Microtribological properties of silicon and silicon coated with self-assembled monolayers: effect of applied load and sliding velocity

Abstract

This paper investigates the influence of the applied load and sliding velocity on the microfrictional properties of native oxide-covered Si(100) and Si(100) coated with octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs) using a precision microtribometer. Microfriction was investigated as a function of the applied load and sliding velocity. As has been confirmed in earlier studies, in the microtribological regime, OTS and FDTS significantly reduce the friction force in comparison to the bare native oxide-covered (hydrophilic) silicon surface. The friction versus applied load curve of the substrate material as well as the SAMs-covered surfaces can be described by a model based on contact mechanics. For the native oxide surface, microfriction is reduced with increasing sliding speed. The friction force of the OTS- and FDTS-covered surfaces increases with load and is proportional to the natural logarithm of sliding speed. The increase with sliding velocity gets larger for higher normal loads. It can be shown that this increase is proportional to the contact area of the counter sample with the SAMs.