Atomic scale frictional aging in silicon carbide due to diffusion and creep

Static friction is the force required to impose sliding on a rested body. The force depends on material properties and external factors such as normal pressure and temperature, but also a time dependent component is important. The frictional aging effect is at origination of the difference between static and dynamic friction, and is also believed to be responsible for the velocity-weakening of sliding friction. Despite immense effort, how microscopic processes affect the macroscopic aging is still not fully understood. We have performed molecular dynamics simulations where we demonstrate that high surface diffusion may provoke rapid contact area growth of an asperity-substrate interface, inducing a strong frictional aging effect. This mechanism differs from elastic and plastic creep in the sense that it occurs even at no normal pressure. The growth of contact area was found to be nearly logarithmic due to an exponentially decaying diffusivity. Furthermore, when applying a normal stress the aging effect is enhanced due to plastic creep. Our work suggests a new explanation of the logarithmic nature of aging and helps bridging the gap between empirical macroscale friction laws and the microscale behavior. While aging due to plastic and elastic creep is well-known and incorporated into most friction laws, diffusion aging has yet to be considered. The ultimate goal is to design or redesign friction laws taking the microscopic behavior into account and conceivably improve the accuracy of the laws. In the long term, this may contribute to improved earthquake forecast.