THE INFLUENCE OF SAMPLE PREPARATION ON THE SURFACE STRESS STATE AND PALMQVIST'S METHOD FOR TOUGHNESS TESTING OF CEMENTED CARBIDES

Abstract : Different views exist on the influence of surface preparation on crack formation at Vickers indentations in the test used by Palmqvist to evaluate the toughness of cemented carbides. The present experimental results clearly show that grinding diminishes the thermal tensile stresses in the cobalt phase and thus reduces the crack length. The deformed surface layer is removed by polishing and a stress pattern is approached corresponding to that in the bulk material. Cracks of maximum length are characteristic of this stress state. Deformation stresses can be removed by annealing above 800C. On the basis of these results, sample preparation for the crack length test can be simplified, the accuracy of measurement is improved and a simple and significant parameter is found to describe crack resistance of cemented carbides.