Failure Modes and Mechanisms of Functionally Gradient Ceramic Tools With High Thermal Shock Resistance

As cutting tool materials, ceramics posses high hardness, wear resistance, heat resistance and chemical stability, with less deformation or dissolution wear in cutting processes. In spite of the advances in strengthening and toughening of ceramic tool materials as well as the improved processing techniques, their applications in intermittent cutting operations are still restricted by their intrinsic drawbacks such as lower strength, lower fracture toughness and lower thermal shock resistance. The introduction of the concept of functionally gradient materials (FGM) into the design and fabrication of ceramic cutting tool materials provides an approach to improving the thermomechanical properties of ceramic tool materials. However, the investigations on the thermal shock resistance evaluation of FGMs for cutting tool applications have been very few, with most work concentrating on the design, fabrication and evaluation of heat-shielding FGMs for space applications. A strength-based fracture criterion for thermal shock resistance evaluation of FGM ceramics is formulated in the present paper, based on which the design rules for FGM ceramics with high thermal shock resistance are presented. An Al₂O₃/TiC and an Al₂O₃/(W, Ti)C functionally gradient ceramic tool materials were consequently designed and fabricated by using hot pressing technique. Their cutting performance, failure modes and mechanisms were investigated via a series of intermittent cutting experiments in comparison with those of common ceramic tools. The results revealed improved tool lives of the FGM tools over that of the corresponding common ceramic tools with the same composition systems. The FGM tools exhibited similar wear and chipping characteristics to that of common ceramic tools at initial and normal cutting stages, but different failure modes from that of common ceramic tools. The failure of common ceramic tools was mainly caused by thermal shock and thermal fatigue, whereas the failure mechanisms of the FGM tools were mainly the mechanically induced wear and mechanical fatigue in virtue of their improved thermal shock resistance.