Stability of alumina- and mullite-based fibers by thermal exposure: experimental study and phase-field modeling

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The efforts to achieve high strength and damage tolerant oxide-based ceramic matrix composites (Ox-CMCs) were, for a long time, focused on searching for new matrix systems and proceeding routes, as well as on the adjustment of the fiber-matrix-interface. Nowadays, such "conventional" methods seem to have reached the limits of their capacity.In the last decade, several studies on oxide fibers and composites revealed another way to enhance the performance of Ox-CMCs by optimal utilization of the oxide fibers. Being used as reinforcements, these fibers are responsible for the mechanical performance of the composites. As it is well-known, oxide fibers present an excellent strength and stiffness in as received conditions. However, commercially available polycrystalline fibers show strength loss when they are exposed to elevated temperatures due to grain growth. Thus, attention has been given to this subject, since such temperatures can be easily reached during processing and application of Ox-CMCs.The goal of the present proposal is to understand the mechanisms and to successively predict microstructural changes of alumina- and mullite-based fibers embedded in ceramic matrices depending on the matrix composition, the processing conditions, and additional heat treatments performed. This will be achieved by combining experiments and phase-field modeling as necessary to bridge the different effects that are involved. Furthermore, the effect of the microstructural changes on the quasi-static and long-term performance of the fibers at high temperatures should be evaluated.For that, minicomposites comprised of one fiber bundle are to be manufactured under different conditions. The grain size distribution and morphology of the fibers in dependency on the matrix composition will be experimentally investigated after exposure to elevated temperatures and mechanical load. The corresponding changes of the strength and the creep performance will be investigated by means of mechanical characterization techniques and correlated to the microstructure evolution. In parallel, the effective growth mechanisms will be studied through phase-field modeling combined with the feedback from the experimental results. The model will take into account the anisotropy of crystal growth, the grain boundary diffusion, the formation and evolution of impurities and pores. Special attention will be given to the abnormal grain size distribution and possible diffusion mechanisms between fiber and matrix.As a main result, it is expected that the enhanced predictability of the fiber properties (in composites) will show a way to determine the matrix composition used. By doing so, it will be possible to reduce the strength loss of the ceramic fibers during composites lifetime. Consequently, an adjusted matrix composition will provide flexibility in the tailoring of composite properties to the target application in terms of strength and durability.