No abstract

International audienceThe rheological behaviour of several fully formulated gear oils is characterized: two mineral-based oils, one poly-alpha-olefin, and three ester-based oils. Experimental traction curves were obtained on a mini-traction machine, where the selected gear oils were submitted to numerous traction tests combining several temperatures, contact loads, and rolling speeds. Having obtained the experimental data from the traction tests, it was least-squares fitted to a rheological model (elasto-viscoplastic rheological model) in order to obtain the model's governing parameters: elastic modulus and limiting shear stress and their dependence on pressure and temperature. This study presents both the traction tests results and the fitted rheological parameters, and discusses the similarities and differences between the oils, as well as the influence of the operating conditions on the behaviour of the oil

International audienceLubricant formulations for manual gear box applications are optimized for gear contacts although rolling element bearings are lubricated with the same fully formulated oil. This may cause fatigue-related damage to these bearings. Several explanations can be considered, but this work focuses on the effect of additives contained in the lubricant. Rolling contact fatigue tests were performed on a twin-disc machine. Artificial dents generated by a Rockwell penetrator were made on the faster surface in order to accelerate the fatigue phenomena. Then, pure rolling and 6.7 per cent slip (slide-to-roll ratio (SRR)) tests were performed with different lubricants (pure base oil, fully formulated oil, and base oil with detergent and anti-foam). Fatigue life results and spalling morphologies are compared. For the sample obtained with the fully formulated oil and 6.7 per cent of SRR, crack analysis was performed. Using focus-ion-beam technique, a spalled sample was milled to reveal a cross-section of a crack. Secondary electron microscopy (SEM) images were taken and energy-dispersive X-ray spectrometry (EDX) analyses along the crack were performed. Additive elements are detected up to the crack tip. Auger electron spectroscopy depth profiling was also performed in the tribofilm generated on the disc surface. The role of additives in rolling contact fatigue is discussed in the light of these results

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