Cohesive failure and film adhesion of PVD coating: Cemented carbide substrate phase effect and its micro-mechanism

For a coated cutting tool, film adhesion and cohesive failure resistance are two equally important technical parameters. To systematically investigate the effect of the hard phase and binder phase on the two parameters, WC–Co cemented carbides with varying average WC grain sizes and Co contents were used as the substrates. A PVD-Ti0.94Si0.06N/TiAlSiN/Al0.52Ti0.48N multilayer coating was deposited on the substrates. The results of quantitative single-point scratch testing show that there is a positive correlation between the film adhesion and the substrate hardness. Nevertheless, at the condition of Co ≤ 6 wt% in the substrates, the cohesive failure resistance declines substantially. The results of high-resolution transmission electron microscopy observation and analysis disclose the micro-strengthening mechanism as follows: (1) the WC hard phase contributes to the enhancement of the film adhesion through the formation of coherent/semi-coherent WC/film interfaces; (2) the binder phase contributes to the enhancement of the cohesive failure resistance through plastic deformation constrained by WC hard phase, fcc to hcp martensitic transformation and formation of disordered atomic layer, which consume the strain energy of the coating. Nevertheless, due to the formation of conical convergence points by grain merging growth of the film on the binder phase, which can lead to stress concentration in the film, there is a suitable range of the cobalt mass fraction. The selection criteria for the substrates of PVD coatings are discussed.