Effect of Deep Cryogenic Treatment on the Properties of TiN Coated 316L Stainless Steel

Abstract

This research studied the effects of deep cryogenic treatment on the microstructure, residual stress, hardness, and film adhesion of Titanium Nitride (TiN) on 316L stainless steel coated by the cathodic arc physical vapor deposition (PVD) process. The results showed voids and particles located on the surface of TiN coating for both cryogenic treated and untreated samples. The process time affected not only the amount of particles and voids that appeared on the surface, but also their size. Compressive residual stress was generated by cryogenic treatment due to the thermal effect creating different induced stress between the coating and substrate. The change in the number and size of voids and particles observed on the surface was likely caused by residual stress. Higher compressive residual stress was observed with longer process time, resulting in more particle detachment and, consequently, more voids on the surface. However, no significant reduction in the number of particles was observed, which suggested that (though not yet fully studied in this work) more particles were expected and that this may be the result of particles being compressed and moved towards the surface. It should be noted that a significant drop in compressive residual stress was observed at 24 hours process time, which may be caused by large particle detachment resulting in stress release in such area. Closer voids were also observed in this case, resulting in smaller void size, although more voids were generated due to small particle detachment caused by process induced stress. The critical normal load corresponding to 1st coating damage (LC1) under the scratch test was found to be higher for the sample with higher compressive residual stress as expected as it could slow down the crack propagation and dislocation motion, leading to lower plastic deformation under a certain load. The compressive strength and fracture toughness (Kc) estimated from the scratch test corresponded with LC1. It was also found to be higher with increased compressive residual stress. However, no significant change in surface hardness was observed. This may be due to the complexity of the combination of different effects on material flow caused by compressive residual stress and the number of voids and particles as well as their size relative to indentation size.