Micro/nano-tribological behaviors of crown-type phosphate ionic liquid ultrathin films on self-assembled monolayer modified silicon

A series of novel imidazolium-based crown-type phosphate ionic liquid ultrathin films were fabricated on silicon substrates modified by a self-assembled monolayer. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nano-tribological behaviors of the films were tested by a homemade colloidal probe, and a ball-on-plate tribometer was used to evaluate their micro-tribological performances in the reciprocating mode. As a result, the dual-layer films show enhanced micro/nano-tribological properties as compared to single-component ionic liquid films coated directly on the silicon surfaces, which is ascribed to synergic effect of the steady self-assembled underlayer as a load-carrying phase and the proper amount of flowable ionic liquid fraction with self-replenishment property. Hopefully, the novel dual-layer ultrathin films based on fluorine-free ionic liquids and self-assembled monolayer produced in the present work might find promising applications in the lubrication of micro-electromechanical systems.