Effect of lubricant additives in rolling contact fatigue

Lubricant 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.