FIBER CREEP AND RUPTURE MODELS FOR DESIGN OF ADVANCED HIGH-TEMPERATURE SIC-BASED CERAMIC MATRIX COMPOSITES

摘要

With the objective of seeking improved temperature capability for SiC/SiC CMC components in future aerospace engines, this presentation discusses the results of recent studies at NASA aimed at understanding the multiple microstructural factors controlling the high-temperature creep strain and rupture strength behavior of current near-stoichiometric SiC fibers and their composites. These polycrystalline fibers include polymer-derived Hi-Nicalon-S, polymer-derived and sintered Tyranno SA3 and Sylramic-iBN, and CVD-derived Ultra SCS fiber. Based on experimental data generated at NASA and within the literature, empirical models have been developed that describe the axial creep strain in the primary and secondary stages for these single fibers as a function of time, temperature, and stress as well as their key microstructural variables. Of fundamental and practical importance is that all of the polymer-derived fibers with nearly equiaxed grains display a long-time steady state creep strain that grows with a stress dependence to the third power and with an inverse linear dependence on average grain size; whereas the CVD-derived Ultra SCS fiber with non-equiaxed grains displays only a primary stage with a time dependence to the 1/3 power and a linear stress dependence. Included in these models are empirically derived creep parameters for each fiber type and their creep activation energy, which for all but the SA3 fiber corresponds to the diffusion energy for carbon in SiC. It is shown that these models also predict the high-temperature axial creep of tows within SiC/SiC CMC (1) when the matrix is either cracked or carries little load due to poor creep resistance and (2) when the fiber tows are not bent or woven, but are axially aligned along the CMC stress direction.