High temperature deformation mechanisms of cemented carbides and cermets
The motivation of this work derives from the need of the cutting tool industry to improve its products in order to support harder and harder working conditions, namely increasing cutting speeds and working on stronger modern materials. The lifetime of the tools is limited by plastic deformation that occurs at the cutting edge under working conditions, which involve high temperatures and stresses. The high temperature deformation of the materials that are used for the production of cutting tools is studied. Two base materials are chosen, a WC-Co cemented carbide and a TiWCN-Co cermet, with the same amount and composition of the cobalt binder. The experimental strategy combines macroscopic deformation tests by three-point bending with microscopical observation and mechanical spectroscopy. We also analyze residual stresses and crystal structure as a function of temperature by neutron and X-ray diffraction. By three-point bending, the transition temperature at which, macroscopically, grain boundary sliding (GBS) of the ceramic phase becomes the dominant deformation mechanism is determined to be 1100 K for the cemented carbide and 1350 K for the cermet. Measurements of residual stresses show that high compressive stresses are present in the hard phase at room temperature and that they relax as the temperature increases. These stresses are much higher in WC-Co, 800 MPa at room temperature, and do not relax completely before 1500 K. In the cermet, they are around 200 MPa at room temperature and relax at 1200 K. Moreover, important changes in the chemical composition of binder and hard phase are observed. In WC-Co, a strong increase of the tungsten content in the cobalt binder is observed above 1100 K. In the cermet, the cobalt composition does not substantially change, but the hard phase exhibits a change in the C/N ratio above 1350 K. Mechanical spectroscopy evidences a number of relaxation phenomena in both materials. At high temperature, three peaks in WC-Co and two peaks in the cermet have been related to grain boundary sliding. A model is proposed that involves two regimes: limited GBS due to cobalt segregation at grain boundaries, and extensive GBS due to fully infiltrated grain boundaries. The latter behavior is similar in WC-Co and cermets and it is associated with a high and very broad relaxation peak. On the other hand, the mechanisms for the limited GBS are different in WC-Co and in cermets. In cermets, cobalt segregation increases continuously with temperature, starting at about 1350 K. In WC-Co, the segregation of cobalt depends on the type of WC/ WC grain boundaries: different relaxation mechanisms can be attributed to general grain boundaries and special grain boundaries, where the special grain boundaries are stabilized by a half-monolayer of segregated cobalt. The effect of tantalum on the plastic deformation behavior is furthermore investigated. Tantalum leads to a beneficial effect in WC-Co below 1273 K, by rejecting cobalt from the grain boundaries and therefore suppressing GBS. At higher temperatures, it leads to an increasing instability of the grain boundary structure both in cermets and cemented carbides, which goes along with a decrease of the tool performance. In different cermet grades, the performance of the tools seems strongly correlated with the stability of the structure of the grain boundaries at high temperatures evidenced by mechanical spectroscopy. Moreover, the impact of tantalum on the deformation behavior of the cermet grades depends strongly on the carbon content. Finally, both the cemented carbide with tantalum and the cermet with tantalum show a phase transformation of the grain boundaries from a crystalline to an amorphous viscous state. Generally, the two basic cutting tool materials, WC-Co cemented carbide and TiWCN-Co cermet, show differences regarding the onset temperature and the low-temperature mechanisms of GBS. However, the GBS of fully infiltrated grain boundaries at high temperature seems to be quite comparable in both materials.