EFFECTS OF ADDITIVE AMOUNT ON MICROSTRUCTURE AND FRACTURE-TOUGHNESS OF SIC-TIB2 COMPOSITES

Powder mixtures of beta-SiC-30 wt% TiB2 containing 5 wt% (ST1), 10 wt% (ST2), and 20 wt% (ST3) Al2O3-Y2O3 as sintering additives were liquid -phase sintered at 1850 degrees C for 1 h by hot-pressing and subseque ntly annealed at 1950 degrees C for 2, 6, and 12 h. These materials ha d a microstructure of 'in situ toughened composites' as a result of th e beta --> alpha phase transformation of SiC during annealing. The int roduction of larger amount of additives accelerated the grain growth o f elongated alpha-SiC grains with higher aspect ratio, resulting in th e improved fracture toughness. The fracture toughnesses of 12-h anneal ed materials with 5, 10, and 20 wt% additives were 5.7, 5.9, and 7.1 M Pam(1/2), respectively. (C) 1998 Elsevier Science Limited and Techna S .r.l. All rights reserved.