New insight into tribofilm composition and structure of iron polyphosphate-rich tribofilm from formulated engine oil by ToF-SIMS

This research aims to track the chemical composition and fine structure of tribofilm, primarily considering sulfur- and phosphorus-related compounds in response to different applied loads and temperatures on the boundary lubrication regime, where the commercially formulated engine oil was adopted as lubricant. The results clearly reveal the formation of iron polyphosphate-rich tribofilm preferentially on the moving cast-iron surfaces, with increasing amount of FeS₂/FeS₂xOx species along with heavier load and higher temperature. Nevertheless, FeS₂/FeS₂xOx species gradually decrease rather than as the intermediate layer or increasingly concentrate on the substrate surface, which is greatly different from the previously reported tribofilm primarily consisting of zinc/iron polyphosphate and sulfide mixtures. More detailed information regarding how sulfur- and phosphorus-related species distribute spatially across the whole tribofilm thickness was given by ToF-SIMS technique.