Strength and fracture toughness of in situ-toughened silicon carbide

Fine beta-SiC powders either pure or with the addition of 1 wt % of alpha-SiC particles acting as a seeding medium, were hot-pressed at 1800 degrees C for 1 h using Y2O3 and Al2O3 as sintering aids and were subsequently annealed at 1900 degrees C for 2, 4 and 8 h. During the subsequent heat treatment, the beta --> alpha phase transformation of SIC produced a microstructure of ''in situ composites'' as a result of the growth of elongated large alpha-SiC grains. The introduction of alpha-SiC seeds into the beta-SiC accelerated the grain growth of elongated large grains during annealing which led to a coarser microstructure. The sample strength values decreased as the grain size and fracture toughness continued to increase beyond the level where clusters of grains act as fracture origins. The average strength of the in situ-toughened SiC materials was in the range of 468-667 MPa at room temperature and 476-592 MPa at 900 degrees C. Typical fracture toughness values of 8 h annealed materials were 6.0 MPa m(1/2) for materials containing alpha-SiC seeds and 5.8 MPa m(1/2) for pure beta-SiC samples.