MICROSTRUCTURE STUDIES OF POLYCRYSTALLINE REFRACTORY OXIDES

Very fine grained, highly dense, pure alumina prepared by higher pressure sintering in graphite dies shows strengths which are less than would be predicted on the basis of decreasing grain size. Such structures appear to be densely packed but with weakly bonded grains, as evidenced by predominantly intergranular fractures. Preliminary results show strengths in excess of 90,000 to 110,000 psi for pure alumina prepared by ceramic die pressure sintering, even with 3% porosity. Creep studies of dense, pure, fine grained alumina revealed a transition in strain rate - stress dependence at about 2000 psi from a linear to a square dependence. An activation energy of 167 k cals/mol was found. Calculated diffusion coefficients were several orders of magnitude higher than self-diffusion values. An additional vacancy creation creep mechanism was suggested which would be independent of and supplement creep due to self-diffusion, to explain the high coefficients and high activation energy.