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Friction of diamond-like carbon films in different atmospheres
Wear 254 (2003) 1070–1075
Friction of diamond-like carbon films in different atmospheres J. Andersson∗ , R.A. Erck, A. Erdemir Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA
Abstract Diamond-like carbon (DLC) films constitute a class of new materials with a wide range of compositions, properties, and performance. In particular, the tribological properties of these films are rather intriguing and can be strongly influenced by the test conditions and environment. In this paper, a series of model experiments are performed in high vacuum and with various added gases to elucidate the influence of different test environments on the tribological behavior of three DLC films. Specifically, the behavior of a hydrogen-free film produced by a cathodic arc process and two highly hydrogenated films produced by plasma-enhanced chemical-vapor deposition were studied. Flats and balls used in these experiments were coated with DLC and tested in a pin-on-disc machine under a load of 1 N and at constant rotational frequency. With a low background pressure, in the 10−6 Pa range, the highly hydrogenated films exhibited a friction coefficient of less than 0.01, whereas the hydrogen-free film gave a friction coefficient of approximately 0.6. Adding oxygen or hydrogen to the experimental environment changed the friction to some extent. However, admission of water vapor into the test chamber caused large changes: the friction coefficient decreased drastically for the hydrogen-free DLC film, whereas it increased slightly for one of the highly hydrogenated films. These results indicate that water molecules play a prominent role in the frictional behavior of DLC films—most notably for hydrogen-free films but also for highly hydrogenated films. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Friction; Tetrahedral amorphous carbon; Diamond-like carbon; Hydrogen; Water; Vacuum
1. Introduction Superlubricity and wearless sliding under dry sliding conditions are often desired for many moving mechanical assemblies (MMAs: be they MEMS devices, spacemechanism assemblies, or ordinary journal bearings) but unfortunately seldom achieved. As one could envision, the implication of near frictionless and wearless sliding for MMAs is enormous. It means reduced maintenance, increased reliability, and lowered energy consumption along with longer wear lives in mechanical components. In this paper, we will use a carbon-based film with the capacity of nearly frictionless and wearless sliding in vacuum. Earlier experiments in inert gas environments have confirmed that this film is capable of affording friction and wear coefficients in the ranges 0.001–0.005 and 10−11 to 10−10 mm3 /Nm, respectively, to sliding steel and ceramic interfaces [1–3]. There exist a few other materials and coatings that can provide super-low friction. For example, MoS2 and WS2 can give friction coefficients less than 0.01, but not in the presence of any appreciable amounts of water. Hence, their ∗ Corresponding author. Present address: Tribomaterials Group, Angstrom Laboratory, P.O. Box 534, SE-751 21 Uppsala, Sweden. Tel.: +46-18-471-3077; fax: +46-18-471-3572. E-mail address: [email protected] (J. Andersson).
use in moist atmospheres for an extended period of time is almost impossible. These materials are nonetheless used in some aerospace applications, i.e. rolling element bearings, sliding bearings and actuator gears, etc. [4]. Another problem is the scratch sensitivity and relatively high abrasive wear-rates of these materials, which puts further restraints on where they can be used [5,6]. Another potentially super-low friction material is boric acid [7,8]. The basis of the low friction mechanism is the reaction of boric oxide with ambient humidity. The use of boric acid as a low friction coating is therefore limited to reasonably humid conditions, precluding its use in vacuum or otherwise dry applications. Kato has recently demonstrated that water lubrication of certain ceramics (such as Si3 N4 and SiC) can lead to superlow friction coefficients. A combination of fluid and tribochemical film formation has been proposed as the controlling mechanism in their cases [9,10]. Tests with oxide-based ceramics did not lead to similar improvements in friction and wear under water lubricated sliding conditions. With recent progress, there are now also diamond-like carbon (DLC) films that exhibit super-low friction [11,12]. This interesting behavior, however, is limited to vacuum and inert atmospheres. The large amounts of hydrogen in these films have been implicated as instrumental for the
0043-1648/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00336-3
J. Andersson et al. / Wear 254 (2003) 1070–1075
super-low friction behavior [13,14]. Recent tribological studies by Umehara et al. on a certain class of carbon nitride films have also yielded friction coefficients less than 0.01 in dry N2 during sliding against Si3 N4 [15]. Another interesting carbon-based material is tetrahedral amorphous carbon (ta-C). So far, not in the super-low friction domain, ta-C can nonetheless exhibit friction coefficients down to 0.08 [16]. Coupled with low wear and very high scratch resistance this is an attractive material from an engineering point of view. However, just like boric acid, the low friction and wear of ta-C has been suggested to be dependent on ambient humidity [17]. Naturally, ultra-low friction regardless of environment is an attractive concept. Various attempts have been made, for example, Voevodin et al. [18] produced a nano-composite of WC/DLC/WS2 , which worked well in vacuum and dry nitrogen and initially in humid air as well. Unfortunately, cycling the different atmospheres gave a progressive increase in friction. With the long-term goal of producing an environmentally benign low friction DLC film, we believe an important step is to understand the relationship between film composition and environment with respect to frictional behavior. In the search for clues on the governing mechanisms, we performed tribological experiments in vacuum, oxygen, hydrogen and water vapor on two kinds of hydrogenated carbon films (HFC) and one hydrogen-free ta-C film. To reduce the interference of transfer films and dissimilar hardness, identical materials were used on both sides of the tribo-couple.
2. Experiments In this study, three different materials were tested; one tetrahedral amorphous carbon film and two hydrogenated carbon films. Approximately 0.5 m ta-C was deposited on AISI H13 steel flats and M50 (9.53 mm in diameter) steel balls using cathodic arc-evaporation. The resulting film is amorphous, mostly sp3 bonded and has very low hydrogen content [19]. The HCF was produced by plasma-enhanced chemical-vapor deposition on similar substrates to a thickness of 1 m. HCF 1 was deposited in a fifty-fifty hydrogen–methane flow rate mixture, whereas the deposition of HCF 2 utilized a 0.75:0.25 ratio [1,2]. These films contained about 39 atm.% hydrogen according to hydrogen forward scattering measurements with 2 MeV He2+ ions. The friction force between the coated flat and the similarly coated ball was measured in a standard pin-on-disc set-up situated inside a vacuum chamber. Valves allowed the introduction of high-purity gases and liquids. For each non-vacuum experiment, 1300 Pa of hydrogen, oxygen or water was added to background pressures no higher than 5 × 10−5 Pa. For water, this corresponds to roughly 50% relative humidity at laboratory temperature. The length of the experiments was limited to 100 revolutions, corresponding to 3 m, mainly because the endurance
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lifetime of ta-C in vacuum was just above 100 revolutions. The hydrogenated films could be tested for thousands of revolutions in vacuum without any noticeable wear. A constant rotational frequency of 60 rpm was used and the linear speeds were of the order of 0.07–0.03 m/s. A normal force of 1 N was applied to the ball. To reach ultra-low friction coefficients for the hydrogenated films, a longer run-in than 100 revolutions was necessary and hence the experiments were extended. As the friction force diminished to very small values, the importance of a correct zero level increased. In an attempt to cancel this uncertainty, the direction of revolution was reversed after run-in and the friction coefficient was calculated by taking the mean of the absolute values of the two different friction forces. The wear was qualitatively examined in a light optical microscope. Special care was taken that the coating on the balls were not worn through or had cracked or spalled off during the experiment.
3. Results The frictional behavior of ta-C films differed quite substantially when tested in different atmospheres as shown in Fig. 1. The highest friction, i.e. 0.65, was observed in high vacuum, while the lowest friction, i.e. 0.07 was recorded in the presence of water molecules in the test chamber. In high vacuum, the wear damage on ta-C-coated ball and disk samples was so severe that the length of the experiments was limited to 100 revolutions. Upon the introduction of 1300 Pa of oxygen, hydrogen or water vapor into the test chamber, the friction seemed to level off at approximately 0.25, 0.15 and 0.07, respectively, and the wear damage on films was significantly reduced. As is clear from Fig. 1, introduction of water into the test chamber had the most beneficial effect on friction. It reduced the friction coefficient from 0.65 to 0.07 and effectively eliminated the rather pronounced stick–slip behavior seen during tests in hydrogen and oxygen.
Fig. 1. Run-in frictional behavior of ta-C in different environments.
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Fig. 2. Frictional behavior of ta-C in vacuum subsequent to run-in in respective environment.
When water, hydrogen, or oxygen was pumped out and the sliding tests were repeated on the same tracks, the friction coefficients increased substantially (see Fig. 2). The track previously exposed to hydrogen provided the smallest increase in friction, while those tracks previously exposed to oxygen exhibited much higher friction. In vacuum, both HCF 1 and 2 exhibited friction coefficients below 0.02 within 100 revolutions of the sliding tests (Figs. 3 and 4). With the addition of hydrogen and oxygen, the friction was just slightly higher and the general behavior during run-in was very similar to that in vacuum. The experiments with 1300 Pa of water were the second in the same track. Hence, the tracks were run-in before the shown data was taken, explaining their initially lower friction. Whereas the friction after 100 revolutions in vacuum, hydrogen and oxygen still seems to be decreasing, the friction in water vapor seems to have stabilized, and for HCF 2 at a higher level than that of the other three environments by the end of the experiment. After an extended run-in in vacuum, an ultra-low friction coefficient, about 0.009, was achieved for HCF 1 (Fig. 5). The friction coefficient in vacuum calculated as the mean
Fig. 3. Frictional behavior of HCF 1 in different environments.
Fig. 4. Frictional behavior of HCF 2 in different environments.
Fig. 5. Frictional behavior of HCF films in an extended experiment in vacuum.
from going in both directions was 0.007 for HCF 2. There was no visible wear on the HCF 1 and 2 coated balls, even after these extended vacuum experiments.
4. Discussion Results presented in Figs. 1–4 clearly demonstrate that environmental species have a very pronounced effect on the frictional behavior of ta-C, and also some effect on that of HCF films. These findings are consistent with earlier studies in high vacuum and/or various gases [20–23]. From the results, it is clear that ta-C can only provide low friction in the presence of water, oxygen, or hydrogen molecules in the test chamber. Water has the most beneficial effect in reducing friction. As for HCF films, the presence of water, oxygen and hydrogen molecules only slightly increase their friction coefficients (see Figs. 3 and 4). The initially lower friction of the HCF surfaces is due to a prior run-in. Apart from the surface roughness effect, covalent interactions may be a major contributor to the frictional force between sliding diamond and/or DLC surfaces. These are
J. Andersson et al. / Wear 254 (2003) 1070–1075
strong interactions that can lead to very high frictional forces [24–29]. The other forces that can give rise to friction in carbon films are –∗ , van der Waals, electrostatic attractions, and capillary forces. The –∗ interactions may occur at sliding interfaces of carbons with substantial amounts of sp2 bonding, such as graphite and glassy carbon. Capillary forces may result from adsorbed molecular species, such as water, but they are not expected to play any significant role for the friction in vacuum. The very high and un-steady friction exhibited by the ta-C film in vacuum can be explained as follows. The surface carbon atoms in these films are believed to be bonded to their near neighbors with three bonds, but the fourth bond may be free and dangling out of the surface. In open air, these free bonds are normally terminated or passivated by such adsorbates as water molecules, oxygen, hydrogen, etc. Some of these species are bonded to carbon atoms physically, but others may have been bonded chemically. When such surfaces are placed in high vacuum, some of the adsorbates (especially those bonded physically) are desorbed, while those species bonded chemically may still remain on the surface. When a sliding contact experiment is performed on such surfaces in vacuum, species adsorbed on the top surface are removed by mechanical wear and/or thermal desorption (due to frictional heating); hence, the strong bonds of surface carbon atoms are freed and available for covalent bond interactions with atoms in the other side of the sliding interface. Such interactions can cause strong adhesion and hence high friction. Therefore, the very high friction coefficient in high vacuum (see Fig. 1) may be due to strong bond interactions at the sliding interfaces. When oxygen, hydrogen, and water molecules were introduced in the test chamber, the friction coefficient dropped significantly. One logical explanation for this is that the bonds of surface carbon atoms of this film were passivated by the introduced molecules and hence the extent of covalent bond interactions became less pronounced. Even if some of the bonds may have been exposed due to wear and/or thermal desorption, they are rapidly repassivated by adsorbing molecules. As a result, the friction coefficient of the hydrogen-free DLC film became relatively low (i.e. 0.07–0.25). This finding is consistent with previous experimental studies, which showed that surface adsorbates (such as water molecules, oxygen, etc.) could have significant influence on the friction and wear properties of DLC films [30,31]. In support of the explanation provided above, upon removal of water, oxygen, and hydrogen from the test chamber, the friction coefficient of ta-C increase substantially as shown in Fig. 2. It is interesting to see that the increase in friction is more gradual and relatively small for the wear track that was exposed to the hydrogen environment, while that for oxygen is more rapid and the largest. Such a disparity could be attributed to the differences in the bond strengths
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between carbon and these two species (i.e. C–H bond energy of 413 kJ/mol versus C–O bond energy of 358 kJ/mol). Because of the high intensity hydrogen-rich plasma, most of the carbon atoms on the surface as well as within the HCF 1 and 2 films may have been paired with hydrogen atoms. It is also conceivable that some unbonded hydrogen (in both atomic and molecular form) may have been accommodated as interstitials among carbon atoms. Additionally, at least on the surface, di-hydrated carbon atoms are feasible [32]. As discussed above, the elimination of free bonds on sliding DLC surfaces may be an essential requirement for achieving low friction coefficients. The presence of di-hydrated carbon atoms on DLC surfaces can certainly provide an even higher degree of chemical passivation for surface carbon atoms and thus lower friction. Consistent with a full hydrogen termination scenario, the friction coefficients of HCF 1 and 2 films are extremely low and very steady, especially in the long vacuum runs shown in Fig. 5. As shown in Figs. 3 and 4, introduction of hydrogen and oxygen into the test chamber has a small effect on the frictional behavior of the HCF films. Also, the much smaller effect from gaseous species on HCF 1 and 2 friction compared to that of ta-C fits with the description of a fully terminated surface. If the surface bonds are saturated, only small effects are anticipated from adsorbed molecules. The slight increase in friction of HFC films in oxygen and hydrogen can be attributed to some physical and/or chemical interactions triggered by these species at the sliding contact interface. Note that the oxygen pressure in the oxygen experiments is about one-fifteenth of that in a standard atmosphere, which means that conclusions on the effect from oxygen in a normal atmosphere should not be drawn from these experiments. Rather, this is a comparison on the relative level of friction moderation from water, oxygen and hydrogen. With friction data only, it is of course impossible to draw far-reaching conclusions on mechanisms that may also include possible effects of third bodies and/or transfer layers present on the sliding surfaces. It is certainly possible to produce third bodies through the wear process, but whether they remained within the contact zone or were pushed aside or out of the contact zone was not clear in our experiments. Coatings being applied on both sliding surfaces, it is rather hard to imagine the formation of such layers on either or both sides. Earlier tests in dry N2 could not verify the presence of such films on sliding contact surfaces [1–3]. The contact surfaces were rather clean, and most of the third body particles were accumulated around the edges of wear scars and tracks. Nonetheless, the “true” friction of these carbon coatings is reported, i.e. the observed friction is not a function of a mechanism connected with a dissimilar counter face material. A recent study by Donnet et al. [33] on steel rubbing against a HCF film revealed that the presence of oxygen and water had different effects on friction. Specifically, they saw a strong effect on friction from water at high pressures, but
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none from oxygen. They attributed such disparate behavior to the formation of a thinner transfer layer on the uncoated steel ball surface in the case of water (i.e. water hinders transfer film buildup) but relatively thicker transfer layer in the case of oxygen. So, it seems transfer layers and/or third bodies are more relevant to and important for sliding pairs with carbon coating in one side only.
5. Conclusions The test results of this study confirm that the presence or absence of environmental species in the test chamber significantly affects the friction and wear performance of ta-C as well as HCF. Specifically, ta-C films exhibit very high friction during sliding in high vacuum, whereas the films grown in highly hydrogenated plasmas provide ultra-low friction. The friction of ta-C was moderated by 1300 Pa of hydrogen, oxygen and water respectively. Water was the strongest moderator of ta-C friction. The friction of the hydrogenated carbon films in this experiment was moderated to a small extent by the addition of 1300 Pa of water. The presence of hydrogen or oxygen had somewhat adverse effects on their friction. Overall, water seems to have the strongest effect on friction of ta-C and HCF. Mechanistically, we proposed strong covalent bond interactions for the high friction behavior of ta-C, a reduction of this effect with added gases, and the near elimination of such interactions by hydrogen in the case of HCF films.
Acknowledgements The Office of Transportation Technologies of the US Department of Energy under contract W-31-109-Eng.-38 provided support for this work. The international collaboration was facilitated by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and the Swedish Research Council (VR).
References [1] A. Erdemir, O.L. Eryilmaz, I.B. Nilufer, G.R. Fenske, Synthesis of super-low friction carbon films from highly hydrogenated methane plasmas, Surf. Coat. Technol. 133–134 (2000) 448–454. [2] A. Erdemir, O.L. Eryilmaz, G.R. Fenske, Synthesis of diamond-like carbon films with super-low friction and wear properties, J. Vac. Sci. Technol. A 18 (4) (2000) 1987–1992. [3] A. Erdemir, Super-low friction carbon films from hydrogen-rich hydrocarbon plasmas, in: Proceedings of the 6th Applied Diamond Conference and 2nd Frontier Carbon Technology Joint Conference, 2001, p. 437. [4] P.D. Fleischauer, Tribology in the space environment, new directions in tribology, in: I. Hutchings (Ed.), Proceedings of the 1st World Tribology Congress, ImechE Publishers, 1997, pp. 217–227.
[5] J.S. Zabinski, M.S. Donley, S.V. Prasad, N.T. McDevitt, Synthesis and characterization of tungsten disulphide films grown by pulsed-laser deposition, J. Mater. Sci. 29 (1994) 4834–4839. [6] J.S. Zabinski, M.S. Donley, S.D. Walck, T.R. Schneider, N.T. McDevitt, Effects of dopants on the chemistry and tribology of sputter deposited MoS2 films, Tribol. Trans. 38 (1995) 894– 904. [7] A. Erdemir, R.A. Erck, J. Robles, The relation of Hertzian contact pressure to friction behavior of self-lubricating boric acid films, Surf. Coat. Technol. 49 (1991) 435–438. [8] A. Erdemir, C. Bindal, C. Zuiiker, E. Savrun, Tribology of naturally-occurring boric acid films on boron carbide, Surf. Coat. Technol. 86–87 (1996) 507–510. [9] M. Chen, K. Kato, K. Adachi, The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3 N4 and SiC under water lubrication, Tribol. Int. 35 (2002) 129–135. [10] K. Kato, Water lubrication of ceramics, in: Proceedings of the 2nd World Tribology Congress, Austrian Tribology Society, 2001, pp. 51–58. [11] I. Sugimoto, S. Miyake, Oriented hydrocarbons transferred from a high-performance lubricative amorphous C–H–Si film during sliding in a vacuum, Appl. Phys. Lett. 56 (19) (1990) 1868– 1870. [12] C. Donnet, J. Fontaine, T. Le Mogne, M. Belin, C. Heau, J.P. Terrat, F. Vaux, G. Pont, Diamond-like carbon-based functionally gradient coatings for space tribology, Surf. Coat. Technol. 121 (1999) 548– 554. [13] A. Erdemir, O.L. Eryilmaz, Y.B. Nilufer, G.R. Fenske, Effect of source gas chemistry on tribological performance of diamond-like carbon films, Diamond Relat. Mater. 9 (2000) 632–637. [14] C. Donnet, A. Grill, Friction control of diamond-like carbon coatings, Surf. Coat. Technol. 94–95 (1997) 456–462. [15] N. Umehara, M. Tatsuno, K. Kato, Nitrogen lubricated sliding between CNx coatings and ceramic balls, in: Proceedings of the International Tribology Conference, Nagasaki, Japan, Japanese Society of Tribology, 2000, pp. 1007–1012. [16] A. Grill, Diamond-like carbon: state of the art, Diamond Relat. Mater. 8 (1999) 428–434. [17] H. Ronkainen, S. Varjus, J. Koskinen, K. Holmberg, Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings, Wear 249 (2001) 260–266. [18] A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, Nanocomposite tribological coatings for aerospace applications, Surf. Coat. Technol. 116–119 (1999) 36–45. [19] R.H. Horsfall, Commercial applications using non-hydrogenated carbon films for industrial uses such as cutting tools and wear components, in: Proceedings of the 41st Annual Technical Conference, Society of Vacuum Coaters, Boston, MA, USA, 18–23 April 1998, Society of Vacuum Coaters, Albuquerque, NM, USA, pp. 60–65. [20] B.D. Strom, D.B. Bogy, C.S. Bhatia, B. Bhushan, Tribochemical effects of various gases and water-vapor on thin-film magnetic disks with carbon overcoats, J. Tribol. 113 (4) (1991) 689–693. [21] A.G. Ramirez, M.A. Kelly, D.B. Strom, R.G. Walmsley, Carbon-coated sliders and their effect on carbon oxidation wear, Tribol. Trans. 39 (3) (1996) 710–714. [22] J.A. Heimberg, K.J. Wahl, I.L. Singer, A. Erdemir, Super-low friction behavior of diamond-like carbon coatings: time and speed effects, Appl. Phys. Lett. 78 (2001) 2449–2451. [23] A. Erdemir, M. Switala, R. Wei, P. Wilbur, A tribological investigation of the graphite-to-diamond-like behavior of amorphous carbon films ion beam deposited on ceramic substrates, Surf. Coat. Technol. 50 (1991) 17–23. [24] F.P. Bowden, J.E. Young, Friction of diamond, graphite, and carbon and the influence of surface films, Proc. R. Soc. London, Ser. A 208 (1951) 444–455.
J. Andersson et al. / Wear 254 (2003) 1070–1075 [25] F.P. Bowden, A.E. Hanwell, The friction of clean crystal surfaces, Proc. R. Soc. London, Ser. A 295 (1966) 233–243. [26] D. Tabor, in: J.E. Field (Ed.), The Properties of Diamond, Academic Press, 1979, p. 325. [27] M.N. Gardos, Tribological fundamentals of polycrystalline diamond films, Surf. Coat. Technol. 113 (1999) 183–200. [28] M. Dugger, D. Peebles, L. Pope, Counterface material and ambient atmosphere, in: Y.W. Chung, A.M. Homolo, G.B. Street (Eds.), Surface Science Investigations in Tribology: Experimental Approaches, ACS Symposium Series 485, American Chemical Society, Washington, DC, 1992, p. 72. [29] S.V. Pepper, Effect of electronic structure of the diamond surface on the strength of the diamond–metal interface, J. Vac. Sci. Technol. 20 (1982) 643–646.
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[30] D.S. Kim, T.E. Fischer, B. Gallois, The effects of oxygen and humidity on friction and wear of diamond-like carbon films, Surf. Coat. Technol. 49 (1991) 537–542. [31] Y. Liu, A. Erdemir, E.I. Meletis, Influence of environmental parameters on the frictional behavior of DLC coatings, Surf. Coat. Technol. 94–95 (1997) 463–468. [32] A. Erdemir, The role of hydrogen in tribological properties of diamond-like carbon films, Surf. Coat. Technol. 146–147 (2001) 292–297. [33] C. Donnet, T. Le Mogne, L. Ponsonnet, M. Belin, A. Grill, V. Patel, C. Jahnes, The respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings, Tribol. Lett. 4 (1998) 259–265.
Friction of diamond-like carbon films in different atmospheres J. Andersson∗ , R.A. Erck, A. Erdemir Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA
Abstract Diamond-like carbon (DLC) films constitute a class of new materials with a wide range of compositions, properties, and performance. In particular, the tribological properties of these films are rather intriguing and can be strongly influenced by the test conditions and environment. In this paper, a series of model experiments are performed in high vacuum and with various added gases to elucidate the influence of different test environments on the tribological behavior of three DLC films. Specifically, the behavior of a hydrogen-free film produced by a cathodic arc process and two highly hydrogenated films produced by plasma-enhanced chemical-vapor deposition were studied. Flats and balls used in these experiments were coated with DLC and tested in a pin-on-disc machine under a load of 1 N and at constant rotational frequency. With a low background pressure, in the 10−6 Pa range, the highly hydrogenated films exhibited a friction coefficient of less than 0.01, whereas the hydrogen-free film gave a friction coefficient of approximately 0.6. Adding oxygen or hydrogen to the experimental environment changed the friction to some extent. However, admission of water vapor into the test chamber caused large changes: the friction coefficient decreased drastically for the hydrogen-free DLC film, whereas it increased slightly for one of the highly hydrogenated films. These results indicate that water molecules play a prominent role in the frictional behavior of DLC films—most notably for hydrogen-free films but also for highly hydrogenated films. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Friction; Tetrahedral amorphous carbon; Diamond-like carbon; Hydrogen; Water; Vacuum
1. Introduction Superlubricity and wearless sliding under dry sliding conditions are often desired for many moving mechanical assemblies (MMAs: be they MEMS devices, spacemechanism assemblies, or ordinary journal bearings) but unfortunately seldom achieved. As one could envision, the implication of near frictionless and wearless sliding for MMAs is enormous. It means reduced maintenance, increased reliability, and lowered energy consumption along with longer wear lives in mechanical components. In this paper, we will use a carbon-based film with the capacity of nearly frictionless and wearless sliding in vacuum. Earlier experiments in inert gas environments have confirmed that this film is capable of affording friction and wear coefficients in the ranges 0.001–0.005 and 10−11 to 10−10 mm3 /Nm, respectively, to sliding steel and ceramic interfaces [1–3]. There exist a few other materials and coatings that can provide super-low friction. For example, MoS2 and WS2 can give friction coefficients less than 0.01, but not in the presence of any appreciable amounts of water. Hence, their ∗ Corresponding author. Present address: Tribomaterials Group, Angstrom Laboratory, P.O. Box 534, SE-751 21 Uppsala, Sweden. Tel.: +46-18-471-3077; fax: +46-18-471-3572. E-mail address: [email protected] (J. Andersson).
use in moist atmospheres for an extended period of time is almost impossible. These materials are nonetheless used in some aerospace applications, i.e. rolling element bearings, sliding bearings and actuator gears, etc. [4]. Another problem is the scratch sensitivity and relatively high abrasive wear-rates of these materials, which puts further restraints on where they can be used [5,6]. Another potentially super-low friction material is boric acid [7,8]. The basis of the low friction mechanism is the reaction of boric oxide with ambient humidity. The use of boric acid as a low friction coating is therefore limited to reasonably humid conditions, precluding its use in vacuum or otherwise dry applications. Kato has recently demonstrated that water lubrication of certain ceramics (such as Si3 N4 and SiC) can lead to superlow friction coefficients. A combination of fluid and tribochemical film formation has been proposed as the controlling mechanism in their cases [9,10]. Tests with oxide-based ceramics did not lead to similar improvements in friction and wear under water lubricated sliding conditions. With recent progress, there are now also diamond-like carbon (DLC) films that exhibit super-low friction [11,12]. This interesting behavior, however, is limited to vacuum and inert atmospheres. The large amounts of hydrogen in these films have been implicated as instrumental for the
0043-1648/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00336-3
J. Andersson et al. / Wear 254 (2003) 1070–1075
super-low friction behavior [13,14]. Recent tribological studies by Umehara et al. on a certain class of carbon nitride films have also yielded friction coefficients less than 0.01 in dry N2 during sliding against Si3 N4 [15]. Another interesting carbon-based material is tetrahedral amorphous carbon (ta-C). So far, not in the super-low friction domain, ta-C can nonetheless exhibit friction coefficients down to 0.08 [16]. Coupled with low wear and very high scratch resistance this is an attractive material from an engineering point of view. However, just like boric acid, the low friction and wear of ta-C has been suggested to be dependent on ambient humidity [17]. Naturally, ultra-low friction regardless of environment is an attractive concept. Various attempts have been made, for example, Voevodin et al. [18] produced a nano-composite of WC/DLC/WS2 , which worked well in vacuum and dry nitrogen and initially in humid air as well. Unfortunately, cycling the different atmospheres gave a progressive increase in friction. With the long-term goal of producing an environmentally benign low friction DLC film, we believe an important step is to understand the relationship between film composition and environment with respect to frictional behavior. In the search for clues on the governing mechanisms, we performed tribological experiments in vacuum, oxygen, hydrogen and water vapor on two kinds of hydrogenated carbon films (HFC) and one hydrogen-free ta-C film. To reduce the interference of transfer films and dissimilar hardness, identical materials were used on both sides of the tribo-couple.
2. Experiments In this study, three different materials were tested; one tetrahedral amorphous carbon film and two hydrogenated carbon films. Approximately 0.5 m ta-C was deposited on AISI H13 steel flats and M50 (9.53 mm in diameter) steel balls using cathodic arc-evaporation. The resulting film is amorphous, mostly sp3 bonded and has very low hydrogen content [19]. The HCF was produced by plasma-enhanced chemical-vapor deposition on similar substrates to a thickness of 1 m. HCF 1 was deposited in a fifty-fifty hydrogen–methane flow rate mixture, whereas the deposition of HCF 2 utilized a 0.75:0.25 ratio [1,2]. These films contained about 39 atm.% hydrogen according to hydrogen forward scattering measurements with 2 MeV He2+ ions. The friction force between the coated flat and the similarly coated ball was measured in a standard pin-on-disc set-up situated inside a vacuum chamber. Valves allowed the introduction of high-purity gases and liquids. For each non-vacuum experiment, 1300 Pa of hydrogen, oxygen or water was added to background pressures no higher than 5 × 10−5 Pa. For water, this corresponds to roughly 50% relative humidity at laboratory temperature. The length of the experiments was limited to 100 revolutions, corresponding to 3 m, mainly because the endurance
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lifetime of ta-C in vacuum was just above 100 revolutions. The hydrogenated films could be tested for thousands of revolutions in vacuum without any noticeable wear. A constant rotational frequency of 60 rpm was used and the linear speeds were of the order of 0.07–0.03 m/s. A normal force of 1 N was applied to the ball. To reach ultra-low friction coefficients for the hydrogenated films, a longer run-in than 100 revolutions was necessary and hence the experiments were extended. As the friction force diminished to very small values, the importance of a correct zero level increased. In an attempt to cancel this uncertainty, the direction of revolution was reversed after run-in and the friction coefficient was calculated by taking the mean of the absolute values of the two different friction forces. The wear was qualitatively examined in a light optical microscope. Special care was taken that the coating on the balls were not worn through or had cracked or spalled off during the experiment.
3. Results The frictional behavior of ta-C films differed quite substantially when tested in different atmospheres as shown in Fig. 1. The highest friction, i.e. 0.65, was observed in high vacuum, while the lowest friction, i.e. 0.07 was recorded in the presence of water molecules in the test chamber. In high vacuum, the wear damage on ta-C-coated ball and disk samples was so severe that the length of the experiments was limited to 100 revolutions. Upon the introduction of 1300 Pa of oxygen, hydrogen or water vapor into the test chamber, the friction seemed to level off at approximately 0.25, 0.15 and 0.07, respectively, and the wear damage on films was significantly reduced. As is clear from Fig. 1, introduction of water into the test chamber had the most beneficial effect on friction. It reduced the friction coefficient from 0.65 to 0.07 and effectively eliminated the rather pronounced stick–slip behavior seen during tests in hydrogen and oxygen.
Fig. 1. Run-in frictional behavior of ta-C in different environments.
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Fig. 2. Frictional behavior of ta-C in vacuum subsequent to run-in in respective environment.
When water, hydrogen, or oxygen was pumped out and the sliding tests were repeated on the same tracks, the friction coefficients increased substantially (see Fig. 2). The track previously exposed to hydrogen provided the smallest increase in friction, while those tracks previously exposed to oxygen exhibited much higher friction. In vacuum, both HCF 1 and 2 exhibited friction coefficients below 0.02 within 100 revolutions of the sliding tests (Figs. 3 and 4). With the addition of hydrogen and oxygen, the friction was just slightly higher and the general behavior during run-in was very similar to that in vacuum. The experiments with 1300 Pa of water were the second in the same track. Hence, the tracks were run-in before the shown data was taken, explaining their initially lower friction. Whereas the friction after 100 revolutions in vacuum, hydrogen and oxygen still seems to be decreasing, the friction in water vapor seems to have stabilized, and for HCF 2 at a higher level than that of the other three environments by the end of the experiment. After an extended run-in in vacuum, an ultra-low friction coefficient, about 0.009, was achieved for HCF 1 (Fig. 5). The friction coefficient in vacuum calculated as the mean
Fig. 3. Frictional behavior of HCF 1 in different environments.
Fig. 4. Frictional behavior of HCF 2 in different environments.
Fig. 5. Frictional behavior of HCF films in an extended experiment in vacuum.
from going in both directions was 0.007 for HCF 2. There was no visible wear on the HCF 1 and 2 coated balls, even after these extended vacuum experiments.
4. Discussion Results presented in Figs. 1–4 clearly demonstrate that environmental species have a very pronounced effect on the frictional behavior of ta-C, and also some effect on that of HCF films. These findings are consistent with earlier studies in high vacuum and/or various gases [20–23]. From the results, it is clear that ta-C can only provide low friction in the presence of water, oxygen, or hydrogen molecules in the test chamber. Water has the most beneficial effect in reducing friction. As for HCF films, the presence of water, oxygen and hydrogen molecules only slightly increase their friction coefficients (see Figs. 3 and 4). The initially lower friction of the HCF surfaces is due to a prior run-in. Apart from the surface roughness effect, covalent interactions may be a major contributor to the frictional force between sliding diamond and/or DLC surfaces. These are
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strong interactions that can lead to very high frictional forces [24–29]. The other forces that can give rise to friction in carbon films are –∗ , van der Waals, electrostatic attractions, and capillary forces. The –∗ interactions may occur at sliding interfaces of carbons with substantial amounts of sp2 bonding, such as graphite and glassy carbon. Capillary forces may result from adsorbed molecular species, such as water, but they are not expected to play any significant role for the friction in vacuum. The very high and un-steady friction exhibited by the ta-C film in vacuum can be explained as follows. The surface carbon atoms in these films are believed to be bonded to their near neighbors with three bonds, but the fourth bond may be free and dangling out of the surface. In open air, these free bonds are normally terminated or passivated by such adsorbates as water molecules, oxygen, hydrogen, etc. Some of these species are bonded to carbon atoms physically, but others may have been bonded chemically. When such surfaces are placed in high vacuum, some of the adsorbates (especially those bonded physically) are desorbed, while those species bonded chemically may still remain on the surface. When a sliding contact experiment is performed on such surfaces in vacuum, species adsorbed on the top surface are removed by mechanical wear and/or thermal desorption (due to frictional heating); hence, the strong bonds of surface carbon atoms are freed and available for covalent bond interactions with atoms in the other side of the sliding interface. Such interactions can cause strong adhesion and hence high friction. Therefore, the very high friction coefficient in high vacuum (see Fig. 1) may be due to strong bond interactions at the sliding interfaces. When oxygen, hydrogen, and water molecules were introduced in the test chamber, the friction coefficient dropped significantly. One logical explanation for this is that the bonds of surface carbon atoms of this film were passivated by the introduced molecules and hence the extent of covalent bond interactions became less pronounced. Even if some of the bonds may have been exposed due to wear and/or thermal desorption, they are rapidly repassivated by adsorbing molecules. As a result, the friction coefficient of the hydrogen-free DLC film became relatively low (i.e. 0.07–0.25). This finding is consistent with previous experimental studies, which showed that surface adsorbates (such as water molecules, oxygen, etc.) could have significant influence on the friction and wear properties of DLC films [30,31]. In support of the explanation provided above, upon removal of water, oxygen, and hydrogen from the test chamber, the friction coefficient of ta-C increase substantially as shown in Fig. 2. It is interesting to see that the increase in friction is more gradual and relatively small for the wear track that was exposed to the hydrogen environment, while that for oxygen is more rapid and the largest. Such a disparity could be attributed to the differences in the bond strengths
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between carbon and these two species (i.e. C–H bond energy of 413 kJ/mol versus C–O bond energy of 358 kJ/mol). Because of the high intensity hydrogen-rich plasma, most of the carbon atoms on the surface as well as within the HCF 1 and 2 films may have been paired with hydrogen atoms. It is also conceivable that some unbonded hydrogen (in both atomic and molecular form) may have been accommodated as interstitials among carbon atoms. Additionally, at least on the surface, di-hydrated carbon atoms are feasible [32]. As discussed above, the elimination of free bonds on sliding DLC surfaces may be an essential requirement for achieving low friction coefficients. The presence of di-hydrated carbon atoms on DLC surfaces can certainly provide an even higher degree of chemical passivation for surface carbon atoms and thus lower friction. Consistent with a full hydrogen termination scenario, the friction coefficients of HCF 1 and 2 films are extremely low and very steady, especially in the long vacuum runs shown in Fig. 5. As shown in Figs. 3 and 4, introduction of hydrogen and oxygen into the test chamber has a small effect on the frictional behavior of the HCF films. Also, the much smaller effect from gaseous species on HCF 1 and 2 friction compared to that of ta-C fits with the description of a fully terminated surface. If the surface bonds are saturated, only small effects are anticipated from adsorbed molecules. The slight increase in friction of HFC films in oxygen and hydrogen can be attributed to some physical and/or chemical interactions triggered by these species at the sliding contact interface. Note that the oxygen pressure in the oxygen experiments is about one-fifteenth of that in a standard atmosphere, which means that conclusions on the effect from oxygen in a normal atmosphere should not be drawn from these experiments. Rather, this is a comparison on the relative level of friction moderation from water, oxygen and hydrogen. With friction data only, it is of course impossible to draw far-reaching conclusions on mechanisms that may also include possible effects of third bodies and/or transfer layers present on the sliding surfaces. It is certainly possible to produce third bodies through the wear process, but whether they remained within the contact zone or were pushed aside or out of the contact zone was not clear in our experiments. Coatings being applied on both sliding surfaces, it is rather hard to imagine the formation of such layers on either or both sides. Earlier tests in dry N2 could not verify the presence of such films on sliding contact surfaces [1–3]. The contact surfaces were rather clean, and most of the third body particles were accumulated around the edges of wear scars and tracks. Nonetheless, the “true” friction of these carbon coatings is reported, i.e. the observed friction is not a function of a mechanism connected with a dissimilar counter face material. A recent study by Donnet et al. [33] on steel rubbing against a HCF film revealed that the presence of oxygen and water had different effects on friction. Specifically, they saw a strong effect on friction from water at high pressures, but
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none from oxygen. They attributed such disparate behavior to the formation of a thinner transfer layer on the uncoated steel ball surface in the case of water (i.e. water hinders transfer film buildup) but relatively thicker transfer layer in the case of oxygen. So, it seems transfer layers and/or third bodies are more relevant to and important for sliding pairs with carbon coating in one side only.
5. Conclusions The test results of this study confirm that the presence or absence of environmental species in the test chamber significantly affects the friction and wear performance of ta-C as well as HCF. Specifically, ta-C films exhibit very high friction during sliding in high vacuum, whereas the films grown in highly hydrogenated plasmas provide ultra-low friction. The friction of ta-C was moderated by 1300 Pa of hydrogen, oxygen and water respectively. Water was the strongest moderator of ta-C friction. The friction of the hydrogenated carbon films in this experiment was moderated to a small extent by the addition of 1300 Pa of water. The presence of hydrogen or oxygen had somewhat adverse effects on their friction. Overall, water seems to have the strongest effect on friction of ta-C and HCF. Mechanistically, we proposed strong covalent bond interactions for the high friction behavior of ta-C, a reduction of this effect with added gases, and the near elimination of such interactions by hydrogen in the case of HCF films.
Acknowledgements The Office of Transportation Technologies of the US Department of Energy under contract W-31-109-Eng.-38 provided support for this work. The international collaboration was facilitated by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and the Swedish Research Council (VR).
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