Microstructural evolution during silicon carbide (SiC) formation by liquid silicon infiltration using optical microscopy

Optical microscopy and quantitative digital image analysis were used to examine the formation of fully dense, net shape silicon carbide by liquid silicon infiltration (LSI) of porous carbon preforms. By examining the phase distribution and structural changes during the reaction, we identified six reaction stages (I–VI) that describe reaction mechanisms and their time scales. The initial stages (0–15 min) of the LSI reaction include (I) liquid silicon infiltration of the carbon preform, (II) dissolution of carbon, and (III) formation of silicon carbide at the liquid–solid interfacial regions. These initial stages occur simultaneously and very rapidly, and culminate in (IV) the completion of a continuous silicon carbide layer of about 10 μm at every liquid–solid interface. Further reaction can only be achieved by (V) carbon diffusion through this layer. The reaction is essentially complete after ∼120 min. Longer reaction times should be avoided because over-reacting causes (VI) long, thin silicon-filled cracks to develop within the continuous silicon carbide matrix.