Influence of Surface Roughness on the Tribological Behavior of Gear Oils in Steel-Steel Contacts: Part I-Boundary Friction Properties

Improving knowledge of the tribological behavior of gear oils under different operating conditions is essential to be able to design efficient lubricants for gear contacts. This article examines how surface roughness influences the friction properties of gear oils in boundary-lubricated steel-steel contacts at different temperatures and contact pressures. Stribeck curves were generated for smoother and rougher substrates. Friction in the boundary lubrication regime from Stribeck curves and the evolution of friction during prolonged rubbing were used to evaluate the abilities of additives to adsorb on steel surfaces, form tribofilms, and withstand severe operating conditions. In general, smoother substrates produce lower friction but higher wear than rougher substrates for the oils and operating conditions investigated in this study. Surface roughness has significant effects on additive adsorption and tribofilm formation at low contact pressures and negligible effects at high contact pressures. In addition, whether or not other surface-active additives present in gear oils affect the typical low friction response of friction modifiers (FMs) was investigated. The results show that severe operating conditions (pure sliding and high contact pressure) could cause disruption of FM films with some gear oils and hence do not reduce friction. In a forthcoming companion article, Part II, these Stribeck curves are used to understand the response of different gear oils in mixed lubrication with varying substrate roughnesses and to understand how tribofilms influence friction in mixed lubrication by evaluating tribofilm-generated surfaces with base oil (Vengudusamy, etal. (1)).