Effect of atomic-scale surface roughness on friction: a molecular dynamics study of diamond surfaces

We have examined by molecular dynamics simulations the friction which arises when two hydrogen-te~inated diamond (111) surfaces are placed in sliding contact. In particular, the effects of load, crystallographic sliding direction, and substitution of surface hydrogen groups by methyl groups on the friction coefkient p were investigated. For the hydrogen-terminated system, it is possible to have F approximately equal to zero for certain sliding directions, while k increases with increasing load for other sliding directions. However, when methyl groups are substituted for some surface hydrogen atoms p generally increases with increasing load for all sliding directions examined. IO this case, the directional anisotropy of p is less than for the hydrogen-terminated system. As a result of the advances in diamond thin film technology, understanding the tribological properties of these films has become crucial. Previously, studies have been undertaken which examine the friction and wear properties of diamond and diamond films on the macroscopic scale [l-5]. Emerging technologies such asthe atomic force microscope [6] and the surface force a~~a~a~~~ f7f have pioneered the investigation of these processes at the microscopic level. In spite of these efforts, relatively little is known about the mechanism of diamond friction and wear at the atomic level. Computational experiments have also been employed to study friction and associated phenomena [8-141. Friction has been examined with a variety of methods, from simple analytic models [S] to first-principles calculations [9]_ Molecular dynamics (MD) simulations have been used to examine the friction which arises from slippage at a solid-solid interface [IO] and between closed-packed films [ll] in sliding contact. Similarly, we have employed MD calculations to investigate the atomic-scale mechanism of friction when two hydrogenterminated diamond surfaces are placed into sliding contact [123. In that work, we examined the dependence of the friction coefficient on load, crystallographic sliding direction, temperature, and sliding velocity. In this work, we examine the effect of atomic-scale roughness on the friction coefficient as a function of load and crystall~graphic sliding direction. 0043-1648/93/$6.00 MD calculations were carried out by integrating Newton’s equations of motion with a Nordsieck predictor corrector [15] and a constant time step of 0.5 fs. The particIe forces are derived from an empirical hydrocarbon potential that is capable of modeling intramolecular chemical bonding in both diamond and grapbite lattices, as well as in a variety of small hydrocarbon molecules t&S]. The potential used here has been modified to include terms which better describe torsional forces [ST] and short-range repulsive terms which may prove important under high compression [18]. Atomic-scale friction has been investigated by bringing two (1 X 1) hydrogen-terminated diamond (111) surfaces into sliding contact. The effect of atomic-scale surface roughness on the friction process was investigated by replacing surface hydrogen atoms on one of these surfaces with methyl groups. The initial atomic positions for the methyl-terminated system investigated here are shown in Fig. 1. Each individual diamond lattice contains 11 layers of atoms; 10 layers of carbon atoms containing 16 atoms each and one layer which is composed of either hydrogen or a mixture of hydrogen and methyl groups, In Fig. l(a) the lower lattice is terminated on the top with 16 hydrogen atoms. The upper lattice is terminated on the bottom with 14 hydrogen atoms and two methyl groups. This creates an atomically rough interface between the surfaces. Ruring the course of the simulation, the inne~ost 4 layers of the upper and lower lattice