Comparison of the tribological behavior of steel–steel and Si3N4–steel contacts in lubricants with ZDDP or ionic liquid

Abstract

Tribological evaluations were conducted on lubricating base oils of different viscosity grades with and without an anti-wear (AW) additive in lubricating steel-steel and ceramic-steel contacts. Two AW additives were applied: a conventional secondary zinc dialkyldithiophosphate (ZDDP) and an oil-miscible phosphonium-phosphate ionic liquid (IL). Tests were carried out using reciprocating ball-on-flat sliding at room temperature. The flat material was AISI A2 tool steel, and the ball material was either AISI 52100 bearing steel or silicon nitride. Four lubricants were tested: Chevron SAE 15W40 and 0W30 base oils, and the 0W30 base oil plus 1 wt% ZDDP or IL. For the steel–steel contact, the lower-viscosity 0W30 base oil resulted in a higher wear rate than did the 15W40 base oil, as expected. Both the ZDDP and the IL substantially reduced wear, and the IL-additized 0W30 base oil was the best performer. For the ceramic–steel contact, the ZDDP provided moderate wear protection for both ball and flat. In contrast, the IL reduced the steel flat wear more effectively but increased the ceramic ball wear rate. Cross-sectional transmission electron microscopy (TEM) examination and x-ray photoelectron spectroscopy (XPS) chemical analysis were used to reveal the thickness, nanostructure, and composition of the tribofilms formed by ZDDP and IL on the steel and silicon nitride surfaces.