Boundary lubrication film formation from phosphorus antiwear additives with application to metal V-belt type continuously variable transmission lubricants

Abstract

This study has focused on the application of sulphur-free phosphorus antiwear additives in metal V-belt pushing type continuously variable transmission fluids to achieve higher friction coefficient plus excellent antiwear performance between the metal—metal contacting surfaces. The metal—metal tribological characteristics of phosphorus compounds were experimentally investigated using a reciprocating ball on plate tribometer, enabling film formation to be monitored during reciprocating sliding tests by an electric resistance method. As a result, sample with acid phosphate possessing C8 alkyl chain gave 20 per cent higher friction coefficient between metal contacting interfaces than C18 alkyl acid phosphate. Notably, an additive formulation composed of C8 acid phosphate with shorter alkyl chain, amine compound, and over-based calcium sulphonate demonstrated a synergism with 9 per cent higher friction and better antiwear performance than acid phosphate alone, which would give a positive outcome in the belt and pulley system of the continuously variable transmission. From surface analysis such as optical interferometry and X-ray photoelectron spectroscopy, it is hypothesized that the variation of tribofilm species produced by a chemical reaction between these additives plays a role in reducing corrosive wear and inhibiting the entrainment of lubricant into interfaces, resulting in higher friction. Moreover, the post-test rubbing tracks still possessed a longitudinally oriented surface pattern, which seemed to contribute to a reduced fluid film effect, giving a higher friction coefficient during rubbing tests.