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

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.