Propriétés mécaniques à haute température de cermets Ti(C,N)-WC-Mo-Co à gradient de composition pour outils de coupe
The materials used for cutting tool applications are principally divided in two classes: the WC-Co cemented carbides, tough but with a limited resistance to deformation at high temperature, and the TiCN-Mo-Co cermets, more refractory but less tough than the WC-Co. The goal of this study of TiCN-WC-Mo-Co cermets is to optimise the combination of the toughness of WC-Co with the resistance to deformation at elevated temperatures of TiCN-Mo-Co. Furthermore, a composition gradient is introduced in these materials to obtain a TiCN-WC-Mo-Co core, resistant to deformation, coated with a tough WC-Co layer. The tough surface layer is added to avoid the formation of cracks which are responsible for the rupture of cutting tools. The scope of this work includes a study of the mechanisms responsible for the deformation in TiCN-WC-Mo-Co cermets and the influence of a WC-Co surface layer on the mechanical properties of cutting tools. For this purpose, low frequency mechanical spectroscopy measurements are conducted on TiCN-WC-Mo-Co cermets, which are completed by three point bend tests as well as by transmission and scanning electron microscope observations. The results obtained from deformation tests makes it possible to define three temperature domains, valid for all the materials used for cutting tool applications: domain I (T < 900 K), in which the hardmetal exhibits a pure elastic deformation with a brittle rupture; domain II (900 K < T < 1300 K), characterised by an increase in toughness and by the beginning of the plastic deformation, first in the binder (Co,Ni) and then in the TiCN grains when present; domain III (T > 1300 K), in which the plastic deformation of hardmetal becomes very high and occurs by grain boundary sliding. These domains are characterised by respective internal friction peaks, localised in the material phase where microplasticity occurs. A good agreement is observed between internal friction peaks and creep mechanisms, particularly when comparing the thermal activation measurements. A grain boundary sliding model is presented to explain the high temperature creep (domain III) of materials used for cutting tool applications. An original method to measure the KIC factor is presented. This method, which uses the presence of a CVD coating characteristic of cutting tools, makes it possible to exhibit the positive influence of a WC-Co surface zone on the macroscopic toughness of hardmetals for cutting tools.