Variation of porosity and pore size during post-sintering of reaction-bonded silicon nitride doped with Y₂O₃ and MgO additives

Abstract

Sintering of reaction-bonded silicon nitride (SRBSN) is a method of preparing Si₃N₄ ceramics with high thermal conductivity and good mechanical properties. In the present study, by using a high purity silicon powder as the starting material and 2 mol % of Y₂O₃ and 5 mol % of MgO as sintering additives, the phenomena including phase transformation, grain growth, densification, and pore size evolution occurred in the SRBSN process were studied. It was found that the α- to β-Si₃N₄ phase transformation could complete at 1700°C. Obvious grain growth started at around 1700°C. Substantial densification took place at temperatures between 1800 and 1900°C. When sintered at 1900°C for 30 min, not only the total number of pores drastically decreased as a result of the shrinkage of the compact, but also a fraction of open pores turned to be closed pores due to microstructural changes such as grain growth and pore coalescence.