Influence of Co-enriched surface zones in cemented carbides on the microstructure and mechanical properties of TiN/TiC0.6N0.4/α-Al2O3 coatings

In metal cutting applications functionally graded near-surface zones in cemented carbide substrates are applied to optimize their properties, in particular toughness and hardness. Thus, the present work focuses on the influence of Co-enriched substrate surface zones and their thickness on the microstructure and mechanical properties of state-of-the-art TiN/TiC0.6N0.4/α-Al2O3 coatings synthesized using chemical vapor deposition. Complementary cross-sectional energy dispersive X-ray spectroscopy and electron back-scatter diffraction maps provided insight into the grain size, preferred orientation and phase composition of coatings and substrates. While the hardness and Young's modulus of the coatings were hardly affected by the Co-enriched surface zone and its thickness, nanoindentation maps performed on the cross-sections of the substrates confirmed a lower hardness and Young's modulus in zones with higher Co content. Furthermore, it was found that the Co-enriched surface zone and its thickness have no effect on the thermal crack networks of the coatings. Rockwell-indentation tests demonstrated a reduction of the coating adhesion with increasing thickness of the Co-enriched surface zone. As determined by X-Ray diffraction, the tensile residual stress in both, the TiC0.6N0.4 and α-Al2O3, decreased with increasing thickness of the Co-enriched surface zone. Complementary finite element method simulations have shown that plastic deformation due to creep in the substrate and the Co-enriched surface zone only has a minor influence on the residual stress in the coating. The results obtained within this work contribute to a better understanding of the influence of a Co-enriched surface zone and its thickness on the performance of TiN/TiC0.6N0.4/α-Al2O3 coated cutting tools.