Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 2: Sintering behaviour and micro-mechanical properties

This second part of the report deals with, how the sintering additives Y₂O₃, Al₂O₃, and MgO influence the sintering behaviour of SRBSN. Paraffin-based feedstocks with varying sintering aid compositions and silicon grain size were used for moulding macro- and micro-scale samples. It was observed that compositions with smaller Si grain size (with correspondingly high SiO₂ content) and containing Al₂O₃ as sintering additive exhibit higher shrinkage and lower residual porosity when sintered at 1700–1800 °C after nitridation. The mechanical properties determined for micro-scale samples were obtained by three-point bending tests, with the resulting characteristic strength values σ₀ ranging from 500 MPa up to 1200 MPa. Surprisingly residual porosity did not play the role of a strength limiting factor; rather it was observed that the presence of crystalline secondary phases – mainly Y₂Si₃O₃N₄ – was responsible for reducing the micro-bending strength. As micro-samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si₃N₄ and volatilization of SiO₂ which is considered to be responsible for the occurrence of secondary phases preferred at the sample surface. The powder bed condition was also found to play a prominent role in the development of the secondary phases during liquid phase sintering.