The formulation for coarse, medium, and fine SiC particles should be 5:1:4. Black SiC is generally selected as the raw material for manufacturing silicon carbide bricks; its chemical composition is as follows: SiC 98.0%, free C 0.5%, Fe 0.2%, and free SiO₂ 0.6%. Manufacturing processes vary depending on the bonding method. Brief outlines of the processes for different bonding methods are as follows:
(1) Silicon nitride-bonded silicon carbide bricks: Green bodies formed from coarse, medium, and fine SiC particles and fine silicon powder are fired in a pure N₂ atmosphere at 1370°C. This produces a brick with an α-SiC skeleton and an α- and β-Si₃N₄ matrix. Small amounts of residual silicon and SiO₂ remain in the matrix; the relative content of these components varies by manufacturer.
(2) Silicon carbide bricks: SiC, silicon powder, and carbon powder are mixed and molded according to a specific ratio, then fired at 1400°C in a reducing atmosphere. In most cases, firing is conducted with the bricks buried in carbon. During firing, a brick with an α-SiC skeleton and a fine-grained β-SiC matrix is produced. The β-SiC is formed through the reaction of fine silicon powder with fine carbon powder during firing; this product also contains small amounts of residual silicon and carbon.
(3) Silicon oxynitride-bonded silicon carbide bricks: When preparing the batch for these bricks, the proportion of fine silicon powder used is lower than that of the silicon carbide.