Post-sintering of reaction-bonded silicon nitride

Reaction-bonded silicon nitride, RBSN, may be used as the starting material for a post-densification process carried out at about 1800°C in nitrogen at atmospheric pressure. The sintering aids used are similar to those employed in hot pressing and pressureless sintering (e.g. MgO, Y₂O₃). They may be added to the silicon powder before nitridation or, in the case of MgO, by diffusing the vapour into preformed RBSN. High densities (98.5% theoretical) have been achieved accompanied by a shrinkage of only 6.2%. The strength at R.T. was about 1000 MN/m²; the strength at H.T. depends on devitrification. The process thus offers complex components to be formed by injection moulding, with strength values after sintering similar to those of hot-pressed silicon nitride.