Experimental strategy to study the mechanical behaviour of hardmetals for cutting tools

Two main families of hardmetals for cutting tool applications can be defined according to the ceramic matrix: WC and TiMoCN based hardmetals with either cobalt or nickel binder. In this paper, we illustrate an original approach to study this kind of materials combining different techniques, macroscopic and microscopic. The morphology of hardmetals can be well described by a structure made of two interpenetrated skeletons: the ceramic and the metal ones. This morphology makes it easy to remove the metal by chemical etching. The comparison between the properties of the ceramic skeleton and those of the whole material allow to locate the mechanisms responsible for a given mechanical behaviour. The macroscopic mechanical behaviour is studied by bending tests made as a function of temperature and composition. Generally, three distinguished domains can be defined while increasing the temperature: brittle, limited plasticity and plasticity. A transition from bulk deformation in the limited plasticity region to grain boundary sliding in the plasticity region is observed in cobalt base hardmetals. Only bulk deformation of TiMoCN and nickel is observed in nickel base hardmetals. The kinetics of the high temperature deformation is studied by temperature jumps leading to better stability of the microstructure compared to traditional creep tests. However, apparent activation energies are often obtained due to changes in the internal composition of the binder. A finer tool to study the dynamics of defects responsible for deformation and their kinetics is the mechanical spectroscopy. The analysis of the thermal activation of damping peaks can be made isothermally as a function of frequency which means without structural changes: this method provides true activation energies with respect to deformation tests. Mechanical tests are supported by electron microscopy imaging. (C) 1999 Elsevier Science Ltd. All rights reserved.