Dependence of strength and toughness of reaction bonded silicon nitride and composites on fabrication routes

Nitridation characteristics of silicon compacts made by uniaxial and isostatic pressing and slip casting, respectively, have been studied. α and β-Si3N4 were the major phases in uniaxially and isostatically pressed specimens, and α phase being over 90%. In slip cast specimens around 20% Si2N2O was detected. The uniaxially pressed and nitrided samples showed a regular rise in MOR and KIC up to 1200°C, falling thereafter. Temperature-independent values were observed for isopressed samples. Slip cast specimens on the other hand showed a rise with temperature, and sharply increased after showing a dip at 1200°C. The MOR and KIC values did not fall at 1400°C. Si3N4 SiC composites show improved values of MOR and KIC when SiC is 5 vol%;Si3N4 BN (3·6 vol%) and Si3N4 TiC (5 vol%) show an inflection in the MOR and KIC curves above 1200°C which do not deteriorate even at 1400°C. Oxidation, pore size distribution and nature of phases present in RBSN and composites have been considered for explanation of the observed behaviour