Influence of microstructural assemblage of the substrate on the adhesion strength of coated PcBN grades

Coated polycrystalline cubic boron nitride (PcBN) tools are increasingly used for industrial applications with stringent requirements. However, information on the mechanical integrity of these systems is quite limited. Within this context, the adhesion strength of two TiN/TiAlN-coated and microstructurally different PcBN substrates – high- and low-cBN content grades with metallic and ceramic binders, respectively – is investigated by means of Rockwell C indentation testing. In doing so, special attention is paid to substrate microstructure effects on emergence and evolution of damage as applied load is increased. For both coated systems, it is found that damage scenario evolves from early adhesive delamination within the contact area towards radial cracking and spalling outside the residual imprint. However, critical loads for transition between damage stages are discerned to depend on microstructural assemblage of the substrate. Higher resistance to radial cracking and delamination is exhibited by the high cBN-content grade, in agreement with its higher hardness and toughness. Failure micromechanisms associated with cracking and spalling are similar for both coated-PcBN systems. Very interesting, delamination outside the residual imprint is determined to be of cohesive nature within the substrate, an experimental fact resulting from interaction among indentation-induced radial, cone-like and lateral fissures in PcBN.