Mechanical integrity of PVD TiAlN-coated PcBN: Influence of substrate bias voltage and microstructural assemblage

Abstract

Polycrystalline cubic boron nitrides (PcBN) have been increasingly used together with PVD coatings, mainly for hard turning operations. Within this context, effectiveness of coated PcBN as cutting tool is usually addressed by evaluation of its machining performance. Meanwhile, studies aiming to assess and understand the correlation between microstructural features and mechanical behaviour of the coating-substrate system are rather limited. Aiming to overcome such lack of information, in this study the influence of substrate bias voltage (−35 V as compared to −60 V) and microstructural assemblage (as a function of cBN content and binder chemical nature) on the mechanical integrity of TiAlN-coated PcBN systems is investigated. In doing so, contact damage response and coating adhesion strength of different coated-PcBNs are evaluated by means of indentation testing using distinct loading conditions (static and sliding) and tip geometries (spherical and conical). Such testing program is complemented by detailed FESEM inspection of the involved failure micromechanisms, as well as microstructural and micromechanical characterization of the deposited films. Results indicate that resistance against crack nucleation and propagation of coated PcBN, induced by either spherical or conical indentation, is enhanced by using harder (high content of cBN particles) and tougher (metallic binder) substrates (H-PcBN). Regarding bias voltage, systems with coatings deposited using a higher value (−60 V as compared to −35 V) show improved adhesive strength, this being particularly true for combinations involving low cBN content and ceramic binder substrate (L-PcBN). Similar beneficial effect was found, but exclusively in coated L-PcBN systems, regarding resistance to radial cracking emergence and to material removal through cohesive-failure chipping induced in Rockwell C tests. Although these findings are linked to the higher compressive residual stresses exhibited by coatings deposited under −60 V bias voltage, the latter does not translate in significant changes in microstructural and intrinsic mechanical properties of the TiAlN coating itself.