FATIGUE OF HARD METALS AND CERMETS

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 sma ll cracks is important for the behaviour of the materials in the techn ical application as cutting tools. In the experiments, the materials h ave been investigated under static, monotonically increasing, and cycl ic loads (22 Hz, 11 Hz) in air at room temperature. For these experime nts a special apparatus was built. The behaviours of the materials und er the different loads are presented and discussed. Electron mciroscop y including SEM, TEM and SAM were used to investigate the micro-mechan isms of damage during fatigue. The results are used to interpret the o bserved mechanical behaviour. The mechanical behaviours of the materia ls under cyclic loads are found to be different compared with that und er static and monotonically increasing loading conditions. The corresp onding damage processes are also completely different. Subcritical cra ck growth under the cyclic loads occurs along special crack paths whic h are different from those under the other loading conditions. From th e mechanical behaviour under the different loads and the results of th e microstructural investigations, conclusions regarding the damage pro cesses responsible for the reliability and for the lifetime under cycl ic loads can be drawn.