Characterization and hardness modelling of alternate TIN/TICN multilayer cathodic arc PVD coating on tool steel

TiCN coatings on the market today are in general multi-layer TiN/ Ti(CxN1−x) coatings. Such multilayered film configuration enables optimisation of the film/substrate as well as the film/worked material interactions, by controlling the internal stress state, fatigue toughness, hardness and superficial composition of these Ti(CxN1−x) coatings. This paper presents the results of the investigations on a wear resistant coating made by alternate layers of TiN and Ti(CxN1−x) (nominal 0.5 μm each), deposited on S600 tool steel by reactive cathodic arc evaporation using a reactive gaseous mixture of methane and nitrogen. Microstructural and compositional characterisation were carried out using ball crater tests, Optical Microscopy, Scanning Electron Microscopy associated with Energy Dispersive Microanalysis and Image Analysis. Micro hardness measurement were evaluated by means of the Chicot and Lesage volume law of mixture model to cope with the problem of the multiple influence of the different layers and the substrate on the real multilayer surface hardness. In order to predict the surface hardness, the model needs the knowledge of the properties of each type of layers and of the substrate (Young modulus, H0=hardness at infinitely small load and n=strain hardening coefficient, or ISE index). These properties were measured using microindentation tests from ad hoc samples of single TiN 0.5 μm layer film, single Ti(CxN1−x) 0.5 μm layer film and the uncoated substrate. Young modulus for the TiN and TiCN were evaluated with load–displacement nanoindentation tests; Young modulus for the substrate is from manufacturer. Thickness, composition profiles and microstructure of each film were used to qualify the data input for the model. Experimental measurements on the composite surface hardness allowed then the verification of the predictions.