Fig xA Scanning electron micrographs showing
It is generally argued that oxide ceramics are prone to slow crack growth because of stress corrosion cracking assisted by water molecules. Indeed, the slow crack growth sensitivity of a given material depends on its affinity to water. Materials presenting ionic bonds are more sensitive to water than AGN 192403 materials. However, when failure occurs at the grain boundary of covalent materials (as it is the case of the two LP-SiC materials of this study), it is the grain boundary chemistry what dominates the sensitivity to water, hence to slow crack growth. There was no evidence of plateau value, i.e. region II, where the crack velocity becomes independent on the applied stress, as corrosion is controlled by the diffusion rate at which water can reach the tip of the crack. Region II develops approximately at crack velocity above 10−4 ms−1, whereas, the measurement of the v-KI of this work was limited to crack growth rates below 10−4 ms−1 and this may explain the absence of region II. In contrast, for the SS-SiC there is no difference in the crack velocities measured in air and in water and the very steep slope of the graphs suggest that this may correspond to fast fracture (region III) only (no effect of water molecules on the crack propagation, even for slow crack rates).