Abstract
Compairing the literature values of fracture toughness of various materials revealed that the value of the same kind of material scatters largely, depending on the authors or measuring ways, but that the temperature dependence is very small and almost common, particularly in the range RT through 1, 200 °C, for most ceramics in spite of the large scattering in the absolute value. The temperature coefficient, (-(dKIc/dT)/KIc(RT)), is(-2. 1-1.6)10-5/K for SiC ceramics, and(0-1.3)10-4/K for Si3N4. The unexpectedly small temperature dependence was explained on the basis of the dependence of the surface free energy, rs.which was found to be approximated by the empirical formular (-(dγs/dT)/γso = 1/(2.1Tm) ; Tm is melting or sublimation point ) and that of Young's modulus, E. On this basis, the value of KIc at 1, 600 °C is expected not to be less than 70% of the value at RT, if the material is not affected by low heat resistant boundary material. Furthermore, the literature values are found to be 2 to 6 times larger than the ideal value calculated from (2γsE)1/2. This large value is related to the generation of rough fractured surface.
