Unveiling Failure Mechanisms of Harsh Environment Ceramic Materials

As sources of energy have expanded from burning wood and coal to a diverse assortment of modern renewable and nonrenewable sources, including hydrothermal, ultra-high temperature combustion, and nuclear, more robust, structural and protective materials are urgently needed to harness this energy. These materials often require excellent thermal stability, must be chemically inert, and must demonstrate reliable mechanical performance to withstand harsh conditions within many energy generation systems, like a nuclear reactor or combustion engine. Here, thermal barrier coatings (TBCs) and silicon carbide fiber / silicon carbide matrix composites (SiC/SiC CMCs) are considered for their wide suitability for multiple extreme environment energy applications. Both materials have been extensively studied within the last few decades, yet there remain many critical questions about their failure mechanisms in relevant environments and loading modes, which represent barriers to their widespread adoption.

This dissertation elucidates these critical failure mechanisms via digital image correlation (DIC) -enabled mechanical testing of TBC and CMC material systems under relevant environmental conditions up to 1200 °C and loading modes via indentation, wear, and flexural testing. DIC techniques enable quantitative measurements of deformation and strain, which provide boundary conditions for predictive analytical models of material stress state and creep response. Material degradation is further characterized via electron microscopy and acoustic emission monitoring. Importantly, both TBCs and CMCs are multi-component systems composed of high temperature ceramic materials, and their failure mechanisms reflect a complicated accumulation of properties of their individual constituents. Thus, it is critical to characterize and quantify the multiscale thermomechanical properties and multi-component interactions inherent in these hierarchically structured material systems, such as thermal property and residual stress mismatch between coating layers or material constituents, which can exacerbate local stress concentrations and promote degradation. DIC techniques are fully capable of quantifying and tracking this degradation to its source.

Through this work, new materials design paradigms are revealed, which support superior performance within these harsh environments. Key findings relating to the TBC system include the need (i) to minimize thermal property mismatch between coating layers, which exacerbate residual stresses within each layer and promote delamination, and (ii) to optimize the coating deposition methodology to reduce instances of porosity, pre-existing cracks, and solid particles, which can serve as fracture initiation sites. Key findings relating to the CMC system include (i) the need for optimization of the fiber tow architecture to create non-uniform microstructure to disrupt crack growth and (ii) the development of a rule-of-mixtures materials model to describe the nanoscale CMC creep response.