Sintering of silicon carbide II. Effect of boron

The results of investigations on the role of carbon in SiC sintering, presented in the first part of this paper, indicate that this additive prevents the mass transport mechanisms ineffective in densification. Thereby they suggest that porosity is eliminated due to boron. In order to verify such a hypothesis we measured the kinetics of SiC sintering at constant temperature of 2150 °C. The samples used in this study, contained carbon at a constant concentration of 3 wt.% and boron up to 4 wt.%. It was found that at the carbon concentration of 3 wt.% the optimum addition of boron was 0.2–0.5 wt.%. Although the results of kinetic measurements did not allow for a unanimous identification of the mass transport mechanism, they revealed an unexpectedly high rate of densification. Within 60 s the system reached 0.9 theoretical density. Microstructural observations combined with the results of kinetic measurements suggest that boron activates the sintering process by promoting the formation of a liquid phase, Si–B–C.