Study on Working Mechanism of Sulfur and Phosphorous Additives by in situ AFM Wear Test under High Temperature Lubrication

Lowering the viscosity of lubricants is important for improving fuel economy, but it decreases the thickness of oil films and may undermine the reliability of hardware. In particular, sulfur and phosphorous additives have been used to ensure reliability, so an understanding of their working mechanisms is important. In this study, to clarify the mechanism, an in situ atomic force microscopy (AFM) wear test was conducted under high-temperature lubrication conditions. The test conditions for causing wear were investigated by changing the AFM tip material, and it was found that steel surfaces were worn under high load conditions with a hard diamond-coated AFM tip. Then, using this method, the wear depths for different additives were examined; it was revealed that addition of sulfur and phosphorous additives separately did not significantly improve antiwear performance, whereas a combination of specific sulfur and phosphorous additives did. Next, the surface hardness and chemical composition were examined, and it was observed that oil with both sulfur and phosphorous additives prevented wear by forming a hard reaction film. Overall, the results suggest that the in situ AFM wear test is a useful technique for investigating nanometer-wear depth and the working mechanism at nanoscale order of additives in lubricants.