1. N. Claussen, Advances in Ceramics (eds. N. Claussen, M. Rühle and A.H. Heuer), Vol. 12, p. 325, The Amer. Ceram. Soc., Columbus, OH, 1984.
2. M. Rühle, N. Claussen and A.H. Heuer, p. 352 in Ref. 1
3. K. Tsukuma, K. Ueda, K. Matsushita and M. Masahiko, submitted to J. Am. Ceram. Soc.
4. M.V. Swain, to be published.
5. R.N. Cannon and A.G. Evans, to be published.