Molecular dynamics simulations of friction between alkylsilane monolayers

Alkylsilane monolayer films are used in the fabrication of microelectromechanical systems (MEMS) as anti-stiction coatings. Recently, these films are also being considered as anti-friction coating for MEMS parts that operate under conditions of friction, such as gears, motors, etc. Study of frictional behaviour of these films is of significant interest from the performance point of view. In this work we use the method of molecular dynamics (MD) to study the friction between two rigid silica substrates coated with alkylsilane monolayers. The friction coefficient, friction force and normal force on the films are obtained as a function of separation between the substrates, temperature of the films and velocity of the substrates. The results of simulations are compared with the thermal activation model of Briscoe and Evans (1982 Proc. R. Soc. Lond. A 380 389). The frictional behaviour of the films as a function of separation between the substrates follows the thermal activation model. The MD simulations in the present work show that the normal force (or pressure) on the films is dependent on the temperature and the substrate velocity. This is in contrast to the thermal activation model in which the pressure on the films is assumed to be independent of these variables. The simulation results are seen to be in agreement with a modified thermal activation model which takes into account the dependence of pressure on the temperature and substrate velocity. (Some figures in this article are in colour only in the electronic version)