Improving the thermal stability of oxide ceramic composites: Study of fiber-matrix interactions by combining experiments and phase-field modeling

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Conceptualized to increase the damage tolerance of ceramics, current oxide ceramic matrix composites (Ox-CMC) are still far from reaching their full potential. The main limitation of Ox-CMCs is related to the thermal stability of polycrystalline oxide fibers. At around 1000°C, commercial oxide fibers show strength loss caused by microstructural changes such as grain growth, grooving of defects and crystal phase transformations. Furthermore, the surrounding matrix can also influence the thermal stability of the fibers due to fiber-matrix element diffusion. This is a very concerning issue since temperatures above 1000°C are expected during the processing and possible target applications of Ox-CMCs. Hence, it can be expected that the properties of the fibers in the composites are different than their as-received state. Thus, the main objective of this project is to understand the microstructural changes and interactions in different oxide fiber-matrix systems at high temperatures and their relation to the macroscopic properties of the resultant composites. This goal shall be achieved by combining experimental study with phase-field modeling. For that, several alumina- and mullite-based fibers will be investigated in oxide matrices with different compositions before and after thermal exposures. The experimental part will cover the evolution of grain size distribution and morphology, crystal phase transformations and element diffusion between fiber and matrix. Furthermore, the result of such microstructural changes will be related to the macroscopic mechanical properties of fibers and composites. In parallel, a phase-field model for the 3D anisotropic grain growth of different fiber-matrix systems will be developed. The model will consider the combined effects of anisotropy, constituents' chemical composition, crystal phases, segregation and the presence of defects on the grain growth mechanisms of oxide fibers. Key model parameters will be determined by comparison with the experimental observations. Having a better understanding of the thermal stability of oxide fibers in different oxide matrix systems, the second objective of this work is to be able to successively predict these microstructural changes to develop Ox-CMCs with tailored properties regarding strength and thermal stability. In other words, the results of modeling will be used to adjust matrix composition in accordance to the used oxide fiber and its target application. This can widen the area of application of Ox-CMCs and improve their reliability. In addition, the results of this project can also potentially help on the development of new oxide fibers.