Low-temperature colossal carbon supersaturation enables anti-wear boundary film formation for austenitic stainless steels in oil-lubricated environment

Abstract

Abstract Austenitic stainless steels are traditionally not good bearing materials due to their tendency to gall when rubbing against other hard alloys. Even worse, they cannot be well-lubricated by hydrocarbon oils. In this study, an alternative carburization process, low-temperature colossal supersaturation (LTCSS), offers a potential solution by demonstrating an improvement of wear-resistance by three orders of magnitude in a fully-formulated engine oil. In our reciprocating sliding test, the untreated Type 304 stainless steel (SS) experienced scuffing at a 120 N load resulting in a sudden friction rise from 0.12 to 0.55 and severe surface damage, while the LTCSS-treated 304 SS maintained low friction and low wear at a 240 N load for six-hour testing. Surface morphology examination indicated that LTCSS changed the wear mode from severe adhesive wear to mild abrasive wear. Surface chemical analysis revealed a protective boundary film containing carbon, metallic elements of the stainless steel, and elements contributed by the oil additives. Specifically, the detected Zn, P, and S contents indicate the involvement of the oil anti-wear additive zinc-dialkyl-dithiophosphate (ZDDP) in the boundary film formation. The case-hardening effect along with the newly observed good compatibility with oil additives makes LTCSS promising for a wide range of tribological applications.