Friction anisotropy at Ni„100...Õ„100... interfaces: Molecular dynamics studies

The friction of surfaces moving relative to each other must derive from the atomic interaction at interfaces. However, recent experiments bring into question the fundamental understanding of this phenomenon. The analytic theories predict that most perfect clean incommensurate interfaces would produce no static friction, whereas commensurate aligned surfaces would have very high friction. In contrast recent experiments show that the static friction coefficient between clean but 45° misoriented Ni~001! surfaces is only a factor of 4 smaller than for the aligned surfaces (u;0°) and clearly does not vanish ~u is defined as the rotation angle between the relative crystallographic orientations of two parallel surfaces!. To understand this friction anisotropy and the difference between analytic theory and experiment, we carried out a series of nonequilibrium molecular dynamics simulations at 300 K for sliding of Ni~001!/Ni~001! interfaces under a constant shear force. Our molecular dynamics calculations on interfaces with the top layer roughed ~and rms roughness of 0.8 Å! lead to the static frictional coefficients in good agreement with the corresponding experimental data. On the other hand, perfect smooth surfaces ~rms roughness of 0 Å! lead to a factor of 34–330 decreasing of static friction coefficients for misaligned surfaces, a result more consistent with the analytic theories. This shows that the major source of the discrepancy is that small amounts of roughness dramatically increase the friction on incommensurate surfaces, so that misaligned directions are comparable to aligned directions.