EVALUATION OF SLOW CRACK-GROWTH RESISTANCE IN CERAMICS FOR HIGH-TEMPERATURE APPLICATIONS

The slow (subcritical) crack growth (SCG) resistance of Si3N4 and SiC ceramics has been evaluated by a stepwise-loading test on bending bars precracked by Vickers indentation. Three highly refractory materials were selected for the evaluation: i.e., (1) high-purity Si3N4 sintered by hot isostatic pressing (HIP) without additives and (2,3) alpha- and beta-SiC pressureless sintered with B and C addition. Under the hypothesis of linear elastic behavior at high temperature, which was found satisfied in the present materials, the SCG resistance was expressed in terms of initial stress intensity factor critical for SCG failure within a predetermined lifetime. The present method was found useful in shortening the testing time and consistent with other traditional fatigue tests (e.g., static-fatigue test): It is recommended as a screening test for materials under research and development. Among the materials tested in the present study, the highest SCG resistance up to 1440-degrees-C was found in the high-purity Si3N4 without additives.