Mechanical strength improvement of electrical discharge machined cemented carbides through PVD (TiN, TiAlN) coatings

Electrical discharge machining (EDM) is an alternative shaping route for manufacturing complex component shapes of hard and brittle materials as WC-Co cemented carbides (hardmetals). However, it results in a heat-affected surface layer with poor surface integrity that often leads to mechanical degradation of these materials. This work focuses on assessing the feasibility of physical vapor deposition (PVD) of hard coatings as a technique for improving surface integrity and mechanical strength of EDMed hardmetals. In doing so, the influence of PVD of TiN and TiAlN coatings on the flexural strength of cemented carbides substrates shaped using multi-pass sequential EDM was investigated. For comparison purposes, coating effects on a reference surface condition attained through grinding and polishing using diamond as abrasive were also evaluated. Experimental results show that hard coating deposition markedly decreases the lessening effect of EDM on the fracture resistance of hardmetals. However, no flexural strength differences between coated and uncoated abrasive-machined samples are discerned. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics. It is concluded that, although coated EDMed cemented carbides exhibit an effectively larger critical defect size, a mechanical strength improvement is finally achieved because an overcompensating effect ascribed to beneficial changes on intrinsic severity (nature) of the critical flaw and residual stress state at the surface. (C) 2003 Elsevier B.V. All rights reserved.