Proceedings of 6th International Symposium. Fracture Mechanics of Ceramics: R-Curve Behavior, Toughness Determination, and Thermal Shock (vol.11), pp. 459-71. Edited by R.C. Bradt, D.P.H. Hasselman, D. Munz, M. Sakai, V. Ya Shevchenko, Plenum Press, New York, NY,1996.

The effect of grain shape on strength variability of alumina ceramics

M.J. Readey

Glass and Electronic Ceramics, Sandia National Laboratories, Albuquerque, NM 87185-0333, USA

Desiderio Kovar

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Abstract

Fine-grained and coarse-grained aluminas containing either equiaxed or elongated grain structures were fabricated from commercial-purity and high-purity alumina powders. Compared to the high-purity aluminas, the commercial-purity aluminas having a coarse grain size and elongated grain structures exhibited significantly more pronounced flaw tolerance and T-curve behavior. Thus, microstructural features other than just mean grain size were found to be important in determining the fracture properties of alumina. T-curve behavior determined from indentation strength tests suggested that only the coarse-grained, elongated-grain alumina had a T-curve sufficient to cause stable crack extension prior to failure, a requirement for any observable improvement in reliability. In the high-purity aluminas as well as the fine-grained commercial-purity aluminas, however, it is likely that little or no stable extension occurs prior failure. Thus strength in these materials is dependent solely on the critical flaw size. Strength tests on polished specimens showed the commercial-purity aluminas had a lower mean strength than the high-purity aluminas and the coarse-grained aluminas exhibited a lower mean strength compared to the fine-grained aluminas. An analysis of the mean strength versus grain size revealed that the differences in the critical flaw size alone could not account for the differences in mean strength.