INFLUENCE OF THE SUBSTRATE HARDNESS AND FRACTURE TOUGHNESS ON THE DYNAMIC WEAR PROPERTIES OF COATED TOOL STEELS

The aim of this work was to determine the impact of various heat-treatment processes and parameters on the substrate hardness and fracture toughness combination and subsequently on the wear properties and subsurface deformation of coated tool steel under dynamic impact loading. Powder-metallurgy steels used as a substrate material were vacuum heat treated under six different conditions and combined with deep cryogenic treatment and nitriding with the aim to modify the hardness and fracture toughness. A TiAlN monolayer with 2-mu m thickness was used for all the substrates. In order to evaluate the impact failure of the coating a ball-on-plate impact fatigue test was designed and used. It was shown that the use of a cryogenic treatment can increase the fracture toughness from 10 % up to 67 % while maintaining the same hardness for some cold-work tool steels, when on others it can have a negative effect. Regarding the static load-carrying capacity, the most important feature of the substrate is its hardness, which at working hardness of 63-64 HRc already provides good static load-carrying capacity of the coated substrate. The increased fracture toughness achieved by the deep cryogenic treatment can at very high and/or very low hardness of the substrate have a negative impact on the dynamic wear properties of the coated surfaces. On the other hand, in the case of vacuum heat treatment that ensures an adequate working hardness of 63-64 HRc, the deep cryogenic treatment improves the impact wear resistance of coated surfaces.