Formation Mechanism of a Low Friction ZDDP Tribofilm on Iron Oxide

In a previous study, an iron oxide layer (mixture of Fe 3 O 4 and FeO) formed by water-vapor treatment on a steel plate resulted in an anomalous low friction (μ < 0.06) when slid against a steel cylinder in a lubricant containing ZDDP. This value is about a half of a steel/steel friction contact under the same condition. The formation of a tribofilm containing zinc and sulfur has been identified on the iron oxide. In this study, the formation mechanism of such tribofilm is discussed. The adsorption characteristics of ZDDP on the iron oxide and steel are investigated experimentally. The adsorption layer on the iron oxide analyzed by XPS is rich in zinc but contains almost no sulfur, while the one on steel includes phosphorous and oxygen, and probably phosphates. The adsorption plays a significant role in increasing the zinc content in the tribofilm on the iron oxide, while other mechanisms such as mechanical mixing are necessary to incorporate sulfur in the tribofilm.