Microstructural and mechanical properties at the submicrometric length scale under service-like working conditions on ground WC-Co grades

Abstract

The unique combination of hardness, toughness, and wear resistance exhibited by heterogeneous hard materials, particularly cemented carbides (WC-Co), has made them preeminent material choices for extremely demanding applications, such as metal cutting/forming tools. The grinding post-processing of WC-Co induces changes in their surface integrity by modifying both constitutive phases through cracking the WC ceramic phase, introducing compressive residual stresses and/or inducing phase transformations (from face-centered cubic to hexagonal compact phase) in the metallic Co binder. A systematic micro- and nanomechanical study of different ground WC-Co grades (with distinct metallic Co binder content and WC particle size) is presented. In general, three different aspects are investigated: (1) assessment of the intrinsic hardness of the deformed layer from room temperature up to 600 °C, (2) correlation of the compressive residual stresses with hardness and elastic modulus maps by using high-speed indentation tests, and (3) evaluation of the oxidation process as a function of the testing temperature for the different ground WC-Co grades. It was found that the mechanical properties of the deformed ground layer for the different WC-Co grades slightly decrease at temperatures ranging between 400 and 500 °C, being this temperature non depending on the amount of metallic binder or WC grain size. This is attributed to different effects which take place simultaneously when the testing temperature increases: dislocation motion, reduction of compressive residual stresses and particularly, the generation of a heterogeneous oxide layer formed of CoWO4, WO3 and Co3O4.