PROCESSING OF COMPLEX-SHAPED MICRO PARTS BY REACTION-BONDING AND SINTERING OF SILICON NITRIDE

摘要

Paraffin-based feedstocks for low-pressure injection molding containing silicon and varying sintering additive percentage were prepared to mold micro-bending test specimens as well as prototypic components of a micro turbine. After thermal debinding samples were nitrided in forming gas atmosphere (N2 + 5% H2) at 1390°C and subsequently sintered pressureless at temperatures between 1700 °C and 1800 °C for periods of 1 or 2 hrs. It was stated that the additives Y2O3, Al2O3 and MgO not only control the sintering behavior of Si3N4, but also influence the nitridation reaction of silicon After sin tenng densities up to 98% were achieved. The investigation of the mechanical properties - based upon micro-bending bars - revealed values for σ0 between 500 and 1200 MPa. It was found that the micro bending strength is rather limited by the presence of crystalline secondary phases (mainly melilite Y2Si3O3N4) at the surface of the micro specimens, than by the residual porosity. As micro samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si3N4 and volatilization of sintering additives. To obtain micro components with high strength and reliability the occurrence of secondary phases especially at the sample surface needs to be avoided within the processing framework.