Structural, mechanical and tribological properties of duplex-treated AISI 5140 steel

The structure, hardness, adhesion, surface roughness and tribological properties of duplex-surface-treated (TiN/plasma nitrided), directly TiN-coated and ion-nitrided AISI 5140 steel were investigated using X-ray diffraction, SEM, optical microscopy, a microhardness tester, a scratch tester and a pin-on-disc tribotester. After the plasma nitriding process, the transition treatments were realized to evaluate the effect of the compound layer. It was found that the relative intensity line of TiN (200) and surface roughness increased, but the coating hardness and critical load (Lc) decreased with an increasing amount of ɛ-Fe2–3N iron nitride in the compound layer. The better tribological properties were obtained with coatings applied on substrates, which included a large amount of γʹ-Fe4N iron nitride after plasma nitriding.