Size, Load, and Velocity Effect on State-and-Rate Friction at Micro/Nanoscale
A model for size-, load-, and velocity-dependence of friction at micro/nanoscale is proposed. During multiple-asperity contact, friction force is equal to the real area of contact multiplied by the interfacial shear strength. The real area of contact depends upon the normal load and may depend also on the apparent size of contact, due to the self-affinity of the surface, and on the sliding velocity, based on a state-and-rate friction law. The interfacial shear strength depends on the average size of individual asperity contacts a, due to the scale dependence of mechanical properties, while a, in turn, depends on the real area of contact, due to surface topography. Therefore, changing the apparent size of contact, normal load (pressure) and sliding velocity affects the coefficient of friction. The model combines effects of size, load and sliding velocity on friction and considers them in complex.