B. Casas

Influence of electrical discharge machining on the sliding contact response of cemented carbides

Abstract The correlation between electrical-discharge machining (EDM) and surface integrity and its influence on the sliding contact response of a fine-grained WC-10% wt Co hardmetal have been studied. Different surface finish conditions were evaluated corresponding to sequential EDM as well as grinding and polishing employing diamond as abrasive. Surface integrity was characterized in detail for all conditions through scanning electron microscopy (SEM). Sliding contact response was determined through microscratch measurements using a nanoindentation system. Contrary to the results obtained from conventional hardness evaluation, the above testing technique allows discrimination of EDM effects on the contact behavior of the studied cemented carbide. The experimental results, given in terms of scratch penetration-load curves and friction coefficient, indicate that microscratch resistance increases with finer-executed EDM, up to values similar to those exhibited by the reference ground and polished condition. The findings are discussed in terms of contact damage features as related to surface integrity and intrinsic microstructural features.

Oxidation-induced strength degradation of WC-Co hardmetals

Abstract The oxidation behavior at 700°C and its influence on room-temperature mechanical strength of three grades of WC–Co cemented carbides containing different binder content were studied. Oxidation kinetics, determined by thermogravimetric measurement, were found to follow a linear time dependence of weight gain per unit area in all the cases. It was also observed to rise with an increase in the oxygen concentration of the atmosphere and a reduction of the cobalt content. Three oxidation times of 10 min, 1 and 6 h were chosen to evaluate changes in the mechanical strength. Fracture resistance of oxidized samples were measured at room temperature under four-point bending and compared to that of non-oxidized specimens. In all cases, a pronounced degradation in strength was observed after oxidation. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics (LEFM). It is conjectured that strength reduction results from a combination of load-bearing section reduction (extrinsic effect) and a concomitant interplay of several factors including increasing size and severity of critical flaws, wedge-effects in the case of heterogeneous oxide penetration and residual tensile stresses associated with volume and thermal differences between the substrate and the oxide scale (intrinsic effects).

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

Abstract 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.

Diamond coatings on electrical-discharge machined hardmetals

Abstract The influence of electrical-discharge machining (EDM) of WC–Co cemented carbides on the structural features and adhesion strength of subsequently deposited chemical vapor deposition (CVD) diamond coatings is investigated. Two different surface conditions are analyzed for a low binder-content hardmetal substrate. One variant corresponds to multi-pass sequential EDM, whereas the second one, used here as a reference, is attained through grinding and polishing. In all the cases, identical substrate chemical etching and arc-plasma CVD standard diamond deposition processes are applied. It is found that fine-executed EDM of hardmetals allows the deposition of diamond coatings exhibiting similar structural and adhesion characteristics to those obtained on conventionally machined cemented carbides substrates. Although surface integrity of the substrate together with quality and residual stresses of the diamond film are slightly affected by EDM, adhesion strength corresponding to this surface condition is retained because EDM-induced isotropically rough surfaces enhance mechanical interlocking of the diamond coating to the hardmetal substrate.