Assessing friction, wear and film formation characteristics in formulated lubricants in severe to moderate boundary lubrication conditions

The ZDDP additive has been the subject of much of tribological/tribochemical research over the last few decades. Most studies have investigated the effect of ZDDP in simple ZDDP/base oil systems and in some cases in ZDDP/MoDTC/detergent combinations. From these studies useful information has been gained to help in the understanding of the mechanism of ZDDP performance. However, in real lubricant formulations there can be 10-15 additives and the interactions between those additives should be taken into account. In this paper the effects of eliminating the friction modifier and friction modifier plus anti-wear additive ZDDP from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics are examined. Tests have been conducted under lubricated wear conditions at relatively low (20°C and 50°C) and elevated (up to 100°C) bulk oil temperatures using a reciprocating pin-on-plate tribometer. The wear film has been examined by Energy Dispersive X-Ray analysis (EDX) and X-Ray Photoelectron Spectroscopy (XPS). In this work two fully formulated lubricants containing the same dispersant/detergent system have been used. One lubricant contains no antiwear (ZDDP) additive and no friction modifier (referred to as no AW oil) and the other contains ZDDP but no friction modifier (ZDDP oil). It has been shown that with both oils the friction coefficient increases with increasing temperature. In the fully formulated lubricants with no ZDDP the friction coefficient is lower than with ZDDP when tested in conditions where the starting lambda is much smaller than 1. With higher starting lambda values of around 1 the trend reverses. In all cases, as expected, the wear when ZDDP is present is lower. The nature of the film formed in the presence and absence of ZDDP confirms the competition between detergents and Zn-containing additives in film formation.