dissociating. This mechanism is predominant for highly ionic solids, e.g. for magnesium halides. Dynamic fatigue testing of various alumina ceramics inIn alumina ceramics processing, sintering additives are humid air, water, acidic, and alkaline environments was used which form specific grain boundary phases that can performed. It is shown that the fatigue resistance of influence or control the subcritical crack growth process. glass bonded alumina in air and in water is significantlyThe main component of this phase is a silicate glass for reduced compared with that for high purity magnesia glass bonded aluminas, or magnesia for high purity alumina doped alumina. T he crack growth velocity exponent is ceramics. The response of the grain boundaries to the stress independent of the composition of the glassy grain enhanced chemical attack of environmental species depends boundary phase, as well as of the water content in the on the composition of both the additive and the environenvironment. Hydrochloric acid slightly lowers the ment, and seems to be complicated. In this context, crack growth velocity exponent for yttria-alumosilicate the present work was aimed at the investigation of some glass bonded ceramics, whereas the exponent decreases aspects of subcritical crack growth in various alumina drastically for the magnesia doped material in an acidic based ceramics. environment. Possible mechanisms controlling slow crack growth in the ceramics are outlined. BCT /373

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