Low-cost preparation of Si3N4–SiC micro/nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix

SiC/Si3N4 nano/micro composites were prepared from a mixture of α-Si3N4, amorphous carbon (carbon black), and Y2O3 by carbothermal reduction of SiO2 present at the surface of Si3N4 matrix grains, or added deliberately to the starting mixture. A special heating regime allowed the outgassing of CO(g) (a product of carbothermal reduction) and reduced the residual porosity to less than 2%. The inter- and intragranular SiC inclusions containing the amorphous oxygen-rich layer at the interface between SiC and the Si3N4 grains were present, originating from the reaction of free carbon with the silica melt. The reaction consumes silica in the grain boundary phase. The change of the grain boundary chemistry influences both room and high temperature properties of the nanocomposite.