PNNL

Abstract

This study investigated the elemental composition and surface morphology of solid tribochemical films formed on steel surfaces. The reversible addition-fragmentation chain transfer (RAFT) method was used to synthesize nine different types of polymers, containing no metals, which were blended into commercial hydraulic oils. The polymers were either homopolymers of dodecyl methacrylate and ethylhexyl methacrylate or were copolymers of these monomers with six polar monomers. The wear behavior of the analogs varied greatly, by about a factor of 30. After tribological testing at 100 °C using the ball-on-flat geometry, the resulting tribochemical films were imaged using SEM and optical microscopy. The resulting tribochemical films seemed to have thicknesses around 50-100 nm. Two of the films corresponding to low (P1) and high (P3) wear, were cross-sectioned using FIB and analyzed for elemental composition using EDX mapping. Oxygen and nitrogen enrichment was observed, consistent with the relative chemical composition of the precursor polymers. TEM evidence suggests that at the worn surface, some organic elements penetrate or are mixed into the steel substrate. TEM demonstrated that P1 tribofim has diffused with the steel substrate more so than P3, suggesting a stronger affinity and contact during tribofilm formation, which is presumably one of the main reasons for its superior antiwear properties.