Behaviour of hard metals and cermets under cyclic mechanical loads

Fatigue of materials is a consequence of a deterioration of strength properties as a result of cyclic loading. The investigation of this phenomenon for cyclic mechanical loads in hard metals and cermets is the topic of this work. These materials are related to ceramics. Therefore a new method has been applied for these investigations, which has been developed to investigate the fatigue of ceramics. One very important aspect of this method is that it yields access to information about the growth of small natural cracks. In the experiments the materials, a typical hard metal and a typical cermet, are investigated under static, monotonously increasing, and cyclic loads (22 Hz) in air at room temperature. For these experiments a special apparatus was built. The behaviour of the materials under the different loads is presented and discussed. The investigations of microscopic features on fracture surfaces and of the microstructure in the volume in front of growing cracks by SEM and TEM give additional information about the processes which occur during the fatigue of these materials under cyclic loads. From the mechanical behaviour under the different loads and the results of the micro-structural investigations, conclusions concerning the processes responsible for the reliability and for the lifetime can be drawn.