Effects of various base oils and additives on the tribological behaviour of lubricated aluminium- on-aluminium and steel-on-aluminium contacts

Abstract

This study investigates the friction and wear characteristics of various base oils and additives in lubricated aluminium-on-aluminium and steel-on-aluminium contacts. Cylinder-on-plate tribological tests were conducted with a reciprocating tribometer. The rubbed aluminium surfaces were examined using scanning electron microscopy, energy dispersive X-ray analysis, secondary neutral mass spectroscopy (SNMS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) to analyse the chemical compositions and structures of tribofilms. The friction and wear tests revealed that hydrocarbon-based fluids such as mineral oil (MO) or polyalphaolefin (PAO) exhibited better tribological characteristics than polar base fluids such as monoester, polyol ester, and polyalkylene glycol. Lubricant composed of MO and zinc dialkyldithiophosphate (ZDDP) demonstrated lower friction and better antiwear performance in steel-on-aluminium contacts than MO alone or MO with a sulphur/phosphorus (S/P) additive. In an aluminium-on-aluminium system, on the other hand, the S/P additive showed better friction and wear behaviour than ZDDP. From surface analysis, it was found that when using ZDDP, the iron transfer played a very important role for a tribofilm to form on an aluminium surface. Moreover, the SNMS and XPS analyses illustrated that the kinds of tribofilm created by a chemical reaction between aluminium and ZDDP were different from those between aluminium and S/P additive, which resulted in different tribological properties.