Zinc-dialkyl-dithiophosphate antiwear films: Dependence on contact pressure and sliding speed

Wear tests were performed to map effects of load and sliding speed on formation and tribological properties of zinc-dialkyl-dithiophosphate (ZDDP) antiwear tribofilms formed on cast iron in a 1.2 wt.% ZDDP model lubricant oil using a high frequency friction machine. Wear film formation was characterized by monitoring the evolution of coefficient of friction (μ), power dissipation, induction period for film growth, and final film thickness. The wear surface was subsequently analyzed using X-ray absorption near-edge structure (XANES) spectroscopy and AFM. A polyphosphate tribofilm of thickness 10-100 nm, depending upon load or rubbing speed, was formed on the wear scar. Three stages of film formation were observed. Stage 1 was an induction period during which tribofilm was nucleated, where μ usually first decreased, then increased, and finally stabilized. Stage 2 consisted of tribofilm build up characterized by a constant μ with increasing film thickness. Stage 3 was characterized by competition between continued film formation and wear. The coefficient of friction, power dissipation, induction period, and final film thickness are found to depend sensitively on average sliding velocity and contact pressure. The tribofilm thickness is found to be inversely correlated with lubricant film thickness. ZDDP principally decomposes via thermo-oxidative processes and interfacial contact.