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.