Thermal cycling of siliconized-SiC at high temperatures

Thermal cycling (between 1340 °C and 1480 °C) experiments were conducted using two types of reaction-bonded (siliconized) silicon carbide. A commercial material (Crystar^TM) and various silicon carbide pieces that had been joined together using electrophoretic deposition (EPD) followed by reaction bonding were evaluated. During the thermal cycling, residual free silicon metal rapidly vaporized from the Crystar^TM and cracks developed within its large SiC grains. In contrast, the EPD/reaction-bonded silicon carbide joints did not lose an observable amount of their residual silicon nor develop cracks. The reduced loss was attributed to reduced silicon content with the silicon residing largely in closed pores of the EPD layer. Reduced vaporization of the silicon that resided in surface-connected pores was engineered by applying a thick SiC surface coating. The morphology of the resulting coating was microscopically evaluated and two sequential growth mechanisms were postulated. An implication of this research is that hermetic (gas-tight) joints could be formed using EPD-derived SiC as a filler material.