Correlation between surface fatigue and microstructural defects of cemented carbides

The main aim of the present theoretical work is to determine numerical dependence between the geometrical parameter of maximum area of structural defect image (proposed by Y. Murakami, 1983) and surface fatigue of cemented carbides. The proposed relations allow making predictions of surface fatigue properties of cemented carbides (WC-Co hardmetals—with 10 wt% Co and 15 wt% Co binder; TiC-based cermets—with 32 wt% Fe/8 wt% Ni and 16 wt% Fe/14 wt% Ni binder) in conditions of sliding, rolling contact and impact cycling loading. Pores are considered being equivalent to small defects. Three comparative defects conditions are distinguished: surface pores, pores just below free surface and interior pores. The Vickers hardness of the binder (as mainly responsible for the fracture mechanism of hardmetals and cermets, due to porosity) is presumed to be the basis of such an assumption. The estimate of this prediction has been done by analyzing the pore sizes using the statistics of extremes. The lower limit of fatigue strength and surface fatigue life can be correctly predicted by estimating the maximum occurring pore size in a critical material volume.