Hot Work Tool Steel Substrates Research Articles

Microstructure and selected properties of hot-work tool steel with PVD coatings after laser surface treatment

The paper presents the effect of HPD laser treatment on the microstructure and selected properties of the PVD CrN, (Ti,Al) and Ti(C,N) coatings deposited onto hot-work tool steel substrates. The microstructure and surface topography of the investigated samples are characteristic of the diversified morphology connected with the applied laser beam power. Employment of laser beam at 0.7 kW power to the laser treatment of samples with Ti(C,N) coatings causes clear coating adhesion growth because of the diffusive processes induced by heat release. Because of the higher value of the (Ti,Al)N absorption coefficient one can state that the observed substrate materials change and finally coatings destruction in case of those samples is the most noticeable. The moderate effect of the laser beam treatment of the hot-work tool steel with the PVD coating was observed for CrN coatings. However, for laser beam power above 0.5 kW differences in the thermal expansion coefficients of the substrate materials and coatings generate coating crackings.

Effect of prior plasma nitriding applied to a hot-work tool steel on the scratch-resistant properties of PACVD TiBN and TiCN coatings

Ternary TiBN and TiCN coatings on a hot-work tool steel substrate with and without plasma nitriding (PN) prior to plasma-assisted chemical vapour deposition (PACVD) were investigated. Compositional analysis with a radio frequency glow discharge optical emission spectroscope (rf-GDOES) showed mixtures of TiBN+TiN and TiCN+TiN in the PACVD TiBN and TiCN coatings, respectively. Each coating layer had a compositional gradient across the coating depth and slightly into the substrate. The microhardness profiles (HV0.1) of the substrate with and without PN from the interface with the coating to the substrate core were determined. The depth of the effective nitrided diffusion layer was confirmed from the examination of its optical microstructure. The adhesion of these two coatings to the substrate was evaluated through scratch tests in the progressive mode. It was found that with increasing load, both of the coatings on the substrate with and without prior nitriding deteriorated in the same failure modes. Critical loads corresponding to the first microcracking related to cohesive failure, spallation related to adhesive failure, breakthrough and worn out were determined and used to quantify the scratch resistance of these coatings. With PN prior to PACVD, both the cohesion and adhesion properties of the TiBN and TiCN coatings were remarkably improved. This improvement was attributed to a functionally gradient hardness configuration from the coating through the nitrided diffusion layer to the substrate.