REACTION-FORMED SILICON-CARBIDE

The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide, The control of reaction rate is always a kev issue in reaction processing. We have studied the kinetics and mechanisms of the liquid Si-C reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface. leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, Whereas aluminum is able to impede the reaction through the formation of an inter-facial carbide layer.