The influence of the depth of a plasma nitrided layer in tool-steel substrate on the scratch-resistant properties of PACVD TiBN coating

A hot-work tool steel was plasma nitrided (PN) to create a diffusion layer beneath the surface. Plasma-assisted chemical vapour deposition (PACVD) was then applied deposit a superhard nano-columnar TiBN coating to the nitrided substrate. The depth of the nitrided diffusion layer was varied to determine its influence on the cohesion and adhesion properties of the coating. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) revealed multilayers of TiBN and TiN compounds with compositional gradients across the TiBN coating layer. Microhardness measurements (HV0.025) across the PN diffusion layer in combination with optical microstructure observation showed that an increase in the depth of the nitrided diffusion layer led to an increase in the maximum hardness at the interface between the TiBN coating and the substrate. The maximum hardness correlated linearly with the scratch-resistant properties determined from scratch tests in the progress mode, notably the critical loads corresponding to the first microcracking related to cohesive failure (LC1), spallation related to adhesive failure (LC2) and worn out (LC4). In addition, the scratching coefficients of the TiBN coating on a thicker PN diffusion layer, determined in a scratch test along a scratch track, were all lower than those on a thinner diffusion layer in the substrate. An excessive depth of the nitrided diffusion layer however caused transverse cracking in the substrate. A nitrided layer with an optimum depth can thus improve the adhesion of the PACVD TiBN coating to the tool-steel substrate and the scratch resistance of the coating.