Sintering of Silicon Carbide and Theory of Sintering.

Abstract

In this paper, the three important aspects of SiC materials, that is, the powder synthesis, the sintering process, and the theory of sintering and grain growth are reviewed. They are summarized as follows.(1) SiC precursors were derived from the liquid materials of silicon-tetraethoxide and phenolic resin. Homogeneous mixing of SiO2 and C could be achieved in the precursor. The pure SiC powders without residual carbon were obtained by carbothermic reduction of the precursors. SiC powders were of the single-phase β (3C) type, and sintered with the addition of carbon and a small amount of B.(2) The low-temperature sintering of SiC powder was developed using AlB2 and C additives. These additives could sinter SiC powder at around 1900°C, and promoted elongation of α-SiC grains during sintering. Al atoms from the additive appear to have dissolved into SiC and partly transformed 6H-SiC into 4H-SiC. This transformation caused anisotropic grain growth. The fracture toughness of the sintered SiC materials increased as the aspect ratio and the size of grains increased.(3) New models and theory for sintering and grain growth were proposed. The theory assumed that the total free energy on the surface and grain boundary of grains drove mass transport. The rate equations for sintering and grain growth were derived from the theory, and the sintering behavior involved in joining two grains was simulated. It is found that the grain boundary energy, which was dependent on the additives used, was important for sintering of SiC, and that low grain boundary energy was necessary for stimulation of sintering and suppression of grain growth.