The development of microstructure in silicon nitride-bonded silicon carbide

The microstructure of a commercial silicon nitride-bonded silicon carbide ceramic composite, formed via the nitridation of silicon powder-SiC preforms, has been characterised using analytical transmission electron microscopy. It comprised coarse particles of SiC dispersed in a matrix which was a mixture of crystalline phases based on Si₂N₂O and β-Si₃N₄, and an amorphous phase. Qualitative microanalysis of amorphous matrix regions revealed the presence of significant concentrations of aluminium, silicon, calcium and oxygen, while aluminium and oxygen were also readily detectable in both of the surrounding crystalline phases. It is thus suggested that the crystalline phases in the matrix are in fact O- and β-sialons respectively. A mechanism, combining reaction bonding and solution-precipitation in a (Ca,K)-aluminosilicate liquid phase at the reaction temperature, is proposed to describe the development of the matrix microstructure.