1. N. CLAUSSEN and M. R. RÜHL, in ?Science and Technology of Zirconia in Advanced Ceramics?, Vol. 3, edited by A. H. Heuer and W. Hobbs (American Ceramic Society, Columbus, Ohio, 1981) p. 137.
2. N. TOSHIKAWA and H. SUTO,J. Jpn Inst. Metals 50 (1986) 108.
3. N. NAKANISHI, T. SHIGEMATSU, T. SUGIMURA and H. OKINAKA, ?Zirconia Ceramics?, Vol. 8 (Uchida Rokkakudo, Tokyo, 1987) p. 71.
4. Idem,,J. Jpn Soc. Powder Powder Metall. 33 (1986) 356.
5. C. A. LEACH,J. Mater. Sci. Lett. 6 (1987) 303.