Frictional Response of a Silane Monolayer to Sliding in a Humid Environment

The effect of humidity has been studied on the friction coefficient of a steel ball sliding in reciprocating mode on a self-assembled monolayer of octadecyltrichlorosilane on silicon. At all relative humidities (RH < 70%), the friction coefficient first increases with sliding time, as defects build up in the monolayer, and then subsequently decreases. The deliberate introduction of defects by heat treatment of the monolayer to 130 °C leads to a substantial increase in the friction coefficient in a dry environment but a decrease in a moist environment. It is proposed that the interaction of water adsorbed to the steel probe with the monolayer results in a softening of the monolayer and a decrease in the relaxation times of the processes responsible for energy dissipation in the monolayer in a dry environment. The friction coefficient of a monolayer in a moist environment can reach values of 0.02: lower than the value of 0.04 observed for a pristine monolayer.