Sintering and hot isostatic post-densification of silicon carbide

Silicon carbide has been mixed with additives of boron and/or aluminium with simultaneous addition of carbon, then it has been formed to bending bars by cold isostatic pressing. In point of density, microstructure and strength, the hot densification by pressureless sintering and hot isostatic post-densification showed comparable results as sintering and hot isostatic pressing in the same cycle (Sinter/HIP). For all chosen additives for SiC the obtained sintered density was sufficient for a post-densification. The boron additive caused increased grain growth during sintering and hot isostatic post-densification, which had a negative result on the bending strength. A too high post-densification temperature causes a decrease in strength while considerably coarsening the grain, too. By hot isostatic post-densification with argon as a pressure gas normally the strength level is increased, while the scattering of the data decreases simultaneously. A post-densification in nitrogen as pressure gas shows no advantage; however, surface defects seem to heal by a long-term annealing in N2-gas. If the Sinter/HIP process is controlled, this procedure offers economic advantages compared with sintering and post-densification in separate units. (orig.)