Effect of annealing conditions on microstructural development and phase transformation in silicon carbide

The effect of annealing with and without applied pressure on the microstruc tural development and phase transformation was investigated in fine-grained beta-SiC ceramics containing alpha-SiC seeds. Materials annealed without p ressure had a microstructure consisting of elongated grains, while material s annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressu re (25 MPa) was found to greatly retard phase transformation from beta --> alpha polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth migh t be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.