Influence of slide-to-roll ratio on tribofilm generation

Abstract

International audienceRolling contact fatigue has become a surface initiated problem. This phenomenon is due to the initiation and propagation of cracks from the surface to the volume of mechanical pieces. A great amount of research has been done since the 1970s to understand its origin, but many aspects remain unclear. Many authors found that the lubricant composition and especially certain additives can either improve or reduce fatigue life, depending on the operating conditions. It is now admitted that additives form a tribofilm in the rubbing track, especially under pure sliding conditions, but no direct relation to rolling contact fatigue has been drawn to this moment. The knowledge of the reaction pathways of lubricant additives under various sliding conditions as well as the tribofilm properties and morphology has become an important issue. In this work, the mini traction machine has been used to study friction under different sliding conditions of a pure mineral base oil and a formulated oil containing five selected additives. The results suggest that friction is likely to mimic tribofilm formation. The slide-to-roll ratio influences the rate of direct asperity contacts and hence tribofilm growth

Experimental observation of zinc dialkyl dithiophosphate (ZDDP)-induced iron sulphide formation

“…Alongside base oil, additives also penetrate into the microcracks. Additive interaction with the crack faces has also been shown previously [49,53,54]. However, key understanding of derived from the interaction is missing.…”

“…Interestingly the elemental distribution of the tribofilm in the area in which a crack is initiated (Figure 6) is different from other part of the FIB sample in which no crack was observed ( Figure 5). In contrast to Alongside the iron sulphide formation induced by the interaction of the ZDDP elements and steel substrate, lubricant additives can interact with surface initiated cracks [6,49].…”

Effect of lubricant additives in rolling contact fatigue

“…First, it was necessary to find out if tribofilms are generated before or after crack initiation. Studies on tribofilm generation on a mini traction machine (MTM -PCS Instruments) [15,24] suggest that a tribofilm is fully formed in less than 5 h, which is much shorter than the fatigue life. Meheux et alhave performed tests at 5 per cent SRR (0.89 GPa Hertzian pressure, 80 • C, and 100 mm/s entrainment speed) with the same fully formulated lubricant like the one used in this article.…”

“…This is reported in The Hertzian contact pressure is estimated at 0. 85 GPa reference [15] concerning tests on an MTM machine with the same fully formulated lubricant. The conditions are close to the ones used in this study even if it is difficult to know exactly the increase of contact pressure induced by the dents for TDM experiments.…”

“…For certain contact conditions, tribofilms consisting of additive elements are generated in the contact. Usually, these films are created under pure sliding conditions, but it is also possible for milder SRR ratios [15]. If a tribofilm is generated (around 100 nm thick), it will contribute to an increase of the whole separation of the friction surfaces [16] (tribofilm + EHL film thickness) reducing contact severity.…”