TiN coating on an electrical discharge machined WC-Co hardmetal: surface integrity effects on indentation adhesion response

Electrical discharge machining (EDM) is an alternative-shaping route for manufacturing complex component shapes of hard and brittle materials such as hardmetals (WC-Co cemented carbides). It is well established that in these materials EDM typically induces a heat affected surface layer with poor integrity. This degradation effect may be compensated through specific post-EDM surface treatments either by thermomechanical means or material surface deposition. In the latter case, a key property for optimal performance is the level of coating to substrate adhesion and how this is affected by the EDM-induced surface. The main objective of this investigation was to evaluate the adhesion strength of TiN coatings on EDMed hardmetals. A series of hardmetal samples that had been subjected to different multi-pass sequential EDM levels were coated with TiN. Adhesion behavior was assessed using the indentation adhesion test and comparing the critical load for crack extension (Pc) and the interfacial fracture toughness (KIc,interface) to those exhibited by the TiN coating deposited on a ground and polished substrate (used as baseline control). Experimental results indicated that indentation adhesion increased with finer-executed EDM, almost reaching baseline level values. The results are discussed on the basis of the compromising EDM influence on both surface integrity of the substrate and tortuousness at the interface, the latter resulting in mechanical anchoring of the TiN coating to the hardmetal substrate.