Velocity dependence and rest time effect on nanoscale friction of ultrathin films at high sliding velocities

Micro∕nanoelectromechanical system devices and components often operate at high sliding velocities, and it requires the investigation of friction at high velocities. In this study, the velocity dependence of friction and the rest time effect on friction of hard diamondlike carbon films, soft perfluorodecyltrichlorosilane, and perfluoropolyether films were investigated up to $2×105μm∕s$ using an ultrahigh velocity stage and a high velocity stage. The velocity dependence of friction was found to vary with films and involve different mechanisms including adhesion due to solid-solid interaction, adhesion due to formation of meniscus bridges, atomic-scale stick slip, high velocity impact of the contacting asperities∕molecules, phase transformation∕tip jump, and viscoelastic shear. The increase in friction as a function of rest time was caused by continued growth and formation of water menisci bridges for the hydrophilic surfaces and continued deformation of soft films.