CRACK-GROWTH AND ACOUSTIC-EMISSION IN CERAMICS DURING THERMAL-SHOCK

It is recognized that microcracks may contribute to failure of ceramics undergoing thermal shock, although there is little direct experimental evidence. The combination of acoustic emission (AE) and SEM observations provides such evidence. In this work AE data were analyzed after rapid quenching of samples in silicone oil. A thermal shock resistant material, alumina, and a material resistant to thermal damage, advanced zirconia refractory, have been examined. Cracks were detected by analysis of AE amplitudes and durations and their growth was monitored by systematic SEM observations as thermal shocks of increasing severity were applied. Three‐point bending strengths were determined in air after quenching. For the first time SEM images are presented showing early stages of crack initiation for temperature differences less than ΔTcrit, i.e., where the fracture was not believed to occur. Further development of the cracks leads to abrupt strength reduction in alumina and controlled strength loss in zirconia, although AE data did not indicate any particular pattern of catastrophic crack propagation when substantial loss of strength occurred. Copyright © 1990, Wiley Blackwell. All rights reserved