Tribofilm properties and microcrack morphologies for a carburized SCM420 steel under high-pressure rolling sliding contact fatigue with lubricants containing high sulfur- and phosphorus-additives

Abstract

Tribofilms and microcracks developed on a carburized SCM420 steel through a two-roller-type pitting test were characterized. Tribofilms composed mainly of calcium phosphate, iron phosphate, and iron oxide were generated on the contact surface. The tribofilms were homogeneously distributed on the contact surfaces that exhibited a longer life, suggesting that the tribofilm distribution affects the rolling sliding contact fatigue (RCF) life. Compounds including the elements from the lubricant were also generated on the surfaces of microcracks formed on the contact surface, suggesting that lubricant flowed into the surface microcracks during the test. The martensitic microstructure at the specimen subsurface was dynamically reconstructed to an ultrafine-grained microstructure during RCF, which resulted in the residual stress relief, especially on the contact surface. After the residual stress release, the fluid pressurization accelerated the formation of a mixed-mode microcrack of mode I and mode II with a high angle to the surface in the later stage, while a shallow microcrack in mode II developed during the early stage.