Fatigue of hard metals and cermets — New results and a better understanding

Abstract

Hard metals and cermets are submitted as cutting materials to complex loadings, wear, mechanical and thermomechanical loadings. Among these the cyclic load conditions occur always with different amplitudes in the technical cutting processes. In contrast to this fact, the cyclic loading and a possible fatigue as a life time limiting process of the cutting tools received relatively little attention in the past. Our investigations since 1992 have proved the existence of fatigue phenomena and their life time limiting influence on components of these materials under cyclic loads at room temperature. These results are obtained from experiments in a simple apparatus under three different loading conditions, static, monotonically increasing and cyclic loads. Beside this quantitative phenomenological description of the fatigue by the measured mechanical properties, investigations of the microstructure of fractured surfaces by SEM and of the volume, especially around the crack tips by TEM, have led to the conclusion that the fatigue processes are located predominantly in the ductile binder phase of these composite materials. This result led to new information about the processes in the microstructure during fatigue. The investigations in this work here were carried out with different hard metals and cermets. They differ in the compositions of the brittle ceramic and of the ductile binder phase. The new results from these two routes of investigations, mechanical properties and microstructure, allow important conclusions to be drawn about the processes and the mechanisms which are responsible for the damage in the materials during fatigue. This knowledge allows the design of improved materials with regard to higher resistance against fatigue under cyclic alternating loads.