RELATIONSHIP BETWEEN THERMAL SHOCK STRENGTH BY LASER IRRADIATION TECHNIQUE AND FRACTURE TOUGHNESS FOR CERAMICS

摘要

The traditional water quenching technique used to evaluate the thermal shock resistance of ceramics is accompanied by an unstable heat transfer coefficient in the quenching process. This leads to an unreliable result of the value of the thermal shock strength of ceramics. Our purpose is to establish a new method using the laser irradiation technique by which the thermal shock strength of ceramics could be evaluated. Defining the laser power density, the so-called critical power density P_L at which the ceramic specimen fractures, the method was evaluated both theoretically and experimentally for several structural ceramic materials. Since thermal shock fractures are caused by the induced thermal stress, the thermal shock strength must be correlated to the fracture toughness of ceramics. This study presents a relationship between the thermal shock strength P_L, which corresponds to the critical powerdensity, by laser irradiation and the fracture toughness K_(IC) by the indentation fracture technique for ceramics as a means to investigate their fracture mechanism. The thermal shock strength P_L can be used as a fracture criterion corresponding to the critical temperature difference in the-quenching method. The thermal shock strength for several ceramics was obtained by the irradiations with CO_2 lasers. On the other hand, their fracture toughness was measured by the indentation technique using a micro Vickers hardness tester. It was concluded that these two quantities, the thermal shock strength and the fracture toughness, were closely correlated; a linear relation was shown in a semi-log plane.