The sintering behavior of the SiC-7wt% $Al_2O_3$ was investigated. Relative density as high as 98% was achieved by sintering at 2000℃ for 1.5h. In this system, in order to examine the effects of free carbon and surface silica layer contained in the as-received powder on the densification, a series of experiments were conducted.
First, the compacts obtained from the as-received powder could be sintered to 96.1% of theoretical density at 2000℃ for 1h with $Al_2O_3$ addition. Second, however, when both of the free carbon and the surface silica layer were removed from the as-received powder, the sintered density was restrictive to 86% of theoretical density at the same sintering condition. At this time, the free carbon was removed by heating the powder in air at 1000℃ for 4h and the surface silica layer was removed by a HF treatment. Finally, on the occasion of sintered specimen from silica layer alone removed, relative density was 95.9% as like as first case. As a result, the silica layer had no effect on the densification of the $Al_2O_3$ doped SiC but the free carbon gave rise to densification through forming the Si-Al-O-C melt.
Variation of the fracture toughness with holding time of the SiC-7wt% $Al_2O_3$ specimen sintered at 2000℃ was also investigated. Fracture toughness increased with holding time and in case of holding time as long as 3h, fracture toughness was $6.3MPa\cdotm^\frac{1}{2}$. Compared with other additives doped SiC, it has improved greatly. This high value result from crack deflection and crack bridging brought due to large platelike α grain formed by β ->α phase transformation at sintering temperature.
