Studies of tribological behavior and separation between surfaces at initial boundary lubrication

The tribological behavior and separation of steel surfaces during sliding at initial boundary lubrication were investigated over a wide range of roughness values. Tests were performed on a disk-on-block machine in an oil bath. The friction coefficient and electrical contact resistance of the interface during step load tests were monitored in situ. The experimental results show that wear increases suddenly at the transition from mixed to boundary lubrication. After this period in the wear process, an increase in the contact load does not induce an increase in the wear volume. The friction coefficient at initial boundary lubrication increases to reach a stable value as the roughness value of block specimen increases. A rather wide range in friction coefficients at the initial boundary lubrication indicates that the surface roughness and sliding speed notably affect the variation of the transition friction coefficient from mixed to boundary lubrication. Using the microcontact model for rough surfaces as a base, a modified formula h/(2.0 sigma(1.39)) less than or equal to 1.0 is developed to analyze the relationship between separation and composite r.m.s. roughness at initial boundary lubrication and to distinguish the onset of boundary lubrication. (C) 1998 Elsevier Science S.A. All rights reserved.