Sintering of Silicon Nitride at Atmospheric Pressure

Abstract

It is well known that highly covalent materials like silicon nitride are difficult to densify by sintering. Even if considerable pressure is applied, as in hot-pressing, sintering aids, e.g.,MgO, Y₂O₃, have to be employed.

Pressureless sintering generally requires a higher temperature than hot-pressing. However, during sintering at such high temperatures a weight loss occurs. This is due to dissociation of the silicon nitride and — what appears so far not to have been fully appreciated — loss of the volatile sintering aid, e.g. MgO. By suitable choice of a powder bed into which the samples are placed, dissociation and volatilization can be suppressed at atmospheric pressure of nitrogen, and high sintered densities can be obtained.

Because of the less drastic decay in strength at high temperatures of samples using Y₂O₃ rather than MgO as sintering aids, a composition 91% Si₃N₄ + 8% Y₂O₃ + 1% MgO has been chosen for the sintering study, High densities were achieved by sintering at 1800°C.

The same sintering aids can also be admixed to silicon before it is subjected to nitridation and final high temperature sintering at 1800°C. This process, which may also be described as “post sintering of reaction sintered silicon nitride”, has resulted in very high room-temperature (~ 1000 MN/m²) with linear shrinkage of only about 6%.

It is also possible to carry out the post-sintering process on reaction-sintered silicon nitride to which no sintering aids have been added in powder form by transferring the sintering aid in the vapour form into the reaction sintered body. In this way silicon nitride of density 3.05 Mg/m³ has been formed, which had a higher strength than the precursor reaction-sintered material between room temperature and 1400°C.