Thermal and mechanical shock resistances of Si3N4/(W, Ti)C graded nano-composite ceramic tool material

Abstract

Thermal and mechanical shock resistances of a newly developed Si3N4/(W, Ti)C graded ceramic tool material were investigated using the water quenching methods and intermittent cutting test, respectively. One homogeneous material was used as a competitor. In the quench-strength tests, the retained flexural strength slightly decreased at first and then dramatically decreased, and after that it decreased gradually once again. The critical temperature difference was 600 °C for the graded material and 500 °C for the homogeneous material. The initiation, propagation and merge of microcracks were the main reasons that leaded to the sharp decrease of bending strength. The graded material had a smaller crack propagation length value than the homogeneous material after the same thermal shock cycles in the indentation-quench tests. In the intermittent cutting test, the tools failed mainly due to ladder-like fracture around the cutting nose. Tool failure mechanisms included mechanical shock microcracks, adhesion, and chipping. After the same number of impacts, the graded tool showed much smaller flank wear. Reasonable distribution of composition resulted in smaller thermal shock tensile stress in the surface layer of the graded material. The residual compressive stress introduced by the graded structure could alleviate some external thermal and mechanical stress. These effects inhibited the growth of microcracks and made the graded material higher thermal and mechanical shock resistances.