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

为了提高陶瓷的损伤容限，目前的氧化物陶瓷基复合材料(Ox-CMC)还远未充分发挥其潜力。Ox-CMCs的主要局限性与多晶氧化物纤维的热稳定性有关。在1000°C左右，商用氧化物纤维的强度损失是由微观结构变化引起的，如颗粒生长、缺陷凹槽和晶体相变。此外，由于纤维基质元素的扩散，周围的基质也会影响纤维的热稳定性。这是一个非常令人担忧的问题，因为在Ox-CMCs的加工和可能的目标应用过程中，温度预计会超过1000℃。因此，可以预期，复合材料中的纤维的性质与它们的初始状态不同。因此，本项目的主要目标是了解不同氧化物纤维-基质体系在高温下的微观结构变化和相互作用，以及它们与所得到的复合材料的宏观性能的关系。这一目标应通过实验研究和相场建模相结合来实现。为此，几种氧化铝和莫来石基纤维将在热暴露前后在不同组成的氧化物基质中进行研究。实验部分将包括晶粒度分布和形貌的演变、晶体相变以及元素在纤维和基质之间的扩散。此外，这种微观结构变化的结果将与纤维和复合材料的宏观力学性能有关。同时，建立了不同纤维基质体系的三维各向异性颗粒生长的相场模型。该模型将考虑各向异性、组分化学成分、晶相、偏析和缺陷的存在对氧化物纤维颗粒生长机制的综合影响。关键的模型参数将通过与实验观测结果的比较来确定。为了更好地了解氧化物纤维在不同氧化物基质体系中的热稳定性，这项工作的第二个目标是能够连续预测这些微观结构的变化，以开发具有定制的强度和热稳定性性能的Ox-CMCs。换句话说，建模的结果将被用来根据所使用的氧化物纤维及其目标应用来调整基质组成。这可以拓宽Ox-CMCS的应用范围，提高其可靠性。此外，该项目的结果还可能有助于开发新的氧化物纤维。

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.