Effect of lubricant on micropitting and wear

Abstract

Micropitting was studied using a three-contact disc machine having a central roller in contact with three harder, annular counter-discs (“rings”) of precisely controlled roughness. Roughness, running conditions, base stock and additive concentration were varied. The response of the same lubricants in a reciprocating sliding wear test operating in the boundary regime was also studied. It was reported in previous work that when a common antiwear additive, secondary zinc dialkyl-dithio-phosphate (ZDDP) is used in solution into a mineral oil, it provides enough protection to the rough surfaces leading to micropitting. In this work, different type of roughness finish, effect of lambda ratio on micropitting, effect of the antiwear concentration, as well as the effect of a friction modifier were studied. Results of experimental studies of the rolling contact behaviour of carburised steel rollers are reported. All the tests with the additive present led to micropitting. This being true for both longitudinal and transverse finish. However, transverse led to most severe case of micropitting wear. For different film thickness, micropitting was observed for various lambda ratio, however, micropitting wear was only observed for a calculated film thickness below 12% of the centre-line average roughness of the rings. It was found that there was an approximately inverse correlation between the micropitting damage in the disc machine test and the mild wear in the reciprocating sliding test. The surface profiles were measured at given intervals during each test and the result confirmed that the nature of the antiwear additive slow-down the run-in of rough surfaces contributing to the onset of micropitting. Indeed, antiwear additives suppress the gradual smoothing of the rough surfaces, which takes place when a pure base stock is used under mild conditions. For cases closer to industry, it is common to use a combination of additives for various reasons. Indeed, antiwear and friction modifier additives are often used in formulated oils, a combination which leads to reduced boundary friction. The effect of the reduction of the friction at the asperity level was studied. A common friction modifier agent, commonly known as molybdenum dithio-carbamate (MoDTC), was added in solution to a mineral oil containing the antiwear additive. It was found that for the oil containing both ZDDP blended with MoDTC the surface of the tested roller exhibited micropitting wear. However, this micropitting wear gradually disappeared with continued running; this was not the case when the mineral oil was tested with ZDDP alone. The optical micrographs of the damaged surfaces showed that the subsurface cracks disappeared on the roller tested with ZDDP and MoDTC. After the test, further surface analysis of the counter-discs showed the presence of MoS2 deposits on the asperity crests. The oil containing the friction modifier was tested using an MTM tribotester and this test confirmed the reduction in friction for a regime close to boundary lubrication. It is speculated that the improved micropitting behaviour resulted from the effect of a reduction in local tensile stress due to reduced asperity friction. This may have reduced the opening of the surface cracks and inhibited their extension.