Formation of a self-lubricating layer by oxidation and solid-state amorphization of nano-lamellar microstructures during dry sliding wear tests

Dry sliding wear tests of steel disks of various compositions were performed with consideration of the effects of microstructure, shear strain and shear strain rate using a ball-on-disk tester with a tungsten carbide ball under ambient atmosphere and room temperature. We found a severe–mild wear transition phenomenon, which we called self-lubrication, and categorized friction behavior into two regimes: a high-friction regime and a low-friction regime. However, self-lubrication occurs only at specific strain levels and rates related to materials with martensite and pearlite structures. We characterized the microstructures of the worn surface, subsurface and matrix and found a thin self-lubricating layer composed of nano-oxide particles with sizes in the range 6–20 nm attached to the surface. The formation of these nano-oxide particles is controlled by lamellar structure (martensite or pearlite) nano-lamination, oxidation and solid-state amorphization during dry sliding. The high density of geometrically necessary dislocations and defects induced by plastic deformation under wear shear strain levels and rates is important in the formation of nano-lamellar microstructures and amorphous oxide. These results both clarify nano-lamellar microstructure solid-state amorphization and oxidation behavior and reveal the mechanism of self-lubricating layer formation observed in this work.