Influence of surface roughness and ZDDP additive on the friction and wear of reciprocating sliding surfaces at high contact pressures

Surface roughness has been proved to be influencing the running-in wear of machined components under dry and lubricated sliding conditions. Zinc dialkyl dithiophosphate (ZDDP) is widely used as an anti-wear additive, which reduces the wear by the formation of a tribofilm on the surface (Spikes, 2004). Factors such as temperature, sliding distance, etc. influence the formation of the film. A significant reduction in the power loss due to friction and wear is possible if a synergy is attained between surface roughness effects and the effectiveness of the tribofilm. The present work aims to study the influence of surface roughness and ZDDP addition on the formation and removal of the tribofilm under high contact pressures.

Samples were prepared by machining. Surface roughness was varied by varying the milling parameters. A reciprocating friction and wear test machine with a ball-on-flat geometry was used for the study. Tests were performed with mineral base oil and base oil added with 1 per cent by weight ZDDP under different operating parameters. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) with energy dispersive X-Ray spectroscopy (EDS) analysis were conducted to study the surface morphology of the tribofilm.

A quasi-steady-state analysis conducted showed that the wear rate was much lower when tested with base oil containing ZDDP after about 65 min. AFM analysis confirmed the presence of chemically reacted films on the surface. SEM analysis revealed agglomeration of crystal like glassy phosphates. However, high contact pressures at the interface caused the removal of the films resulting in variations in the coefficient of friction. A comparison of the wear rates among the samples of different roughness values tested at 100°C showed that the anti-wear performance of ZDDP was not effective due to high contact pressures.

The findings in this study regarding the tribofilm formation with ZDDP additive and its failure due to high contact pressures will be beneficial for further investigation on wear resistant boundary films developed under such extreme conditions.