Interfacial fatigue stress in PVD TiN coated tool steels under rolling contact fatigue conditions

Abstract

Titanium–nitrogen (TiN) films were Physical Vapour Deposited (PVD) on tool steel substrates with different hardness and surface roughness, in a Bai 640R unit using a triode ion plating (e-gun) with a high plasma density. The coated substrates were submitted to a rolling contact fatigue test technique (modified pin-on-ring test) to obtain some clarifications of the mechanism of interfacial failure. Tests were run using PVD-coated rings finished by polishing or grinding to produce different surface roughnesses. From the results, it appears that the fatigue durability is at lower stress levels significantly influenced by both the pre-treatment and the final surface roughness of the material. The polished and smoother surfaces are associated with a longer fatigue life. However, at a higher contact stress, there appears to be very little influence of pre-treatment and surface roughness. Two mechanisms of crack propagation under pure rolling conditions were found, depending on the substrate hardness. For the softer substrates, the cracks propagate mainly perpendicular to the surface, whereas for the harder substrate, the cracks generally originate at the interface and progress in the coating parallel to the surface.