Friction mechanisms in hydrogenated amorphous carbon coatings

DIAMOND ELSEVIER

Diamond and Related Materials 4 (1995) 1267-1270

Friction mechanisms in hydrogenated

amorphous carbon coatings

H. Mohrbacher, J.-P. Celis Department of Metallurgy and Materials Engineering (MTM),

Katholieke Uniuersiteit Leuven, de Croylaan 2, B-3001 Leuven, Belgium

Received 1 February 1995; accepted in final form 19 June 1995

Abstract

The friction behaviour of unlubricated contacts between a hydrogenated amorphous carbon coating and corundum counterbodies was investigated under reciprocating sliding conditions in ambient air. Tests were performed under various normal forces at controlled relative humidity. The coefficient of friction is shown to decrease as the normal contact force increases. At low (5%) and medium (50%) relative humidities, linear correlations between the coefficient of friction and the inverse of the hertzian contact pressure were observed. A physical interpretation for this behaviour is proposed by considering interfacial shear mechanisms accommodating the sliding motion. Keywords: Amorphous

hydrogenated

carbon; Friction mechanism; Interfacial shear strength; Humidity effects

Hard amorphous carbon coatings have attracted considerable interest for mechanical applications because of their favourable tribological properties. The friction behaviour of hydrogenated amorphous carbon (a-C:H) coatings has been characterized in sliding tests against various counterbody materials over recent years [l-6]. The general conclusion of these measurements was that the coefficient of friction has low values (below 0.2) in humid atmosphere but attains very low values (below 0.05) in dry air, dry nitrogen, or ultrahigh vacuum. A convincing interpretation for this humidity effect in terms of friction mechanisms, however, has not yet been given. Graphitization of the amorphous carbon was occasionally considered as a possible origin of low friction. The fact that graphite must absorb moisture in order to become lubricious [7], however, contradicts the higher friction observed with a-C:H coatings tested in presence of humidity. In some cases, microprobe IR spectroscopy revealed films of hydrocarbon molecules that were transferred to the counterbody [4,8]. It thus seems that the friction behaviour of amorphous carbon coatings is to a large extent controlled by surface chemistry. In this study, the influence of contact pressure and humidity on the friction behaviour of a-C:H coatings subjected to unlubricated sliding against corundum (a-A&O,) was investigated. The a-C:H coating was produced by plasma-assisted chemical vapour deposition on a highly polished flat M2 steel substrate hardened to 0925-9635/95/$09.500 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00306-l

63 HRC [9]. The capacitively coupled r.f. reactor was operated at 13.56 MHz in a CH,-H, atmosphere at a bias of 400 V and a substrate temperature of 150 “C. The a-C:H coating was approximately 2 pm thick and had a surface roughness R, of 0.10 ,um. The hydrogen concentration determined by forward recoil spectroscopy was around 40 at.%. A coating hardness of 11 GPa and a Young’s modulus of around 125 GPa were determined by nanoindentation measurements on the a-C:H coating. High density corundum balls of 10 mm diameter and roughness R, of 0.20 ,um were loaded on top of the as-deposited a-C:H coatings at normal forces between 0.5 and 20 N. These contacts were subjected to reciprocating sliding conditions in an instrumented precision tribometer operated without external lubrication [lo]. The linear displacement stroke of 500 pm was oscillated at a frequency of 10 Hz resulting in an average sliding speed of 10mm s-l. The test duration was 10000 vibration cycles. The relative humidity (RH) in the test environment (air at 23 “C) was kept constant at either 5% or 50% using a climate control chamber [ 111. The tangential force due to friction in the vibrating contact was measured with a piezoelectric force transducer. Forceeclisplacement hysteresis loops as shown in Fig. 1 were recorded at increments of 300 vibration cycles. The ratio of the dissipated friction energy represented by the loop area to twice the sliding distance was taken as the average coefficient of sliding friction [ lo]. The evolution

H. Mohrbacher, J.-P. Celis/Diamond and Related Materials 4 (1995) 1267-1270

friction is calculated as the average value of all data measured after the initial 2000 vibration cycles. The steady state coefficient of friction has the lowest values in dry air in agreement with the previously reported friction behaviour of a-C:H coatings [l-6]. In addition, it decreases as the normal force and, thus, the contact pressure increases. This effect appears to be more pronounced at an RH of 50%. The dependence of friction on the contact pressure was similarly observed on other low friction materials such as MO&, boric acid, and polymeric films and has been explained by a shear mechanism accommodating the sliding motion [ 12-151. In that case, the coefficient of friction ~1is determined by the relationship p = S,/P + a

0

100

200

contact Fig. 1. Normalized 2000 reciprocating (5% RH).

300

400

displacement

(pm)

500

force-displacement hysteresis loops acquired after sliding cycles. The tests were performed in dry air

(1)

in which S, is the interfacial shear strength and P is the contact pressure. The coefficient a represents the pressure coelhcient of the interfacial shear strength. However, the contact pressure in a ball-on-flat contact can be related to the normal contact force F,, by hertzian theory [ 161 so that p = S,7t( 3R/4E*)2’3F, 1’3+ a

of this average coefficient of sliding friction over the test duration is shown in Fig. 2. A running-phase is observed in each test after which the coefficient of friction reaches a nearly constant level. That steady state coefficient of 0.4

0.2

L

* 0.0 % 2

.a, .;

F,:20N

0.4

z 0.3 8

F,:l

with R being the ball radius. The reduced elastic modulus of the ball-on-flat contact defined as 1 1 - vf __L+.-.--_L E” E,

1 - v? (3)

E,

was estimated to be approximately 98 GPa using the elastic moduli of the a-C:H coating (E = 125 GPa, 2,z 0.3) and corundum (E = 310 GPa, v =0.27). This value was experimentally confirmed by measuring the width of wear tracks on the coating which corresponds to the diameter of the hertzian contact circle. The steady state coefficient of friction between the a-C:H coating and corundum is plotted against the inverse of the initial contact pressure in Fig. 3. In this representation, the data can be fitted by three linear relationships whose slopes and intercepts are summarized in Table 1. The slope is interpreted as the shear strength of an interfacial film accommodating the sliding motion. This mechanism seems to be fully active in the steady state after the interfacial film has been generated

0.3

c 0.1 .g 5

(2)

N

0.2

0.1

Table 1 Least squares fit data of the interfacial shear strength S, and its pressure dependence TVin Eq. (1) observed during the steady state friction phase for an a-C : H coating sliding against corundum

z;

0.0

I 0.0

I 0.2

I

I

0.4 vibration

0.6 cycles

Fig. 2. Evolution of the coefficient of friction against corundum. Steady state coefficients averaging the data acquired after 2000 vibration

I 0.8

I 1.o

RH, contact pressure range

S, (MPa)

AS, (MPa)

c(

ACC

92.9 91.1 21.4

f 2.4 f 6.8 * 1.9

-0.08 - 0.23 - 0.02

& 0.01 & 0.03 +_0.02

X104

of a-CH vibrating were evaluated by cycles.

50%, 0.2-0.8 5%, 0.2-0.3 5%, 0.3-0.8

GPa GPa GPa

H. Mohrbacher, J.-P. Celis/Diamond and Related Materials 4 (1995) 1267-1270

0.4

0.3 E J g +? : t;

0.2

0.1

0.0 I 1 inverse

I 2 contact

I 3 pressure

I 4 (1 /GPa)

I 5

Fig. 3. Steady state coefficient of friction vs. the inverse of the initial mean Hertzian pressure. The least-squares fit data are summarized in Table 1.

during the running-in phase of the reciprocating sliding tests. In medium humid air, the interfacial shear strength is around 90 MPa over the whole range of applied contact pressure (0.2-0.8 GPa). A very similar shear strength is observed in dry air below a contact pressure of around 0.3 GPa. However, a much lower interfacial shear strength (below 30 MPa) is established above that contact pressure. This transition must be related to a substantial change in the properties of the interfacial film. The intercept characterizing the pressure dependence of the interfacial shear strength clearly has negative values in humid air and also in dry air below the contact pressure of 0.3 GPa (Table 1). This is in contrast to many other materials where a was found to be positive or close to zero [ 12,14,15,17]. The meaning of a negative pressure dependence is that the interfacial shear strength becomes smaller when the applied normal force increases. The shear strength becomes zero at a sufficiently high contact pressure. This may be interpreted as the collapse of the accommodation mechanism. Essentially, friction must then be controlled by another mechanism. The change in the interfacial shear strength observed during sliding in dry air at 0.3 GPa average contact pressure could be related to this phenomenon. In humid air, the interfacial shear strength is less sensitive to contact pressure. A similar transition in the shear strength could not be observed within the range of contact pressure applied in the reciprocating sliding experiments. The pressure dependence of the low shear strength mechanism is nearly zero. Thus, this mechanisms could accommodate sliding up to very high contact pressure without collapsing.

1269

Shearing of interfacial films can involve different rheological processes accounting for changes in the shear strength [ 181. According to Yoshizawa et al. [ 191, the friction of boundary layers is controlled by their molecular configuration. It was shown that very low friction can be obtained once the molecules are forced into a favourable configuration by the so-called shear ordering [ 19,201. This process is activated above a critical sliding velocity and contact pressure which may depend on environmental conditions such as temperature and RH. Adsorbed water is thereby influencing the mobility of the molecules. The transition in the interfacial shear strength observed at low RH during the present reciprocating sliding experiments between a-C:H and corundum is possibly due to shear ordering of molecules in the interfacial film. Although this interpretation is still hypothetical, polarized IR spectra obtained on a transfer film by Sugimoto and Miyake [8] indeed indicated the alignment of hydrocarbon molecules along the sliding direction after friction tests on amorphous carbon coatings in vacuum. The coefficient of friction that they measured on the shear ordered layer was below 0.01. The significant influence of the humidity on the observed friction behaviour of the present a-C:H coating must be related to the adsorption of water to the interfacial zone. The absence of the transition to a low shear strength in humid air suggests that shear ordering in the interfacial film is obstructed. This could be due to a stabilization of the unordered molecular configuration by adsorbed water molecules. It would also explain the reduced pressure dependence of the interfacial shear strength. Some recent IR studies by other researchers indeed proved that adsorbed water molecules wear present in the wear zone on amorphous carbon coatings after sliding in humid air [4,21]. In conclusion, the friction behaviour of a-C:H coatings in sliding contacts appears to be determined by shear processes occurring in an in situ generated interfacial film. Contact pressure and humidity in the test atmosphere influence the shear strength of that film as well as its pressure dependence. This effect is apparently related to the molecular configuration of the film.

Acknowledgments

Sincere thanks are addressed to Dr. E. Dekempeneer (VITO, Mol, Belgium) for preparing the a-C:H coatings. One of the authors (H.M.) is grateful to the Commission of European Communities for providing a Human Capital and Mobility Fellowship. This work was partly financed by a BRITE/EURAM project (BREZ CT92-0224) and presents results obtained within the programme on Interuniversity Poles of Attraction initiated by the Belgian state.

1270

H. Mohrbacher, J.-P. CelisJDiamond and Related Materials 4 (1995) 1267-1270

References Cl1 K. Enke, H. Dimigen and H. Htibsch, Appl. Phys. Lett., 36 (1980) 291. c21 R. Memming, H.J. Tolle and P.E. Wierenga, Thin Solid Films, 143(1986)31. c31 A. Grill, V. Pate1 and B. Meyerson, Surf. Coat. Technol., 49 (1991) 530. c41 D.S. Kim, T.E. Fischer and B. Gallois, Surf. Coat. Technol., 49 (1991) 537. c51 K. Miyoshi, R.L.C. Wu and A. Garscadden, Surf. Coat. Technol., 54-55 (1992), 428. C61D. Klaffke, Diamond Films Technol. 3 (1994) 149. c71 R.H. Savage, J. Appl. Phys., 19 (1948), 1. CSI I. Sugimoto and S. Miyake, Appl. Phys. Lett.. 56 (1990) 1868. c91 E. Dekempeneer, R. Jacobs, J. Smeets, J. Meneve, L. Eersels, B. Blanpain, J. Roos and D.J. Oostra, Thin Solid Films, 217 (1992) 56. Cl01 H. Mohrbacher, J.P. Celis and J.R. Roos, Tribal. Ink, (1994) in press.

[ 111 H. Mohrbacher, B. Blanpain, J.P. Celis and J.R. Roos, Wear, I80 (1994) 43. [12] I.L. Singer, R.N. Bolster, J. Wegand, S. Fayeulle and B.C. Stupp, Appl. Phys. Lett., 57 (1990) 995. [13] A. Erdemir, R.A. Erck and J. Robles, Surf. Coat. Technol., 49 (1991) 435. [ 141 R.C. Bowers, J. Appl. Phys., 42 (1971) 4961. [lS] B.J. Briscoe, B. Scruton and F.R. Willis, Proc. R. Sot. London, Ser. A, 333 (1973) 99. [16] K.L. Johnson, Proc. R. Sot. London, Ser. A, 230 (1955) 531. [17] B.J. Briscoe, p. 167. [ 181 I.L. Singer, in I.L. Singer and H.M. Pollock, (eds.) Fundamentals of Friction: Macroscopic and Microscopic Processes, NATO AS1 Series E, Vol. 220, Kluwer, Dordrecht, 1991, pp. 237-260. [19] H. Yoshizawa, Y.-L. Chen and J. Israelachvilli, Wear, 168 (1993) 161. [20]

H. Yoshizawa, P.M. McGuiggan and J. Israelachvilli, Science, 259 (1993) 1305. [21] K. Oguri and T. Arai, J. Mater. Res., 7 (1992) 1313.