Investigating the effects of cryogenic treatments on PVD TiN coated – AISI M2 high speed steel substrate system

Deep Cryogenic treatment (DCT) is an extension to bulk heat treatment processes, aimed at enhancing engineering materials beneficial for applications. Despite the promising results, there is limited published work on the effects of cryogenic treatment on hard coated steels, such as industrial cutting tools. Much debate surrounds the topic due to contradictory results and lack of understanding of the mechanisms responsible. Therefore, the effect of cryogenic treatment was studied on PVD - TiN coated AISI M2 high speed steel (HSS) substrate.

For non-coated substrate, DCT treated AISI M2 HSS demonstrated a 4.1% increase in hardness with lower levels of retained austenite compared with non-DCT samples. The lowest reduction (approximately 3.1%) was obtained when DCT was employed between quenching and tempering.

On coated DCT samples, it was determined that the effect of DCT could contribute to the improvement in average hardness (5.16%) and elastic modulus (14.81%).

A comparison of scratch test results showed that non-DCT samples had average critical load values 3.6% lower than DCT coated samples. However, this was within one standard deviation and has a low level of statistical confidence.

Further examination on the coating – substrate system showed no difference in the lattice parameter between the non DCT coated and DCT coated sample, implying a minimum level of mismatch at the interface and, contrary to the literature, no evidence of any damage at the interface. No clear microstructural evidence to pinpoint the exact mechanism(s) responsible for the improvement in the mechanical properties was obtained. The results of rigorous testing, characterisation and analysis indicate that cryogenic treatment has little to no significant effect and the mechanical properties of this material.