Engine oils contain additives that provide wear protection to prolong engine life. In a previous study using direct acting mechanical bucket valve train components, we found that aged oil provided better wear protection and friction reduction under certain circumstances. To understand this effect further, friction and wear performance of fresh and laboratory-aged oils with 0.1% phosphorus was studied with ball-on-flat and cylinder-on-flat rigs. Test durations were chosen according to the electrical contact resistance (ECR) values observed between the contacting surfaces.
Anti-wear films were characterized primarily by UV and visible Raman spectroscopy, and results were corroborated by Auger electron and infrared spectroscopies. The greatest compositional differences occurred between films formed by fresh and aged oils. The degree of ECR response or the length of oil aging generally did not affect the type of component observed in the films. In films formed by fresh oil, FeS2, orthophosphates, and carbonates were present, while another type of phosphate and little or no carbonate were present in the films formed with aged oil. Additional components detected in both films were Fe3O4, amorphous hard carbon, and a trace of hydrocarbon.
The combination of UV and visible Raman spectroscopy enabled the detection of various components at different depths in the anti-wear films. Lastly, the films can be heterogeneous when viewed microscopically, as revealed by the relative Raman intensities of various components sampled at different locations across the films.