Properties of reaction bonded silicon-nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (> 1.2 g/cm3) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59-85% alpha-Si3N4, 7-22% beta-Si3N4 and 7-23% Si2N2O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (alpha-Si3N4) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000-degrees-C or 1400-degrees-C or show slight increase up to 1000-degrees-C or 1200-degrees-C. In some cases a little dip around 1200-degrees-C, then a sharp increase in MOR up to 1400-degrees-C was observed. K(ic) values are almost constant up to 1000-degrees-C, and thereafter increase sharply. Pore size distribution, existence of Si2N2O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour