EVIDENCE OF GRAIN-BOUNDARY-SLIDING-INDUCED CAVITATION IN CERAMICS UNDER COMPRESSION

Detailed microscopy of two crept aluminas, one with (AD99) and one wit hout (Lucalox) a grain boundary glassy phase, has been performed to de termine the pertinent damage mechanisms during creep. Evidence is pres ented for a nucleation-controlled cavitation process where creep cavit ies nucleate primarily on two-grain facets, followed by cavity growth and coalescence to form grain-facet-sized cavities and microcracks. A variety of creep cavity morphologies were observed in Lucalox, includi ng spheroidal and irregularly shaped cavities. The latter finding impl ies a strong influence of crystallographic orientation and the corresp onding surface energy of the cavitated planes on the cavity shape. In contrast, classical spheroidal cavities were observed in AD99 due to t he presence of a viscous phase along grain boundaries. Direct evidence for grain boundary sliding as the process driving force for cavitatio n in Lucalox is presented together with evidence for the nucleation of creep cavities at grain boundary ledges. These findings are compared to the grain boundary sliding (GBS) and small-angle neutron scattering (SANS) measurements performed previously on the same systems. Based o n this study, the cavity nucleation process in the glassy-phase- and n on-glassy-phase-containing aluminas is apparently similar as both invo lve the nucleation of rows of equally sized and equally spaced cavitie s.