WC-Co cemented carbides are composites, which combine a hard phase consisting of WC grains and a metallic ductile phase as a binder. Their excellent mechanical properties, combining high hardness, toughness and refractory properties, make them excellent materials for cutting processes such as turning, drilling and milling. For such applications, cemented carbides are coated with thin ceramic films that increase their resistance to thermal stress as well as to the mechanical and chemical wear inherent in extreme machining conditions. Three main characteristics determine the performance and wear resistance of cutting tools: the cohesion of the hard phase above 1200 K, the ductility of the cobalt binder, and the mechanical and thermal resistance of the coatings.
This work focuses on the impact of the microstructural behaviour of the cobalt binder on the tool-life of the cutting tools. The aim is to develop new binders for cemented carbides with improved impact resistance.
The techniques of transmission electron microscopy at room temperature and in-situ at high temperature, as well as mechanical spectroscopy in a forced torsion pendulum have proved to be essential for the identification of the microstructure of cobalt and for the study of the relaxation phenomena occurring therein.
Mechanical spectroscopy has evidenced the presence of two relaxation peaks, P1 and P2 in the cobalt phase and a peak (P3) related to WC-WC grain boundary sliding. The peak P1 appears to be correlated with the cutting tool life measured in interrupted cutting tests.
In this work, the role of the binder is highlighted in particular by the identification of a glass-type transition observed for cobalt. The thermodynamics of the transition is evidenced by the behavior of P1 whose relaxation time diverges according to a Vogel-Fulcher-Tammann model, which is characteristic of glass transitions. The state of the material corresponding to temperatures below this transition shows a strong dependence on its thermomechanical history. This dependence is confirmed by a divergence in plastic deformations over two identical temperature cycles with different thermomechanical histories. Such a divergence on a standard protocol, called (ZFC/FC), seems to indicate a break in the thermodynamic hypothesis of ergodicity, even though it could not be attributed solely to the behaviour of cobalt.
Transmission electron microscopy at room temperature has determined that the low temperature phase of the cobalt binder has a face-centred cubic structure and is composed of twinned nanodomains containing nanotwins. This state is therefore characterised by a short-range order without the long-range order, which is characteristic of glassy materials. In-situ observations have highlighted a detwinning process corresponding to the temperatures of the cobalt relaxation peak identified by mechanical spectroscopy. This detwinning process continues until the appearance of a long-range ordered face-centred cubic phase above 1050K, which is favourable to deformation.
Mechanically, this transition is similar to a brittle-to-ductile transition, greatly influencing the properties of the material and its toughness. The cutting tool life could benefit from lowering the brittle to ductile transition temperature.EPFL