Properties of nitrocarburised and oxidised steel surfaces and the correlation with their tribological behaviour under unlubricated sliding conditions

Abstract In the present study, DIN 31CrMoV9 (1.8519) steel samples were initially treated by salt bath- (SNC), gas- (GNC) and plasma-nitrocarburising (PNC) and afterwards oxidised for increased corrosion protection. The thickness and hardness of the oxide and compound layer as well as the diffusion zone were measured. Surface topographies and microstructures were analysed with optical microscopy, laser scanning microscopy and scanning electron microscopy (SEM), while the detailed chemical compositions of the sample surfaces were determined with energy-dispersive X-ray spectroscopy (EDX), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Tribological tests for characterisation of wear and frictional behaviours of the treated samples were performed under unlubricated sliding conditions. Results show that, although different nitrocarburising processes yield very similar hardness profiles and compound layer thicknesses, their surface morphology and oxide layer properties can be significantly different. On SNC samples, the thickest oxide layers were formed, which had a different chemical composition than those formed on GNC and PNC samples. Furthermore, SNC samples provided the highest wear resistance and only on these samples, the oxide layers were not removed during tribological testing. It is thus possible that a correlation between the oxide properties and the favourable tribological behaviour of the SNC samples exists.