Effect of Diesel Soot on the Distribution, Composition and Mechanical Properties of ZDDP Tribofilm

To investigate the effect of diesel soot on the distribution, composition and mechanical properties of ZDDP tribofilm, a HFRR tribometer was applied to study the tribological performance. Worn surfaces lubricated with ZDDP and soot were analyzed by laser microscopy, SEM/EDS, Raman spectroscopy, XPS, and a nano-indentation equipment. Results show that soot scrapes off the ZDDP tribofilm and can be embedded into the worn surface, leading to the reduction of film thickness and non-uniform distribution of tribofim. The phosphate structure in ZDDP tribofilm changes from short chain pyrophosphate to long chain metaphosphate due to the increased contact stress caused by the soot abrasive wear, which can promote the cross-linking of ZDDP. The hardness (H) and elastic modulus (E) of the worn surfaces increase, while the ratio of hardness to elastic modulus, H/E, decreases, which indicates that the reduction of wear resistance is caused by the soot.