Fatigue of hard metals and cermets

Abstract

The mechanisms of failure in hard metals and cermets have been studied at room temperature under cyclic loads with a new method which yields information about the growth of small cracks. The growth of these small cracks is important for the behaviour of the materials in the technical application as cutting tools. In the experiments, the materials have been investigated under static, monotonically increasing, and cyclic loads (22 Hz, 11 Hz) in air at room temperature. For these experiments a special apparatus was built. The behaviours of the materials under the different loads are presented and discussed. Electron microscopy including SEM, TEM and SAM were used to investigate the micro-mechanisms of damage during fatigue. The results are used to interpret the observed mechanical behaviour. The mechanical behaviours of the materials under cyclic loads are found to be different compared with that under static and monotonically increasing loading conditions. The corresponding damage processes are also completely different. Subcritical crack growth under the cyclic loads occurs along special crack paths which are different from those under the other loading conditions. From the mechanical behaviour under the different loads and the results of the microstructural investigations, conclusions regarding the damage processes responsible for the reliability and for the lifetime under cyclic loads can be drawn.