Processing of Reaction-Bonded and Post-Sintered Silicon Nitride Micro Parts

摘要

Components for a micromechanical system comprising micro turbine and micro planetary gear were manufactured of silicon nitride. Owing to the near-net-shape capability and in view of mass production injection molding was applied as forming technology. In the field of Microsystems Technology very narrow tolerances of few μm are required; therefore it was the aim to reduce sintering shrinkage significantly as it is considered to be one of the main sources for size deviation. Reaction-bonded silicon nitride (RBSN) can be produced nearly without shrinkage, but it offers only poor mechanical strength because of high residual porosity. Post-densification by liquid phase sintering of RBSN is an appropriate way to improve the mechanical properties. Even though this is at the expense of increasing shrinkage - and thereby potential size deviation - the total amount of sintering shrinkage is clearly reduced when compared to conventional processing of silicon nitride powder. In the present work a paraffin based feedstock for low-pressure injection molding containing silicon powder and Y_2O_3/Al_2O_3 as sintering aid was applied. Nitridation of the debindered components was performed in N_2/5%H_2 atmosphere (0.15MPa) at a temperature of 1390°C. The reaction-bonding process is driven by a solid/gas phase reaction where nitrogen diffusion into the Si-powder compact limits the Si_3N_4 formation. It was found that for the micro samples already after 2-4 h dwell time 90% conversion of silicon to silicon nitride was obtained. Therefore this process is especially suited for the fabrication of micro components with inherently high surface-to-volume ratio and small wall thickness. After subsequent sintering in pure nitrogen samples could be densified to 97% of theoretical density. XRD measurements confirmed a complete conversion from - to β-Si_3N_4. The overall linear shrinkage from green body to sintered reaction bonded silicon nitride accumulates to 11%.