Amorphous carbide layer formation on medium-carbon steel via short pulse laser in PAO oil for enhanced tribological performance

This study presents a laser-assisted surface modification method enabling in-situ carburization of medium-carbon steel in hydrocarbon oil. Sub-nanosecond pulses generate transient high-temperature zones, with the oil acting as both carbon source and quencher. XRD shows a weak broad peak at ~41.4° assignable to Fe 3 C/Fe 5 C 2 , consistent with TEM observation of a 50–200 nm amorphous carbide layer. The process maintains surface smoothness (Ra < 0.10 µm), nearly doubles hardness, and yields extended low-friction lifetimes in oil as well as reduced friction in dry sliding. Raman analysis further indicated that the amorphous Fe–C layer contains minor carbonaceous species which evolve into a carbon-rich tribofilm during sliding, supporting the intrinsic lubricity of the amorphous phase. • Sub-ns laser irradiation in oil enables in-situ carburization of medium-carbon steel. • An amorphous iron carbide layer (~200 nm) is formed under ambient pressure conditions. • The laser-modified surface exhibits ~2× hardness and superior wear resistance. • Stable low friction is achieved under both lubricated and dry sliding conditions.