The effects of deep cryogenic treatment on PVD-TiN coated AISI M2 high speed steel

In this study, the effect of deep cryogenic treatment (DCT) on PVD-TiN coated AISI M2 high speed steel was investigated. DCT has been cited to improve hardness, toughness, and wear resistance in martensitic steels. Despite these promising results, there is limited published work on the effects of DCT on hard coated steels, such as industrial cutting tools. Hence, the aim of this study is dedicated to providing experimental data on the effects of PVD-TiN coated AISI M2 high speed steel subjected to DCT. A combination of microstructural and mechanical techniques, such as optical and scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nanoindentation hardness and scratch testing have been applied to determine the changes observed. The result of the nanoindentation hardness showed that an improved result in terms of hardness and elastic modulus were obtained for DCT coated test samples with increases of 5.2 % and 14.8 % compared to conventionally prepared coated non-DCT samples. Upon further examination, it was interesting to note that there was a significant change of 14.8 % in the elastic modulus of the DCT coated samples as compared with the untreated samples. The reason for the difference could be attributed to the contribution of the substrate or changes in the substrate due to DCT. For adhesion testing, optical and SEM examination revealed that DCT coated samples exhibited promising results as the transverse cracks observed for the DCT coated samples appeared denser, more extensive, and could suggest good adhesion as when the mechanical work is applied, energy is better absorbed rather than coating flaking. A comparison of the critical failure points (Lc) revealed that DCT coated samples had Lc values 3.62 % high than the conventionally prepared samples, which could be attributed to the elastic modulus mismatch between the coating and substrate.