The authors examine the boric oxide—ammonia route with special stress on the yield and composition of the intermediate addition compound (BN)_x(B₂O₃)_y(NH₃)_z. It has been concluded that B₂O₃ and NH₃ present in the addition compound formed between 350°C and 900°C cannot be further reacted to convert the B₂O₃ into BN and the BN yield remains at around 66%. A formula (BN)_12·7(B₂O₃)_7·5NH₃ has been suggested for the addition compound.

Silicon nitride-silicon carbide (Si₃N₄-SiC) composites were prepared by varying the percentage of silicon nitride at temperatures of 1350 to 1450°C. The mechanical and thermal properties of these composites were determined. The modulus of rupture of the composites increases with increase of temperature whereas the thermal expansion decreases. Composites with 10% and 50% Si₃N₄ have modulus of rupture of 49 and 86 MPa at 1400°C and thermal expansion coefficients (25°–1000°C) of 4·4 × 10⁻⁶ and 3·2 × 10⁻⁶°C⁻¹ respectively.

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% α-Si₃N₄, 7–22%β-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 (α-Si₃N₄) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K_ic values are almost constant up to 1000°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.

The oxidation kinetics of reaction-sintered silicon carbide has been studied over the temperature range 1200° to 1350°C. The material has a bulk density of 3·00 g/cm³ and the unreacted Si content is 22·5% (v/v). The activation energy for oxidation is 28·75 ± 2·61 kcal/mol. It is proposed that the diffusion of oxygen through the growing oxide film is the rate-controlling process.