DIAMOND RELATED MATERIALS

Diamond and Related Materials 4 (1995) 1009-1019

Tribological properties of smooth polycrystalline diamond films

U. Biigli a, A. Blatter b, S.M. Pirnenov ‘, E.D. Obraztsova ‘, A.A. Smolin ‘, M. Maillat d, A. Leijala d, J. Burger d, H.E. Hintermann d, E.N. Loubnin e

a Institute of Applied Physics, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland b M + FT, Allmendstr. 74, CH-3602 Thun, Switzerland

’ General Physics Institute, Russian Academy of Sciences, 38 Vavilov str., 117942 Moscow, Russia d Centre Suisse d’Electronique et de Microtechnique SA, Maladitre 71, CH-2007 Neuchdtel, Switzerland

’ Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky prospekt 31, 117915 Moscow, Russia

Received 13 October 1994; accepted in final form 20 December 1994

Abstract

We have used a pin-on-disk txibometer with either a monocrystalline ruby or a lOOCr6 steel ball to evaluate the tribological properties of a series of polycrystalline chemical vapour deposition diamond films. Different polishing treatments of the as-grown films resulted in marked differences of their tribological behaviour.

The friction and wear of smooth nanocrystalline diamond films was dependent on the surface morphology and roughness as well as the material transfer from the ball to the film. Microcrystalline films polished by laser irradiation, though being substantially rougher, showed a lower friction and minimal ball wear. Films polished by Ar-microwave plasma etching showed a similar behaviour as the laser-irradiated samples. However, the film with roughness comparable with the laser polished samples produced significantly higher ball wear. An um-ivalled tribological performance was obtained with mechanically polished diamond films.

Keywords: Diamond films; Tribology; Laser irradiation

1. Introduction

Polycrystalline diamond coatings grown by low- pressure chemical vapour deposition (CVD) are extremely hard and wear resistant, but have faceted surfaces. The associated roughness strongly affects their tribological behaviour, smooth surfaces are demanded for the best performance I[ l-41. Recently, Bhushan et al. [S] reported that the friction and wear properties of chemo-mechanically polished diamond films approach those of single-crystal diamond.

In the present paper we compare the tribological behaviour of diamond films which were smoothed by different techniques in an attempt to correlate friction and wear properties with film surface properties. Smooth diamond films were prepared either by growing nanocrystalline diamond films (NDF) or by post-growth polishing of the usual microcrystalline diamond films. Excimer laser surface a’blation and argon microwave plasma etching were the polishing techniques employed. The laser irradiation not only smooths the surface, but also causes surface graphitization which is known to have a dry lubrication effect [6]. A mechanically polished sample served as a reference.

0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI0925-9635(95)00272-3

2. Experimental details

2.1. Synthesis of ultrathin diamond$lms

The growth of nanocrystalline continuous diamond films of micron and submicron thickness requires both an enhanced nucleation density and an accurate thick- ness control of the growing layer. A high nucleation density (> 10’0cm-2) was achieved by the ultrasonic pretreatment of Si substrates using ultrafine diamond powder of 5 nm sized particles. Laser interferometry was employed to measure in situ the thickness of the diamond film during deposition [ 71. Deposition was from a methane/hydrogen mixture using d.c. arc discharge plasma CVD. The methane concentration was kept high (20 vol.%) during the initial period of deposition until the intensity of the reflected He-Ne laser beam reached the first minimum. Subsequently, the concentration was reduced to 5 vol.% for the continuing film growth to the final thickness. Thin continuous diamond layers of thicknesses 0.2 urn, 0.5 urn, 1 urn, 1.5 pm and 2 urn were produced.

These diamond films are essentially nanocrystalline

1010 U. Biigli et al./Diamond and Related Materials 4 (199.5) 1009-1019

,.‘.“~~~“~~~“~~.‘II(C I 1100 1300 1500 1700 l!

Raman shift, cm-’ I

Fig. 1. Laser micro-Raman spectra of nanocrystalline diamond films, with excitation wavelength 488 mn and laser spot size 2 urn: curve 1, the ti grown at 20% CH4/H, 65 nm thick; curves 2-5, the films grown at 5% CHJH, during the two-step deposition (see text) with thicknesses 0.2 urn (curve Z), 0.5 urn (curve 3), 1 urn (curve 4) and 2 pm (curve 5). The curves are plotted on the same arbitrary scale but are displaced with respect to each other for clarity.

and smooth, with grain sizes of typically 50 nm [S]. Fig. 1 shows the Raman spectra of NDFs of different film thicknesses. It is worth noting that the peak at 1140 cm-‘, considered as a feature of “amorphous dia- mond” [9], is observed for all the NDFs. The intensity of the diamond peak at 1332 cm-’ increases with the film thickness, indicating the presence of larger grains. As is seen in Table 1, this tendency is also manifested in an increasing roughness. The smoothest NDF has an R, value as low as 7 nm.

2.2. Polishing of diamond$lms

Laser polishing of microcrystalline diamond films (lo-30 urn thick), was accomplished by irradiating either with an ArF excimer laser (193 nm wavelength, 20 ns pulse duration) or with a KrF laser (248 nm wavelength, 15 ns pulse duration) as described elsewhere [lo,1 I]. The smoothest surface, with a uniform R, w 120 nm over the surface area of 1 cm’, was obtained with an oblique incident ArF laser beam.

Two NDF 1 urn thick, one NDF 1.5 urn thick, and a microcrystalline diamond film 10 urn thick were polished with an Ar plasma in an electron cyclotron resonance microwave plasma reactor [12]. Typically, the Ar gas pressure was 2 ubar and the bias voltage was -500 V. The etched depths were 100 nm, 500 nm and 400 nm, respectively, for the 1 urn, 1.5 pm and 10 urn thick films.

For comparison a few diamond films 30-50 urn thick

were polished by the conventional abrasive technique to a roughness R, ~35 nm.

2.3. Tribological characterisation

The surface roughness was measured with a Tencor AlphaStep profilometer with a diamond tip of radius 1 urn. The roughness R, is defined as the mean of the absolute deviations of height from the average height of the surface profile. The values for R, given in Table 1 are afflicted with a statistical error of lo-20%. There is a systematic error in addition because the profilometer tip cannot resolve narrow cracks at the surface. Surely, the relevant parameters for determining whether the test balls notice the features are their roughness (R,) and asperity angles, and the apparent contact area of the ball. The ball R, value is comparable with the coating R, values, so the features might be seen. However, the contact area is much larger than the R, values, indicating that many asperity contacts will occur, averaging out such features.

The tribological tests were performed with a pin-on- disk tribometer. A monocrystalline ruby or a lOOCr6- steel test ball with a diameter of 6 mm and a roughness R, less than 0.05 urn was balanced on the diamond film surface and then loaded with 1 N. The diamond film was mounted on a turnable table with an offset distance of 2 or 3 mm to the turning axis. Sliding speeds between film and test ball were 1 cm s-l for the NDF and 10 cm s-’ for polished samples. The coefficient of friction (COF) was obtained from the measurement of the tangential force exerted to the ball during sliding. The COF signal was integrated with a time constant of 0.7 s. The measurements were made at ambient temperature and at a relative humidity (RH) of (50 + 5)%. The COF did not change notably upon varying the sliding speed between 1 cm s-l and 10 cm s-l or upon varying the RH to <3% or 100%. After the test, the worn volume of the ball was determined by means of an optical microscope. The wear rates W given in Table 1 corre- spond to this volume loss per load and per sliding distance.

3. Results

For better comprehension a summary of the test samples is presented in Table 1. The NDF samples are denoted Nl-NlO where Nl-N5 is the series measured against the ruby ball and N6-NlO the series measured against the lOOCr6 steel ball. Ll-L4 denote laser irradi- ated samples, Al-A4 Ar plasma etched samples, and Ml is a mechanically polished sample.

U. Biigri et al. JDiamond and Related Materials 4 (1995) 1009-1019 1011

Table 1 Summary of the used samples

Name Thickness

(run)

Roughness

R, (nm)

Surface treatment COF (min-typ”-max)

Wear of ball ( 1O-15 mz N-r) (distance (m))

Sliding speed (mm s-l)

Ball material

Nl 0.2 N2 0.5 N3 1 N4 1.5 N5 2

N6 0.2 N7 0.5 N8 1 N9 1.5 NlO 2

Ll 20

L2 10

L3 10

L4 10

Al 1

A2 1

A3 1.5

A4 10

Ml 30

10.3 as-deposited 0.52-0.73-0.89 7.1 as-deposited 0.73-0.80-0.86

19 as-deposited 0.69-0.72-0.78 30 as-deposited 0.57-0.64-0.74 65 as-deposited 0.16-0.22-0.66

9 as-deposited 0.54-0.65-0.77 8 as-deposited 0.61-0.73-0.79 8.2 as-deposited 0.47-0.63-0.74

16.8 as-deposited 0.55-0.58-0.65 13.6 as-deposited 0.62-0.74-0.86

123

185

116

228

16.2

19.3

26.7

129

35

9.7 (28.3) 9.42 monocryst. ruby 7.7 (28.3) 9.42 monocryst. ruby 28 (28.3) 9.42 monocryst. ruby 1061 (28.3) 9.42 monocryst. ruby 1388 (28.3) 9.42 monocryst. ruby

42.4 (28.3) 1.0 (37.7) 11.6 (26.4) 41.6 (33.6) 70.6 (41.5)

10 100-Cr6 steel 10 100~Cr6 steel 10 lOO-Cr6 steel 10 lOO-Cr6 steel 10 lOO-Cr6 steel

laser polished laser polished laser polished and heat annealed laser polished and heat annealed laser polished and heat annealed polished by Ar-MW plasma polished by Ar-MW plasma polished by Ar-MW plasma polished by Ar-MW plasma mechanically polished

0.14-0.34-0.36 0.12-0.2~0.21 0.15-0.17-0.48 0.19-0.21-0.48 0.22-0.23-0.35 0.31-0.47-0.55 0.16-0.19-0.40 0.34-0.39-0.47 0.24-0.28-0.35

0.21-0.31-0.77

0.19-O. 17-0.23

0.15-0.17-0.48

0.03-0.05-0.08

6.4 (20.8) 10 6.1 (36.0) 10 39.2 (189.6) 100 22.2 (188.6) 100 7.9 (189.2) 100 8.26 (195.0) 100 17.6 (188.6) 100 9.6 (193.5) 100 6.1 (28.3) 10 1.9 (188.9) 100 20.3 (28.9) 10 5.4 (188.6) 100 5.3 (28.3) 10 1.0 (190.9) 100 2683 (28.3) 10 652 (190.9) 100 0.3 (189.3) 100

lOO-Cr6 steel monocryst. ruby monocryst. ruby monocryst. ruby monocryst. ruby monocryst. ruby monocryst. ruby monocryst. ruby monocryst. ruby

monocryst. ruby

monocryst. ruby

monocryst. ruby

monocryst. ruby

a typ, typical value after 15 m (Nl-NlO, and Ll) and 100 m (L2-L4, Al-A4, and Ml) sliding distance.

3.1. Diamondjilms against monocrystalline ruby ball

3.1.1. Nanocrystalline diamond films The development of the COF of the NDF is presented

in Fig. 2. Generally, the COF is lower the thicker and

-5 0.8 3 * ‘s 0.8

E .g 0.4

E 3 0.2

5 IO 15 20 25 30

sliding distance [m]

Fig. 2. Evolution of the coefficient of friction between a monocrystal- line ruby ball and the NDF.

rougher the films are. Increasing fluctuations in the COF signal indicate unstable sliding. While the COF is high and fairly constant for Nl-N4 (there is only a slight decrease with increasing distance) it drops rapidly from 0.66 to 0.16 for N5. This indicates a qualitatively different friction mechanism. A presentation of the COF data as a dependence on the film roughness R, is shown in Fig. 3. There is a clear trend of lowering the COF with increasing roughness. The special behaviour of N5 is manifested in the broad range of the COF and a low typical value at half the sliding distance, reflecting the quick drop of the COF during the initial stage of sliding.

The wear rate W of the ruby ball is a function of the sliding distance. This is seen in Fig. 4 which shows the differential wear rate per sliding distance for a sample of the same type as N5. Similar to the COF, there is a rapid decrease at the beginning of the test followed by a nearly constant level after 30 m. The variation of W mirrors the change in contact pressure which results from the increasing contact area due to the continuing ball wear (inset Fig. 4). Debris transferred to the film forms a surface layer which may reduce further the ball wear. A prominent layer was observed on N5 which

1012 U. Biigli et al/Diamond and Related Materials 4 (1995) 1009-1019

10 roughness R, [nm]

Fig. 3. Plot of the minimum, the typical (after 15 m sliding), and the maximum COF of the samples Nl-N5 with their roughness value R,.

8, 1 .

9 E a

“0 4 i

. ;::r.-.l

= P 0 20 40

. sliding distance [m]

. . .

0 10 20 30 40

sliding distance [m]

Fig. 4. Differential wear rate at different sliding distances as measured with a NDF of thickness Zum. The inset shows the mean contact pressure between the ball and the NDF.

may explain its low final COF. The wear rate of the ball increases drastically from the thin (smooth) film Nl ( lo-i4 mz N-r) to the thick (rough) film N5 (1.4 x lo-l2 m2 N-l) as is evident in Fig. 5. The sudden increase occurs at a film roughness R, of about 20 nm.

After the test the wear tracks left on the diamond

10000

4 IWO n’ E

z b loo

G p! E

10

2 % ’

0.1 I

1 10 100 1000

roughness R. [nm]

Fig. 5. Wear rates of the monocrystalline ruby ball measured with various films versus their corresponding roughness R,. The higher values of the A series correspond to a sliding distance of 30 m, the lower values to 190 m.

films were investigated with micro-Raman spectroscopy (uR), scanning electron microscopy (SEM) with energy dispersive X-ray analysis, Auger electron spectroscopy (AES), and atomic force microscopy (AFM). It is impor- tant to emphasise that all the NDFs were highly adherent to the Si substrate and were not damaged during the tests.

A scanning electron micrograph of a typical wear track is presented in Fig. 6. The white scratches and spots originate from material transfer from the ball onto the diamond surface. The debris dimensions range from 0.1 urn to a few micrometers for all NDFs.

In order to analyze the structure and composition of the transferred material, Auger depth profiles were made in an area of accumulated debris and compared with a neighbouring region without debris. A V6 scientific Auger microscope HBlOO equipped with an ion gun for sputtering was used. AES survey scans were typically taken from lOC-1000 eV using a 20 keV, 30 nA electron beam. Depth profiles were obtained using a fine focus argon ion gun less than 5 urn in diameter. The Al (KU, 1373 eV), C (KU, 270 eV), 0 (KU, 508 eV) and N (KLL, 385 eV) lines were measured with a multiplexer system. To approximate the etch rate of the debris a calibration with bulk alumina was made yielding 6.5-6.7 nm min-‘. In the area of accumulated debris, the Al and 0 signals decrease only slowly during sputter- ing for 30 min and they were stronger than the C signal (Fig. 7(a)). This indicates a thickness of ruby debris of more than about 200 nm. The ratio of the AI and 0 signal corresponds to Ai,O,. The carbon signal dimin- ishes during the measurement but stabilises after about 10 min. A constant N signal suggests that nitrogen had been incorporated into the film during the chemical vapour deposition. The Auger depth profile for a neigh- bouring spot without visible ruby debris is shown in Fig. 7(b). It still indicates the presence of a ruby surface layer, however, this time it is much thinner (of the order of 100 nm) as the Al and 0 signals decrease to low values within 15 min. It should be noted that the formation of aluminium hydroxide during sliding as was suggested by Hayward and Singer [13] cannot be excluded. Hydroxide formation from A1203 in a non- dry atmosphere depends on the particle size; oxide particles0 > 1 urn are stable whereas particles in the range 100-10 A convert immediately. In the present case the scale of the debris is ~0.1 urn, just in between these two extremes. In contrast to Hayward and Singer, however, we did not observe a dependence of the friction on the humidity in the range down to ~3% RH.

The diamond film morphology was found to deter- mine the initial stage of friction and wear. To study in detail the NDF morphology with increasing film thick- ness (Nl-N5), we employed an AFM which provides high resolution 3-D images of the dielectric surface. With the atomic scale tribometer “AST-CSEMEX@” both the

U. Biigri et al./Diamond and Related Materials 4 (1995) 1009-1019 1013

Fig. 6. Scanning electron micrograph of the wear track in N4. The white lines consist of accumulated ruby debris.

0 10 20

sputter time [min]

30

Fig. 7. Auger depth profile taken in a wear track: (a) region of accumulated ruby debris; (b) region without accumulated ruby debris.

lateral (friction) and the vertical (topography) forces between the sharp AFM tip and the film surface were measured.

Squares of 4 pm sides were scanned with high-quality Si Wolterlever tips of length 20-25 urn, aperture angle 18-20”, and radius 10 run. The AFM image of Nl taken in an off-track region reveals small diamond grains in an early stage of columnar growth (Fig. 8). AFM images of the series Nl-N5 show a morphological change with increasing film thickness from dense nanocrystalline grains as seen in Fig. 8 ‘to larger crystallites, as large as 1.3 urn for N5. The image of the wear track reveals a

much smoother surface (Fig. 9). Obviously, the tips of the crystallites did not withstand the tangential forces during sliding. Fractured diamond tips were observed in the wear tracks of all the films Nl-N4 but this tends to diminish with increasing film thickness. Such fracturing has a smoothing effect as is manifest in Figs. 8 and 9. The roughness value R, of Nl evaluated by AFM decreased from 28.2 nm outside (Fig. 8) the wear track to 3.0 nm inside (Fig. 9). Fractured tips and related smoothing were absent in the coarse N5, as becomes obvious when comparing Fig. 10 (original surface) with Fig. 11 (wear track). In the as-deposited area the grain boundaries are discernible in the friction image which mirrors the topography image. The contrast probably originates from a different content of sp2-carbon in grains and grain boundaries. The friction image of the wear track (Fig. 11) is more uniform which may be due to the ruby debris layer.

3.1.2. Laser polished$lms The as-deposited film Ll exhibited a pyramidal (111)

texture whereas L2-L4 had a ( 110) “roof” texture. The samples were polished to different degrees as summarised in Table 1. Ll was irradiated with an ArF laser under oblique incidence whereas L2-L4 were polished under normal incidence. L4 was irradiated with only a few laser pulses in order just to smooth the edges of the crystallites.

The COF data of the laser polished samples measured against the monocrystalline ruby ball are presented in Fig. 12. All the curves evolve smoothly without pro- nounced fluctuations. The COF of Ll immediately jumps from -0.1 to 0.2 and then stays at that value during

1014 l? Bijgli et al./Diamond and Related Materials 4 (1995) 1009-1019

300 nm

1°cf”m”

Fig. 8. AFM images of Nl. Top, 3-D view of a 4 pm x 4 pm area; bottom, corresponding topography (left) and friction (right). In the friction mode, the normal force was 5 nN and the contrast full scale 5 nN.

continuous sliding. The COF of L2-L4 start at some higher value between 0.2 and 0.5 but then continuously decrease to a constant value between 0.15 and 0.35 after about 50 m. In comparison with the NDF films (which must be done within the first 30 m of sliding) the laser- polished samples have a lower COF. Laser polishing is known to produce a thin graphitic surface layer [lo], and such a lubrication layer might explain the difference. The lubrication effect of similar layers produced by C-ion implantation was shown by Wu et al. [SJ. In order to test this hypothesis, the graphitic layers were removed by heat annealing the samples L2-L4 in air at a temperature of 500 “C for 4 h [ 1 l] and a new test was run. Indeed the COF of the heat-treated samples (L2h, L3h, and L4h) were substantially higher as seen in Fig. 12. Moreover, the COF of L3h shows extreme fluctuations.

The wear rates of the ruby balls caused by the laser- polished samples are in the range of (10-40) x lo-” m2 N-’ (Table 1). In contrast to the COF, they do not change significantly upon heat treatment. It is worth noting that the wear rates are comparable with that observed in the test of the smoothest NDF Nl in spite of the large difference in film roughness. Compared with

the other NDF, which are still substantially smoother, laser-polished samples cause a much lower wear of the ruby ball.

3.1.3. Diamondjilms polished in an Ar plasma The morphologies of the samples smoothed in the

Ar-MW plasma are shown in Fig. 13. Sample A2 (Fig. 13(b)) still shows some small tips, remnants of partly etched crystallites. The development of the COF during the first 30 m sliding distance is similar to that of N3 as is seen in the inset of Fig. 14. This may indicate a similar friction mechanism, the fracturing of the remaining crystallites tips in A2. Al (Fig. 13(a)), by contrast, presents a more uniformly smoothed surface. However, there are still sharp edges compared with A3 and A4 (Fig. 13 (c)). The COF signals are presented in Fig. 14. The COF of Al decreases slowly from 0.35 to 0.24. Interestingly, the COF of A2 decreases strongly after a sliding distance of 30 m and it matches the curve of Al after 100 m. Sample A3 is comparable with Ll as is seen in the inset of Fig. 14. Although the morphology of A3 and A4 appear identical their initial COF differ. However, the same value is approached with increasing sliding distance. It seems that the higher edges due to

U. Biigli et al./Diamond and Related Materials 4 (1995) 1009-1019 1015

Fig. 9. AFM images of the wear track in Nl. Top, 3-D view of a 4 pm x 4 pm area; bottom, corresponding topography (left) and friction (right). In the friction mode. the normal force was 5 nN and the contrast full scale 300 pN.

the larger scale in A4 are smoothed out during a run- in period.

The wear rates of the ruby ball caused by Al, A2, A3, and A4 are, respectively, 6.1 x lo-l5 m2 N-‘, 20.3 x lo-l5 m2 N-‘, 5.3 x lo-l5 m2 N-l, and 2683 x lo-” m2 N-’ a:fter a sliding distance of 30 m and 1.9 x lo-l5 m2 N-‘, 5.4 x lo-l5 m2 N-l, 1.0 x lo-l5 m2 N-‘, and 652 x lo-l5 m2 N-’ after a sliding distance of 190 m. The wear rate with Al is relatively small, comparable with that with the NDF of the same film roughness. Again, the wear is drastically higher for the rough film (R,= 129 nm) than for the smooth films (R, c 30 nm).

3.1.4. Mechanically polishedjilm The mechanically polished sample Ml showed the

lowest COF of 0.05 during the whole sliding distance. The COF remained at this low value even for a relative humidity of ~3% or 100%. The associated wear rate of the ruby ball was also lowest, 0.3 x lo-l5 m2 N-‘. With these values the mechanically polished film clearly showed the best performance among all the examined samples. However, such samples are not necessarily the

most economic ones in all applications as mechanical polishing is extremely time consuming.

3.2. Diamondjilms measured against lOOCr6 steel ball

The NDF series N6-NlO, as well as Ll, were tested against a lOOCr6 steel ball. As seen in Fig. 15 the COF of the NDF lie in the range 0.55-0.8. This is close to the value measured for steel-steel sliding friction (0.6), indicating substantial material transfer from the ball to the NDF. The COF of sample Ll, by contrast, increases slowly from a low initial value (0.14) during continued sliding.

The wear rates of the steel ball lie in the range (l-70) x 10-‘5 m2 N-’ (Table 1). In contrast to the tests with the ruby ball there was no correlation observed between wear and film roughness.

The observation of brownish debris in the wear track indicated the transfer and oxidation of ferrous ball material during the test which consequently affects the friction behaviour. The oxidation states of the debris were determined by X-ray photoelectron spectroscopy measurements. Iron was found to be present as Fe and

1016 U. Biigli et al. JDiamond and Related Materials 4 (1995) 1009%1019

500 nm

Fig. 10. AFM images of N5. Top, 3-D view of a 4 pm x 4 pm area; mode, the normal force was 5 nN and the contrast full scale 10 nN.

Fe(I1) but no Fe(II1) was found. This implies the pres- ence of FeO.

4. Discussion

The experiments show that a single roughness parame- ter alone cannot explain the tribological behaviour of diamond films in a given test configuration. Moreover, the surface morphology of diamond films is an important aspect which particularly affects the COF [ 33. For instance, the smooth NDF show among the highest friction coefficients, in apparent contradiction to the more usual low &-low COF relationship. The compari- son of the friction results (Figs. 2 and 3) with the AFM images (Figs. 8-10) nicely documents the influence of the morphology which changes from an early stage of columnar growth (thin films) to larger crystallites (thick films).

The morphology can be characterised by the density of the crystallites and their asperity angle. The high COF of Nl compared with N5 can be correlated with the 100 times higher crystallite density and their lower

jottom, corresponding topography (left) and friction (right). In the friction

asperity angle (see Figs. 8 and 10) which makes them more susceptible to fracture. An additional measurement with a diamond film having about four times the crystal- lite density of N5 showed a final COF of 0.4 which fits reasonably into the gap between the curves for N4 and N5 (Fig. 2). The effect of the morphology fades away with the growth of the ruby debris layer. As this layer grows, it (i) increasingly covers the sharp edges of the diamond crystallites, and (ii) changes the chemistry of the sliding pair. The consequence of the ruby layer was most obvious in a prolonged test performed with sample N2 at a sliding speed of 10 cm s-l. The sample showed the same behaviour as in the first test during the first 28 m. After a distance of about 42 m, however, the COF decreased rapidly from 0.8 to 0.4 and then continued to decrease to 0.11 after 125.7 m. It is worth noting that no wear of the ruby ball was measurable any more after 56.7 m, but the wear track was covered by a smooth carpet of firmly adhering ruby debris.

An increase of COF due to the reverse mechanism, so to speak, occurred in laser-polished samples upon heating. The removal of the graphitic layer (i) changes the morphology by uncovering tiny but sharp edges

U. B6gli et al./Diamond and Related Materials 4 (1995) 1009-1019 1017

Fig. 11. AFM images of the wear track in N5. Top, 3-D view of a 4 pm x 4 pm area; bottom, corresponding topography (left) and friction (right). In the friction mode, the normal force was 5 nN and the contrast full scale 8 nN.

0.6 , ,

5 0.6

z E 0.4

B E 0.3 .r g 0.2

8 0 0.1

0

0 60 loo 150 200 260

sliding distance [m]

Fig. 12. Evolution of the coefficient of friction between a monocrystal- line ruby ball and the laser-polished samples before (Ll-L4) and after (L2h-L4h) the removal of the :surface graphite layer by heat annealing.

[14], and (ii) changes the chemistry so as to stop the dry lubrication action.

In contrast to the COF, the wear of the ruby ball was more directly dependent on the film roughness parameter R,. In particular, there exists a threshold value of R,

above which a sudden increase of the ball wear is observed. The threshold itself, however, apparently depends on the film morphology as it is different for each series of samples, i.e. dependent on the polishing technique.

5. conclusioBs

Ultrathin diamond coatings show good adhesion and wear resistance during sliding tests. The friction and wear behaviour is strongly dependent on the surface roughness and film morphology. The decrease of the coefficient of friction with increasing R, of nanocrystal- line diamond films was found to be mainly related to a change in the film surface morphology. A higher asperity angle and a correspondingly lower density of crystallites produced a lower friction.

The transfer of abraded material from the ruby ball to the film produced an interfacial debris layer which reduced both the friction coefficient and the wear. For the smoothest nanocrystalline film (R,= 7 nm), an

1018 U. Biigri et al./Diatnond and Related Materials 4 (1995) 1009-1019

Fig. 13. SC :annin crys ~talline tips.

3n micrographs of the diamond films etched in an Ar-MW plasma: (a) Al; (b) A2; (c) A4. Arroa is in (b) point to rem

U. Biigli et al./Diamond and Related Materials 4 (1995) 1009-1019 1019

1 0.6

0.2

0

0 50 100 150

sliding distance [m]

Fig. 14. Evolution of the COF between a monocrystalline ruby ball and samples Al-A4. The inset is an expansion of the first 30 m and includes samples Ll and Ml for comparison.

to be substantially smoother than laser-polished samples to cause a comparable low ball wear.

Mechanically polished diamond films are character- ised by the lowest friction coefficient and the lowest wear rate as compared with all other diamond coatings tested in the present experiments.

Acknowledgements

We are indebted to H.P. Weber and V.I. Konov for helpful discussions. We wish to thank E. Krahenbtihl for technical assistance, W. Hanni and P. Ahlers for supplying additional diamond films and Raman meas- urements, and T.V. Kononenko for plasma etching. This work was supported in part by the Swiss Commission for the Encouragement of Scientific Research.

References

0.6 g,

0 10 20 30

sliding distance [m]

Fig. 15. Evolution of the coefficient of friction between the samples N7, N9, and Ll and the lOOCr6 steel ball.

extended run-in period characterised by a high friction coefficient was followed by wearless sliding with a COF as low as 0.1. Generally, the ball wear was found to increase suddenly when the film roughness exceeded a certain value, and this critical value was dependent on the polishing technique. For instance, laser-polished samples with R, - - 100 nm caused as low a ball wear as smooth nanocrystalline films with R, M 10 nm. The laser- polished samples also showed a lower COF than the NDF due to their morphology and the additional dry lubrication effect of the graphitic surface layer produced during irradiation.

Diamond films etched in an Ar-microwave plasma behave similar to the laser irradiated films. Like the nanocrystalline samples, however, these films also need

Cl1 CZI

c31

141

CSI

C61

c71

PI

c91

Cl01

Cl11

Cl21

Cl31

Cl41

I.P. Hayward, St& Coat. Technol., 49 (1991) 554. I.P. Hayward, I.L. Singer and L.E. Seitzmann, Wear, 157 (1992) 215. P.J. Blau, C.S. Yust, L.J. Heatherly and R.E. Clausing, in D. Dowson, C.M. Taylor and M. Godet (eds.), Mechanics of Coatings, Leeds-Lyon Symp. 16 (Tribology Series 17), Elsevier, Amsterdam, 1990, p. 399. MS. Wong, R. Meilunas, T.P. Ong and R.P. H. Chang, Appl. Phys. Lett., 54 (1989) 2006. B. Bhushan, V.V. Subramaniam, A. Malshe, B.K. Gupta and J. Ruan, J. Appt. Phys., 74 (1993) 4174. R.L.C Wu, K. Miyoshi, S. Miyake, H.E. Jackson, A.G. Choo, A.L. Korenyi-Both, A. Garscadden and P.N. Barnes, Diamond Films Technol., 3 (1993) 17. A.A. Smolin, SM. Pimenov, V.G. Ralchenko, T.V. Kononenko, V.I. Konov and E.N. Loubnin, Diamond Films Technol., 3 (1993) 1. V.I. Konov, Proc. 3rd lnt. Business Development Conf on The Global Business and Technical Outlook for CVD Diamond and Diamond-like Carbon Coating Market, Orlando, FL, March 6-8, 1994. R.J. Nemanich, J.T. Glass, G. Lucovsky, and R.E. Shroder, J. Vat. Sci. Technol. A, 6 (1988) 1783. U. Bogli, A. Blatter, S.M. Pimenov, A.A. Smolin and V.I. Konov, Diamond Relat. Muter., 1 (1992) 782. SM. Pimenov, A.A. Smolin, V.G. Ralchenko and V.I. Konov, Diamond Films Technol., 2 (1993) 201. SM. Metev, G. Sepold, V.I. Konov, E.D. Obraztsova, SM. Pimenov, V.G. Ralchenko, A.A. Smolin, Proc. 8th ClMTEC- World Ceramics Congress and Forum on New Materials, Florence, June 28-July 4, 1994, to be published. 1.P Hayward and LL. Singer, New Diamond Sci. Technol., 1991, MRS Int. Conf. Proc., p. 785. U. Bogli, A. Blatter, A. Bachli, R. Lnthi and E. Meyer, Diamond Relat. Muter., 2 (1993) 924.