Development of Si-SiC hybrid structures for elevated temperature micro-turbomachinery

The design of the Massachusetts Institute of Technology (MIT) microengine is limited in part by the material capability of Si primarily due to the pronounced thermal-softening and strain-softening at temperatures higher than the brittle-to-ductile transition temperature (BDT), approximately 550°C. In order to circumvent this limitation, it has been proposed to reinforce the Si with chemical vapor deposited (CVD) SiC in strategic locations to create a Si-SiC hybrid microengine turbine spool. Detailed design of Si-SiC hybrid structures for high temperature micro-turbomachinery, however, has been hampered by the lack of understanding of the mechanical behavior of Si and SiC hybrid structures at elevated temperatures and by the unavailability of accurate material properties data for both Si and SiC at the temperatures of interest. In this work, a series of initial thermomechanical FE analyzes have been performed to assess the advantage of the hybrid structures, and to provide structural design criteria and fabrication requirements. Then, the feasibility of the Si-SiC hybrid structures concept for elevated temperature micro-turbomachinery was verified based on more rigorous mechanical testing at high temperatures. Finally, the Si-SiC hybrid spool design was critically reevaluated with regard to creep using a Si constitutive model developed as a separate effort.