced from 19 nm to 16 nm for A1 and from 33 nm to 27 nm for A2. The 30-50 gm thick films were polished by a conventional abrasive technique. The Ra of the mechanically polished film ( M 1 in Table l) was 35nm (before polishing it was 0.8 gin). Friction tests were performed with a pin-on-disk tribometer. A monocrystalline ruby ball of a radius r = 3 mm and roughness Ra less than 50 nm was balanced on the diamond film surface and loaded with 1 N. The sliding speed was l c m s -I and in several tests 10 cm s-~. The friction coefficient did not depend on the sliding speed in the range 2 mm s -1 to 10 cm s -I. The measurements were done at ambient atmosphere and at a relative humidity of (50 _+ 5)%. The size of the wear flat on a ball was determined with an optical microscope. The wear rate of the ball (W) is taken as the worn volume (V) per load (F) and sliding distance (L), i.e., W= V/(FL).
3. Results and discussion
3.I. Effect of roughness and surface morphology on the friction and wear properties of thin nanocrystaIline diamond films The dependences of the friction coefficient on sliding distance for the diamond films N I - N 5 are shown in Fig. 2. All the diamond coatings revealed high adhesive and wear resistant properties during sliding. It follows from Fig. 2 that the smoother the film was, the higher was the friction coefficient observed. The AFM surface images in Fig. 1 (we compare the films with the lowest and highest roughness) show that the density of crystallites (contact points) is about 100-times higher for the smoother film and the slope angle of asperities relative
to the ball surface is larger. This could explain the higher friction coefficient of the smoother film in the frame of the asperity friction model r 1,2,10]. Besides, the friction coefficient behaviour of the film N5 of the maximum roughness was characterized by its rapid decrease from 0.66 to 0.16. Identical results were reported [14,151 for sapphire and silicon nitride sliders under similar experimental conditions. A reason for such friction behaviour is abrasive wear of the ruby ball during sliding, which was observed for all the films tested. The wear rate of the ball (for the films N I - N 5 ) increases with roughness, as plotted in Fig. 3. The wear of the ball leads to transfer of ruby debris onto the diamond film surface, thus gradually changing the sliding conditions and influencing the friction behaviour. The AFM images of wear tracks revealed a difference in the track morphology upon 28 m sliding distance, as shown in Fig. 4 for the smoothest (N2) and roughest (N5) films, respectively. For the film Nh, no fracture of the tips of crystallites was observed during sliding, and large wear of the ball was accompanied by a decrease in the mean contact pressure by 3 orders of magnitude. In addition to the decrease in the contact pressure the ruby debris filled the valleys between asperities, reducing the ploughing component of friction. For the films (NI-N3) characterized by the low wear rates the track surface was smoothed, indicating that either the tips of asperities were fractured or the film surface was covered by a thin layer of the worn material. Auger electron spectra (AES) from the tracks revealed the debris layer to consist of carbon and alumina. The AES spectra of carbon showed their own specific shape as compared to diamond, and such changes reflected a loss of ordering in the crystalline diamond structure. A repeated friction test on the film N2 (on a new track) but for a longer sliding distance has been done to
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Fig. 3. Wear rate of a ruby ball versus surface roughness, R~, of the diamond films: closed squares, the films NI-Nh; open squares, the plasma etched films A1 and A2; circle, the laser-polished film L1; triangle, the mechanicallypolishedfilmM1. The wear rates correspond to the sliding distance 28 m for the films NI-Nh, A1, A2; 36 m for L1; and 189 m for M1.
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clarify the friction and wear performance of the smoothest film. The friction curve of this test is plotted in Fig. 5. The initial stage of sliding (28 m distance) was very close to the friction curve in Fig. 2, whereas after ~ 40 rn the friction coefficient rapidly decreased and reached the low value of 0.11. What is also very importantisthat no wear of the ruby ball was measurable between L = 56 m and L = 126 m. AES analysis combined with Ar ion etching of the two tracks on the film N2 (upon 28 m and 126 m sliding distance) showed that the layer thickness was about 50 nm in both tracks, whereas the friction coefficient was about 0.8 and 0.1, respectively. This may indicate a dominant role of surface smoothing
achieved by damaging the sharp tips of asperities. In addition to smoothing, multicyclic loading during sliding friction can be supposed to result in sintering of ruby particles with the fractured diamond debris to form a composite material with fine-grained structure which would govern the friction between itself and ruby and minimize the wear as well. An even lower (as compared to the film N2) friction coefficient of 0.05 was characteristic of the regime of wearless sliding for the 200 nm thick film N1 (Fig. 6). Three friction tests were performed on the same wear track (curves 1-3 in Fig. 6): (1) an initial period of sliding to form a wear track, providing wearless sliding,
576
S.M. Pimenov et aL/Surface and Coatings Technology 76-77 (1995) 572-578 1.0
the low friction coefficient even at higher mean contact pressures (curve 2 in Fig. 6(a,b)). If the track surface is disturbed by ultrasonic cleaning, the film surface quickly returns to the steady-state low friction and wearless sliding (curve 3 in Fig. 6(a,b)). Thus, diamond films of submicron thickness have been found to exhibit self-lubricating properties in sliding friction against ruby, providing low friction and wearless sliding via "self-regulating" surface modification of the materials in contact.
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3.2.1. Plasma etching Plasma-assisted etching of micron thick diamond films was found to be a very effective method to reduce both the friction coefficient at the initial stage of sliding and the ball wear. The comparison of friction curves of the plasma-etched films with those of the as grown films is shown in Fig. 7, and the values of the wear rate are plotted in Fig. 3. As was evident in the SEM images [16], the plasmainduced surface modification consists in rounding off sharp edges of the diamond crystallites, i.e., in reducing the slope angle of surface asperities. Similarly to the friction testing of films NI-N5, the surface morphology determined by the size and shape of asperities manifests itself as one of the main parameters affecting the friction and wear during ruby sliding on diamond coatings.
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(2) continued sliding on the same track with an unworn ruby ball, (3) continued sliding with a newly replaced ruby ball after ultrasonic cleaning of the film in isopropanol. From the friction and ball wear behaviour it follows t h a t o n c e t h e interfaciat l a y e r b e t w e e n r u b b i n g surfaces
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value 0.2. For other films tested [161 and even for M1, a wear-in stage of sliding was always observed and was characterized by a higher friction coefficient than the steady-state friction value. The "absence" of the wear-in period for L1 is thought to be caused by a lubrication effect of a thin surface layer of a graphite-like material which is then pushed out of the track. This is confirmed by the Raman spectrum from the track in comparison with the Raman spectrum of the diamond surface outside the track (Fig. 9). The influence of surface graphitization on friction was also evidenced by removal of the graphitized surface layer via heating the laser-polished film at 500 °C for 4 h in air [161.
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4. Conclusions
The ultrathin diamond films revealed high adhesive and wear resistant properties during sliding. The friction and wear behaviour was strongly influenced by surface roughness and film morphology. The value of the friction coefficient of the thin films at the beginning of sliding was determined by the density of surface asperities and asperity angle. The abrasive wear of the softer ruby ball resulted in a transfer of ruby debris onto the diamond film surface, gradually changing sliding conditions and influencing friction and wear behaviour. For the smoothest nanocrystalline films it was found that the initial stage of sliding with a high friction coefficient was followed by wearless sliding with a low friction coefficient. It is concluded therefore that the diamond films of submicron thickness exhibit self-lubricating properties in sliding friction against ruby, implying that low friction and wearless sliding are achieved via "self-regulating" surface modification of the sliding materials. All the polished films were characterized by improved tribological performance. ECR microwave plasma etching of thin films by argon ions was found to significantly reduce the friction coefficient of thin films at the wearin period of sliding and the wear rate of the softer ball material. The laser-polished diamond films showed low friction and low wear properties. A lubrication effect caused by laser-induced surface graphitization was found to be essential to the tribological performance of the laser-polished films. The mechanically polished diamond films were characterized by the lowest friction coefficient and the lowest wear rate as compared to diamond films tested under similar sliding conditions.
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References
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S.M. Pimenov et al./Szoface and Coatings Technology 76-77 (1995) 572-578 Dowson, C.M. Taylor and M. Godet (eds.), Mechanics of Coatings, Leeds-Lyon Symp. 16 (Tribology Series 17), Elsevier, Amsterdam, 1990, p. 399. B. Bhushan, V.V. Subramaniam, A. Malshe, B.K. Gupta and J. Ruan, J. AppI. Phys., 74 (1993) 4174. M.A. Tamor, in M. Yoshikawa, M. Murakawa, Y. Tzeng and W.A. Yarbrough (eds.), Applications of Diamond Films and Related Materials, MYU, Tokyo, i993, p. 221. B. Bhushan, V.V. Subramaniam and B.K. Gupta, Diamond Films TechnoI., 4 (1994) 71. D. Tabor and J.E. Field, in J.E. Field (ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992, p. 547. A.A. Smolin, S.M. Pimenov, V.G. Ralchenko, T.V. Kononenko,
V.I. Konov and E.N. Loubnin, Diamond Films Technol., 3 (1993) 1. [12] U. B~Sgli,A. Blatter, S.M. Pimenov, A.A. Smolin and V.I. Konov, Diamond Relat. Mater., I (1992) 782. [13] S.M. Pimenov, A.A. Smolin, V.G. Ralchenko and V.I. Konov, Diamond Films Teehnol., 2 (1993) 201. [141 I.P. Hayward and I.L. Singer, New Diamond Science and Technology, MRS Int. Conf. Proc., 1991, p. 785. [15.] R.L.C. Wu and K. Miyoshi, in M. Yoshikawa, M. Murakawa, Y. Tzeng and W.A. Yarbrough (eds.), Applications of Diamond Films and Related Materials, MYU, Tokyo, 1993, p. 221. ['16.] U. B6gli, A. Blatter, S.M. Pimenov, E.D. Obraztsova, A.A. Smolin, M. Maillat, A. Leijala, J. Burger, H.E. Hintermann and E.N. Loubnin, Diamond Relat. Mater., 4 (1995) 1009.