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The effects of oxygen and humidity on friction and wear of diamond-like carbon films
Surface and Coatings Technology, 49 (1991) 537— 542
537
The effects of oxygen and humidity on friction and wear of diamond-like carbon films D. S. Kim, T. E. Fischer and B. Gallois Department of Materials Science and Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 (U.S.A.)
Abstract Diamond-like carbon (DLC) films were prepared on Si( 100) wafers by plasma-assisted chemical vapor deposition at a pressure of 900 mTorr. Their wear rates and friction coefficients against a silicon nitride ball were measured in a pin-on-disk tribometer in argon and air with varying relative humidities. In 50% relative humidity, the measured wear rates of the ball and DLC were 3 N-’ m’ and l0~mm3 N’ ~ respectively. In dry argon, dry air and 100% humid air, the wear of the order of l0” mm rates of DLC were l0~, l0~ and 10” mm3 N—’ m’, but that of the ball was below the detection limit. The measured friction coefficients were 0.06 in dry argon. 0.08 in 50’A humid air and 0.09 in l00°/humid argon, and around 0.2 in 50% humid argon. dry and 100% in humid air. In dry argon, the contact area of the ball was covered with material transferred from the DLC film during sliding; the low friction coefficient and wear rate measured in dry argon are attributed to this material. In dry and humid air, surface layers of DLC were oxidized by a tribochemical reaction forming a C=O bond. They covered the contact area of both the DLC film and the ball. This film increased friction coefficients, but it acted as a protective coating when its thickness was sufficient to prevent direct contact of the DLC film against the ball in 100% humid air.
1. Infroduction Various products such as thin film magnetic recording devices, cutting tools and bearings require protective coatings with a low friction coefficient and a high wear resistance. Diamond-like carbon (DLC) films satisfy such demands because of their excellent hardness, friction coefficient and wear resistance. There have been many studies conducted on the applications of DLC, but few of them have included extensive study of its tribological properties [1—4]. Friction was to found increase with relative humidity (from f = 0.05 f = to 0.3) [3], and extremely low friction coefficients (f = 0.2) were measured in vacuum [1]. This is contrary to other carbonaceous materials such as graphite, glassy carbon and diamond, which show a low friction coefficient with increasing humidity, and a high friction coefficient in vacuum [1]. This unusual frictional behavior of DLC has not yet been explained, Recently, it was proposed that the increase in friction is probably associated with changes in the chemistry of the contact surfaces (oxidation of carbon) [2, 4], but the frictional behavior as a function of humidity and oxygen is still open to debate. In the present study, the effects of oxygen and relative humidity (RH) on the frictional behavior and the wear rate of DLC films were investigated. It was found that a film is formed by tribochemical reaction (oxidation); this film increases the friction coefficient but decreases the wear rate when it acts as a protective
0257-8972/9l/$3.50
coating. The kinetics of the film formation depend on the RH and oxygen content in the environment,
2. Experimental methods 2.1. Preparation and characterization of the films DLC films were deposited on a silicon wafer by a capacitively coupled 13.56 MHz glow discharge sustamed by a mixture of hydrocarbon and hydrogen (CH 3 min~ H 4 flow rate standard cm 2 flow rate,an5 3 mm20-‘) at 900 mTorr and 340 K with standard r.f. powercm density of 0.13 W cm-2. The bias voltage was 110 V. The films presented compressive intrinsic stresses of 1.07 ±0.2 GPa, determined from the curvature of the DLC and the silicon wafer measured with a profilometer [5]. Fourier transform JR (FTIR) spectrometry was used to characterize the bonding structure of the DLC films. It showed that the hydrogenated carbon was mainly bound in sp2 and sp3 configurations. The ratio of sp2 to sp3 orbitals in the films determined by curve fitting the FTIR spectrum was 0.18. The content of hydrogen incorporated in the film, determined by elastic recoil detection of forward-scattered hydrogen atoms, was 55%. 2.2. Tribology tests The wear rates and friction coefficients of DLC against a silicon nitride ball (Norton NC-132) were
© 1991
Elsevier Sequoia, Lausanne
D. S. Kim et a!. / Friction and wear of diamond-like Cfilms
538
measured using a pin-on-disk tribometer. The station-
TABLE I. Friction coefficients of diamond-like carbon against Si
ary ball was loaded by a dead weight of 9.8 N and the frictional force was continuously monitored with a load cell. Quantitative wear measurements were performed on the ball by calculating the volume removed. The wear volume of the disk was calculated by multiplying the cross-sectional area of the track measured by the profilometer by the circumference of the track. The experiments were performed at room temperature in a glove box controlling the environmental gases such as air and argon in dry (less than 3% RH), intermediate ((50 ±lO)% RH) and saturated ((95 ±4)% RH) humidity conditions. The sliding speed was kept at 1.87 ±0.08cm s’ (track 0.711 cm in diameter at 50 rev mm - I) to avoid frictional heating. The sliding distances were as high as 1200 m. Scanning electron microscopy (SEM) and optical microscopy were used to examine the worn surfaces of the ball and the disk and microprobe FTIR spectrometry was used to analyze the wear debris,
in air and argon as a function of relative humidity A A ir r 0% RH 50% RH 100% RH 0% RH 50% RH
3. Results 3.1. Wear As shown in Fig. 1, the wear rates of DLC and Si3 N4 show a great dependence on environments and humidity. For the DLC, the lowest wear rate was obtained in dry environments. Humidity increases the wear rates of DLC by roughly a factor of five in 50% and 100% RH argon, and in humidity-saturated air. The highest wear rate was obtained in air at 50% RH. No wear of the Si3N4 ball was observed in dry air, dry argon and 100% RH air. High wear rates were found in 50% RH air and 100% RH argon. The details will be discussed in following sections.
_____________________________
io~~
Ar on g ~ ‘4 50% RH
0% RH
£~10’
Air
N
.!I11j~1 ~ I ~j
0.16 .
0.08
0.19
0.06
0.19
100% RH 0.99a
.
aFriction coefficient remains stable with values around 0.2 for an hour and then decreases to 0.09.
3.2. Friction coefficient The steady friction coefficients of the DLC sliding against the silicon nitride ball measured in dry and humid air and argon are presented in Table 1. When sliding begins, friction coefficients start at 0.2 for all cases, reaching the values shown in Table 1 in a short time except in 100% RH argon, where it took a few thousand turns to lower the friction coefficient from f = 0.2 to f = 0.9. These variations in friction can be attributed to third-body layers, produced by friction, that covered the contact surface of the ball in all cases.
4. Discussion 4.1. Tribochemical reaction of diamond-like carbon films As suggested in a previous study [4], the DLC films are oxidized in air and humid argon by a tribochemical reaction. This reaction is expected to be similar to the oxidation of hydrocarbon polymers described by Scott [6]: (1) The C—H bonds on the top layer of the DLC are mechanically broken by frictional force and oxygen molecules chemisorb (in the absence of oxygen they recombine). (2) These chemisorbed peroxides become —COOH or stay as active radicals such as COO—. (3) Further shearing of C—H bonds in the neighbors donates electrons to form C=O and the carbon network is terminated. The above basic oxidation theory of hydrocarbon
50% RH
RH
3N4
allows us to divide wear and friction modes into four 0% RH
~
RH
.~
.~
—
L.
Fig. 1. Wear rates of DLC and Si 3 N4 sliding against each other in air and argon as a function of relative humidity (Si3 N4 ball 7.94mm in diameter; sliding velocity, 1.87 ±0.08cm s’; load, 9.8 N; room temperature).
cases the observation of SEM images and the analysis of by debris. 4.2. Wear and friction in dry air The measured wear rates of in the dry DLCair.films of the 3 N~m~ Thearefriction order ,of lO~mm coefficient is intermediate (f=0.16). Figures 2(a) and 3(a) show the wear track of the DLC film and the wear .
.
.
scar of the Si3 Ni4 ball respectively. The wear track is covered with a loosely adhering layer, which is bright in Fig. 2(a). This new surface layer rolls up as the ball
D. S. Kim et al. / Friction and wear of diamond-like Cfilms
(a)
(b)
(c)
(d)
539
Fig. 2. (a), (b), (d) Scanning electron and (c) optical micrographs of the wear tracks on a DLC disk in dry and humid air: (a) 0% RH, rolls are oxidized hydrocarbon; (b) 50% RH. clean and smooth surface of the track; (c) 100% RH. compact wear debris of DLC accumulated on the sides of wear track, showing the trace of profilometer’s stylus; (d) cross-sectional view of (c), the thickness of the layer of the DLC wear debris is 0.7 ~jm outside the track and decreases toward the center of the track.
slides, leaving fresh surfaces of DLC film behind the trace of the rolls. When they reach a maximum size, the rolls twist and break into small pieces and accumulate on the sides of the wear track as shown in Fig. 2(a). Microprobe FTIR analysis of these rolls of debris (Fig. 4(a)) shows that they are carbonyl compounds produced by the oxidation of the DLC; the spectrum shows a hydrocarbon (C—H stretching vibration, centered around 2900 cm-’), and an oxidized hydrocarbon (C=O, centered around 1714 cm—’) [7]. A spectrum outside the track is presented in Fig. 4(d) for comparison. As shown in Fig. 3(a), the contact area of the Si3N4 ball is covered with a very thin layer, which is a
tures of the wear debris produced in high humidity. The debris accumulated on the outside of the track is dense and compact and maintains its thickness, around 0.7 jim as shown in Fig. 2(d). A microprobe FTIR spectrum taken from the debris shows the same species of wear debris (oxidized hydrocarbon) as observed in dry air. We conclude that water molecules produce strong adhesion of the wear debris of the DLC film. A dense layer covers the wear track of the DLC and reduces the oxidation rate. It causes the formation of a compact and dense layer of wear debris on the ball that prevents direct contact with the disk as shown in Fig.
transferred layer of oxidized DLC, because no wear was observed on the ball by profilometry. It is concluded that the low wear rate and the intermediate friction coefficient of the DLC film in dry air are due to this soft and dry oxidized layer of DLC.
3(c). Thus sliding of the DLC occurs against this thirdbody layer. No tribochemical reaction of the Si3 N4 ball occurs and no wear is found in the ball even in saturated humid air. The high friction coefficient, f = 0.19, is attributed to the sliding between these compacted layers of debris covering the two bodies.
4.3. Wear and friction in humid air 4.3.1. In 100% relative humidity air. An optical micrograph of the wear track (Fig. 2(c)) shows the fea-
4.3.2. In 50% relative humidity air. In air with 500/u relative humidity, both the ball and 3the disk show N-’ m’. In high this wear rates of the order of l0~mm
540
D. S. Kim et a!.
/ Friction
and wear of diamond-like C films
(a)
(b)
(c)
(d)
Fig. 3. (a). (b), (d) scanning electron and (c) optical micrographs of the wear scars on the Si
3N4 ball in dry air, dry argon and humid air. The contact areas of the balls are covered with a third-body layer in all cases. The profilometer traces are also shown: (a) 0% RH air; (b) 50% RH air, detail of the scar, covered with mixed wear debris of the DLC and the ball; (c) 100% RH air, DLC debris (oxidized DLC) covering the ball; (d) dry argon, DLC debris (unoxidized) covering the ball.
case, the oxidation of the DLC proceeds without a protecting layer. This exposure to the tribochemical reaction gives rise to the high wear rates of the DLC film and the ball. Micro-probe FTIR analysis (Fig. 4(b)) revealed that the debris produced in this case is a mixture of an oxidized hydrocarbon from the DLC film and a hydrated silica from the ball (Si02 xH~0, around 3400, 1620, 1120 and 430 cm-’) [8]. The low friction coefficient (f = 0.08) in 50% RH air is attributed to the mixed tribochemical wear products. .
4.4. Wear and friction in dry argon In the dry argon environment, the dominant wear mechanism is adhesive wear. Figure 3(d) shows the wear scar of the Si3N4 ball after friction in dry argon. It is completely covered with a glossy film of wear debris transferred the wear DLCsurface film. with The debris DLC showed a clean andfrom smooth accumulated on the sides of the track similar to Fig. 2(b). Microprobe FTIR analysis (Fig. 4(c)) indicates that the wear debris is pure and unoxidized DLC. A microfractured DLC film is formed by adhesive wear.
The mechanically scissored particles recombine to produce macrowear debris in the absence of a radical trap such as oxygen [61 and agglomerate to form a thick protective layer on the ball in dry argon as illustrated in Fig. 3(d); it is worth noting that this protective layer is soft, uniform and thick. It is scratched easily by the stylus of a profilometer. This thick and soft transferred film prevents direct contact between the ball and the disk (actual sliding occurs on DLC against DLC); it also reduces the stress concentration on the contact area. The low friction coefficient, low wear rate of the DLC film and the absence of wear on the ball are attributed to this layer of DLC wear debris. 4.5. Wear and friction in humid argon The wear rates of 3DLC have in intermediate N-’ m’) both 50%values and (around x 108because mm the tribochemical reaction rate 100%, RH 2argon of the DLC is low owing to a lack of oxygen. The wear rate of the Si 3N4 ball in 100% RH argon is one order higher than in 50% RH argon. This is explained by the tribochemistry of Si3 N4 [9]; little tribochemical reaction
D. S. Kim et a!. / Friction and wear of diamond-like C films
541
~cY~OSIO2xH2O
4000
2000
1600
1200
800
400
Wavenumber (cm1)
~)
~ 4000
(c)
2000
1600
1200
Wavenumber (cm~)
800
4000
(b)
400
4000
(d)
2000
1600
1200
600
400
Wavenumber (cm’)
2000
1600
1200
800
400
Wovenumber (cm’)
Fig. 4. Microprobe FTIR spectra of the DLC film and the wear debris accumulated on the sides of the track (DLC; C—H stretching vibration centered at around 2900 cm’; C—CH 3 deformation frequencies at around 1450 and 1375 cm’; oxidized hydrocarbon, C=O centered at around 1714 cm’; hydrated silica, centered at around 1100 cm’): (a) 0% RH air; (b) 50% RH air; (c) 0% RH argon; (d) DLC film, outside the track. The spectrum includes contributions from the silicon substrate.
was observed at 50% RH argon, but it was active enough in 100% RH argon to dominate the tribochemistry. In 100% RH argon, the friction coefficient shows an unusual behavior; at first it remains stable with values around 0.2 for 1 h and then decreases to 0.09. Microprobe FTIR analysis shows the presence of oxidized wear debris of DLC and hydrated silica from the ball. The slow decrease in friction indicates that the formation of the third-body layer (hydrocarbon with C=O) in 100% RH argon is slower than in air because of the lack of oxygen; the oxygen in C=O is provided by water molecules adsorbed on the wear debris.
5. Conclusions 5.1. Friction (1) The friction of DLC against Si3 N4 is determined by transfer layers of wear debris. (2) DLC sliding against an unoxidized DLC debris layer presents low friction (f= 0.06). (3) The oxidized DLC formed in air and humid argon increases the friction coefficient.
5.2. Wear (I) There is no correlation between friction coefficient and wear rate. (2) Wear is dominated by tribochemistry in air and humid environments, but in dry argon it is governed by adhesive wear.
Acknowledgments This work was partially supported by the Army Research Office, Division of Materials Science, under Contract DAAG29-85-K-0 124. The authors thank Dr. P. Zanzucchi for the microprobe FTIR measurements, B. Wilkens for the hydrogen profiling and D. Moffatt for the curve-fitting programs used in the FTIR analysis.
References R.Memrning, H. J. Tolle and P. E. Wierenga, Thin Solid Films. 2 B. Marchon, N. Heiman and M. R. Khan, IEEE Trans. Magn.. 26 (1) (1990) 168. 3 K. Enke, Thin Solid Films, 80 (1981) 227.
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D. S. Kim et a!. / Friction and wear of diamond-like C films
4 R. S. Timsii and G. Stratford, STLE Special Pub!. 25, Vol. V. 1988, p. 17. 5 G. G. Stoney, Proc. R. Soc. London, Ser. A, 82 (1909) 172. 6 G. Scott, Polym. Eng. Sci.. 24(13) (1984) 1007. 7 N. B. Colthup, L. H. Daly and S. E. Wiberley, Introduction to
Infrared and Raman Spectroscopy, 2nd edn., Academic Press, New York 1975, p. 239. 8 Anon., Sadtler Standard Spectra Series, Inorganic. JR Grading Spectra, Vol. 1, Spectrum Y126K. 9 T. E. Fischer and H. Tomizawa, Wear, 105 (1985) 29.
537
The effects of oxygen and humidity on friction and wear of diamond-like carbon films D. S. Kim, T. E. Fischer and B. Gallois Department of Materials Science and Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 (U.S.A.)
Abstract Diamond-like carbon (DLC) films were prepared on Si( 100) wafers by plasma-assisted chemical vapor deposition at a pressure of 900 mTorr. Their wear rates and friction coefficients against a silicon nitride ball were measured in a pin-on-disk tribometer in argon and air with varying relative humidities. In 50% relative humidity, the measured wear rates of the ball and DLC were 3 N-’ m’ and l0~mm3 N’ ~ respectively. In dry argon, dry air and 100% humid air, the wear of the order of l0” mm rates of DLC were l0~, l0~ and 10” mm3 N—’ m’, but that of the ball was below the detection limit. The measured friction coefficients were 0.06 in dry argon. 0.08 in 50’A humid air and 0.09 in l00°/humid argon, and around 0.2 in 50% humid argon. dry and 100% in humid air. In dry argon, the contact area of the ball was covered with material transferred from the DLC film during sliding; the low friction coefficient and wear rate measured in dry argon are attributed to this material. In dry and humid air, surface layers of DLC were oxidized by a tribochemical reaction forming a C=O bond. They covered the contact area of both the DLC film and the ball. This film increased friction coefficients, but it acted as a protective coating when its thickness was sufficient to prevent direct contact of the DLC film against the ball in 100% humid air.
1. Infroduction Various products such as thin film magnetic recording devices, cutting tools and bearings require protective coatings with a low friction coefficient and a high wear resistance. Diamond-like carbon (DLC) films satisfy such demands because of their excellent hardness, friction coefficient and wear resistance. There have been many studies conducted on the applications of DLC, but few of them have included extensive study of its tribological properties [1—4]. Friction was to found increase with relative humidity (from f = 0.05 f = to 0.3) [3], and extremely low friction coefficients (f = 0.2) were measured in vacuum [1]. This is contrary to other carbonaceous materials such as graphite, glassy carbon and diamond, which show a low friction coefficient with increasing humidity, and a high friction coefficient in vacuum [1]. This unusual frictional behavior of DLC has not yet been explained, Recently, it was proposed that the increase in friction is probably associated with changes in the chemistry of the contact surfaces (oxidation of carbon) [2, 4], but the frictional behavior as a function of humidity and oxygen is still open to debate. In the present study, the effects of oxygen and relative humidity (RH) on the frictional behavior and the wear rate of DLC films were investigated. It was found that a film is formed by tribochemical reaction (oxidation); this film increases the friction coefficient but decreases the wear rate when it acts as a protective
0257-8972/9l/$3.50
coating. The kinetics of the film formation depend on the RH and oxygen content in the environment,
2. Experimental methods 2.1. Preparation and characterization of the films DLC films were deposited on a silicon wafer by a capacitively coupled 13.56 MHz glow discharge sustamed by a mixture of hydrocarbon and hydrogen (CH 3 min~ H 4 flow rate standard cm 2 flow rate,an5 3 mm20-‘) at 900 mTorr and 340 K with standard r.f. powercm density of 0.13 W cm-2. The bias voltage was 110 V. The films presented compressive intrinsic stresses of 1.07 ±0.2 GPa, determined from the curvature of the DLC and the silicon wafer measured with a profilometer [5]. Fourier transform JR (FTIR) spectrometry was used to characterize the bonding structure of the DLC films. It showed that the hydrogenated carbon was mainly bound in sp2 and sp3 configurations. The ratio of sp2 to sp3 orbitals in the films determined by curve fitting the FTIR spectrum was 0.18. The content of hydrogen incorporated in the film, determined by elastic recoil detection of forward-scattered hydrogen atoms, was 55%. 2.2. Tribology tests The wear rates and friction coefficients of DLC against a silicon nitride ball (Norton NC-132) were
© 1991
Elsevier Sequoia, Lausanne
D. S. Kim et a!. / Friction and wear of diamond-like Cfilms
538
measured using a pin-on-disk tribometer. The station-
TABLE I. Friction coefficients of diamond-like carbon against Si
ary ball was loaded by a dead weight of 9.8 N and the frictional force was continuously monitored with a load cell. Quantitative wear measurements were performed on the ball by calculating the volume removed. The wear volume of the disk was calculated by multiplying the cross-sectional area of the track measured by the profilometer by the circumference of the track. The experiments were performed at room temperature in a glove box controlling the environmental gases such as air and argon in dry (less than 3% RH), intermediate ((50 ±lO)% RH) and saturated ((95 ±4)% RH) humidity conditions. The sliding speed was kept at 1.87 ±0.08cm s’ (track 0.711 cm in diameter at 50 rev mm - I) to avoid frictional heating. The sliding distances were as high as 1200 m. Scanning electron microscopy (SEM) and optical microscopy were used to examine the worn surfaces of the ball and the disk and microprobe FTIR spectrometry was used to analyze the wear debris,
in air and argon as a function of relative humidity A A ir r 0% RH 50% RH 100% RH 0% RH 50% RH
3. Results 3.1. Wear As shown in Fig. 1, the wear rates of DLC and Si3 N4 show a great dependence on environments and humidity. For the DLC, the lowest wear rate was obtained in dry environments. Humidity increases the wear rates of DLC by roughly a factor of five in 50% and 100% RH argon, and in humidity-saturated air. The highest wear rate was obtained in air at 50% RH. No wear of the Si3N4 ball was observed in dry air, dry argon and 100% RH air. High wear rates were found in 50% RH air and 100% RH argon. The details will be discussed in following sections.
_____________________________
io~~
Ar on g ~ ‘4 50% RH
0% RH
£~10’
Air
N
.!I11j~1 ~ I ~j
0.16 .
0.08
0.19
0.06
0.19
100% RH 0.99a
.
aFriction coefficient remains stable with values around 0.2 for an hour and then decreases to 0.09.
3.2. Friction coefficient The steady friction coefficients of the DLC sliding against the silicon nitride ball measured in dry and humid air and argon are presented in Table 1. When sliding begins, friction coefficients start at 0.2 for all cases, reaching the values shown in Table 1 in a short time except in 100% RH argon, where it took a few thousand turns to lower the friction coefficient from f = 0.2 to f = 0.9. These variations in friction can be attributed to third-body layers, produced by friction, that covered the contact surface of the ball in all cases.
4. Discussion 4.1. Tribochemical reaction of diamond-like carbon films As suggested in a previous study [4], the DLC films are oxidized in air and humid argon by a tribochemical reaction. This reaction is expected to be similar to the oxidation of hydrocarbon polymers described by Scott [6]: (1) The C—H bonds on the top layer of the DLC are mechanically broken by frictional force and oxygen molecules chemisorb (in the absence of oxygen they recombine). (2) These chemisorbed peroxides become —COOH or stay as active radicals such as COO—. (3) Further shearing of C—H bonds in the neighbors donates electrons to form C=O and the carbon network is terminated. The above basic oxidation theory of hydrocarbon
50% RH
RH
3N4
allows us to divide wear and friction modes into four 0% RH
~
RH
.~
.~
—
L.
Fig. 1. Wear rates of DLC and Si 3 N4 sliding against each other in air and argon as a function of relative humidity (Si3 N4 ball 7.94mm in diameter; sliding velocity, 1.87 ±0.08cm s’; load, 9.8 N; room temperature).
cases the observation of SEM images and the analysis of by debris. 4.2. Wear and friction in dry air The measured wear rates of in the dry DLCair.films of the 3 N~m~ Thearefriction order ,of lO~mm coefficient is intermediate (f=0.16). Figures 2(a) and 3(a) show the wear track of the DLC film and the wear .
.
.
scar of the Si3 Ni4 ball respectively. The wear track is covered with a loosely adhering layer, which is bright in Fig. 2(a). This new surface layer rolls up as the ball
D. S. Kim et al. / Friction and wear of diamond-like Cfilms
(a)
(b)
(c)
(d)
539
Fig. 2. (a), (b), (d) Scanning electron and (c) optical micrographs of the wear tracks on a DLC disk in dry and humid air: (a) 0% RH, rolls are oxidized hydrocarbon; (b) 50% RH. clean and smooth surface of the track; (c) 100% RH. compact wear debris of DLC accumulated on the sides of wear track, showing the trace of profilometer’s stylus; (d) cross-sectional view of (c), the thickness of the layer of the DLC wear debris is 0.7 ~jm outside the track and decreases toward the center of the track.
slides, leaving fresh surfaces of DLC film behind the trace of the rolls. When they reach a maximum size, the rolls twist and break into small pieces and accumulate on the sides of the wear track as shown in Fig. 2(a). Microprobe FTIR analysis of these rolls of debris (Fig. 4(a)) shows that they are carbonyl compounds produced by the oxidation of the DLC; the spectrum shows a hydrocarbon (C—H stretching vibration, centered around 2900 cm-’), and an oxidized hydrocarbon (C=O, centered around 1714 cm—’) [7]. A spectrum outside the track is presented in Fig. 4(d) for comparison. As shown in Fig. 3(a), the contact area of the Si3N4 ball is covered with a very thin layer, which is a
tures of the wear debris produced in high humidity. The debris accumulated on the outside of the track is dense and compact and maintains its thickness, around 0.7 jim as shown in Fig. 2(d). A microprobe FTIR spectrum taken from the debris shows the same species of wear debris (oxidized hydrocarbon) as observed in dry air. We conclude that water molecules produce strong adhesion of the wear debris of the DLC film. A dense layer covers the wear track of the DLC and reduces the oxidation rate. It causes the formation of a compact and dense layer of wear debris on the ball that prevents direct contact with the disk as shown in Fig.
transferred layer of oxidized DLC, because no wear was observed on the ball by profilometry. It is concluded that the low wear rate and the intermediate friction coefficient of the DLC film in dry air are due to this soft and dry oxidized layer of DLC.
3(c). Thus sliding of the DLC occurs against this thirdbody layer. No tribochemical reaction of the Si3 N4 ball occurs and no wear is found in the ball even in saturated humid air. The high friction coefficient, f = 0.19, is attributed to the sliding between these compacted layers of debris covering the two bodies.
4.3. Wear and friction in humid air 4.3.1. In 100% relative humidity air. An optical micrograph of the wear track (Fig. 2(c)) shows the fea-
4.3.2. In 50% relative humidity air. In air with 500/u relative humidity, both the ball and 3the disk show N-’ m’. In high this wear rates of the order of l0~mm
540
D. S. Kim et a!.
/ Friction
and wear of diamond-like C films
(a)
(b)
(c)
(d)
Fig. 3. (a). (b), (d) scanning electron and (c) optical micrographs of the wear scars on the Si
3N4 ball in dry air, dry argon and humid air. The contact areas of the balls are covered with a third-body layer in all cases. The profilometer traces are also shown: (a) 0% RH air; (b) 50% RH air, detail of the scar, covered with mixed wear debris of the DLC and the ball; (c) 100% RH air, DLC debris (oxidized DLC) covering the ball; (d) dry argon, DLC debris (unoxidized) covering the ball.
case, the oxidation of the DLC proceeds without a protecting layer. This exposure to the tribochemical reaction gives rise to the high wear rates of the DLC film and the ball. Micro-probe FTIR analysis (Fig. 4(b)) revealed that the debris produced in this case is a mixture of an oxidized hydrocarbon from the DLC film and a hydrated silica from the ball (Si02 xH~0, around 3400, 1620, 1120 and 430 cm-’) [8]. The low friction coefficient (f = 0.08) in 50% RH air is attributed to the mixed tribochemical wear products. .
4.4. Wear and friction in dry argon In the dry argon environment, the dominant wear mechanism is adhesive wear. Figure 3(d) shows the wear scar of the Si3N4 ball after friction in dry argon. It is completely covered with a glossy film of wear debris transferred the wear DLCsurface film. with The debris DLC showed a clean andfrom smooth accumulated on the sides of the track similar to Fig. 2(b). Microprobe FTIR analysis (Fig. 4(c)) indicates that the wear debris is pure and unoxidized DLC. A microfractured DLC film is formed by adhesive wear.
The mechanically scissored particles recombine to produce macrowear debris in the absence of a radical trap such as oxygen [61 and agglomerate to form a thick protective layer on the ball in dry argon as illustrated in Fig. 3(d); it is worth noting that this protective layer is soft, uniform and thick. It is scratched easily by the stylus of a profilometer. This thick and soft transferred film prevents direct contact between the ball and the disk (actual sliding occurs on DLC against DLC); it also reduces the stress concentration on the contact area. The low friction coefficient, low wear rate of the DLC film and the absence of wear on the ball are attributed to this layer of DLC wear debris. 4.5. Wear and friction in humid argon The wear rates of 3DLC have in intermediate N-’ m’) both 50%values and (around x 108because mm the tribochemical reaction rate 100%, RH 2argon of the DLC is low owing to a lack of oxygen. The wear rate of the Si 3N4 ball in 100% RH argon is one order higher than in 50% RH argon. This is explained by the tribochemistry of Si3 N4 [9]; little tribochemical reaction
D. S. Kim et a!. / Friction and wear of diamond-like C films
541
~cY~OSIO2xH2O
4000
2000
1600
1200
800
400
Wavenumber (cm1)
~)
~ 4000
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Fig. 4. Microprobe FTIR spectra of the DLC film and the wear debris accumulated on the sides of the track (DLC; C—H stretching vibration centered at around 2900 cm’; C—CH 3 deformation frequencies at around 1450 and 1375 cm’; oxidized hydrocarbon, C=O centered at around 1714 cm’; hydrated silica, centered at around 1100 cm’): (a) 0% RH air; (b) 50% RH air; (c) 0% RH argon; (d) DLC film, outside the track. The spectrum includes contributions from the silicon substrate.
was observed at 50% RH argon, but it was active enough in 100% RH argon to dominate the tribochemistry. In 100% RH argon, the friction coefficient shows an unusual behavior; at first it remains stable with values around 0.2 for 1 h and then decreases to 0.09. Microprobe FTIR analysis shows the presence of oxidized wear debris of DLC and hydrated silica from the ball. The slow decrease in friction indicates that the formation of the third-body layer (hydrocarbon with C=O) in 100% RH argon is slower than in air because of the lack of oxygen; the oxygen in C=O is provided by water molecules adsorbed on the wear debris.
5. Conclusions 5.1. Friction (1) The friction of DLC against Si3 N4 is determined by transfer layers of wear debris. (2) DLC sliding against an unoxidized DLC debris layer presents low friction (f= 0.06). (3) The oxidized DLC formed in air and humid argon increases the friction coefficient.
5.2. Wear (I) There is no correlation between friction coefficient and wear rate. (2) Wear is dominated by tribochemistry in air and humid environments, but in dry argon it is governed by adhesive wear.
Acknowledgments This work was partially supported by the Army Research Office, Division of Materials Science, under Contract DAAG29-85-K-0 124. The authors thank Dr. P. Zanzucchi for the microprobe FTIR measurements, B. Wilkens for the hydrogen profiling and D. Moffatt for the curve-fitting programs used in the FTIR analysis.
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