Low-friction tribofilm formed by the reaction of ZDDP on iron oxide

An iron oxide layer (mixture of Fe3O4 and FeO) was formed by water-vapor treatment on tool steel plates. Bearing steel cylinders were slid against the plates in PAO with and without 1% zinc dialkyldithiophosphate (ZDDP) at 80 °C. The friction coefficient was below 0.06-0.08 with ZDDP, which is more than 20% lower than that without ZDDP and about a half of a steel/steel contact under the same condition. The formation of multi-layered tribofilm of 30-130 nm on the iron oxide was identified by TEM. The bottom part of the tribofilm is a layer of 10-30 nm that contains Zn, Fe, S, P, and O with a gradient composition, where distribution peaks of Zn and S were found by EDX.