Abstract-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.[0988]Index Terms-Finite element analysis, mechanical testing, Massachusetts Institute of Technology (MIT) microengine, Si-SiC hybrid structures.

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Abstract--Silicon in single crystal form has been the material of choice for the first demonstration of the MIT microengine project. However, because it has a relatively low melting temperature, silicon is not an ideal material for the intended operational environment of high temperature and stress. In addition, preliminary work indicates that single crystal silicon has a tendency to undergo localized deformation by slip band formation. Thus it is critical to obtain a better understanding of the mechanical behavior of this material at elevated temperatures in order to properly exploit its capabilities as a structural material. Creep tests in simple compression with n-type single crystal silicon, with low initial dislocation density, were conducted over a temperature range of 900 K to 1200 K and a stress range of 10 MPa to 120 MPa. The compression specimens were machined such that the multi-slip <100> or <111> orientations were coincident with the compression axis. The creep tests reveal that response can be delineated into two broad regimes: (a) in the first regime rapid dislocation multiplication is responsible for accelerating creep rates, and (b) in the second regime an increasing resistance to dislocation motion is responsible for the decelerating creep rates, as is typically observed for creep in metals. An isotropic elasto-viscoplastic constitutive model that accounts for these two mechanisms has been developed in support of the design of the high temperature turbine structure of the MIT microengine.Index Terms-single crystal silicon, constitutive model, finite element analysis, microengine.