Kabel, Joseph; Hosemann, Peter
PhD Tesis (2020)
Silicon carbide / silicon carbide (SiC/SiC) composites have emerged as a leading candidate for accident tolerant fuel cladding in light water reactors (LWR)s. Their high temperature strength and oxidation kinetics offer marked improvement for enduring severe loss of coolant accidents, thereby providing additional coping time for proper response. A key factor for qualification and implementation of these composites is the ability to model and predict their behavior over ranges of temperature, irradiation, and cyclic fatigue. To do so requires a fundamental understanding of the constituent property evolution and interplay at the fiber/matrix interphase. This dissertation asserts that small-scale mechanical testing provides the resolution and versatility required to capture structure-property relationships of the interphase, at its engineered length scale. This was explored by developing four novel experiments to probe the interphase properties that control composite toughness. First, high-resolution SEM DIC quantified the microscale elasticity across the pyrolytic carbon (PyC) bond layer, finding a gradient in Young’s Modulus and Poisson’s ratio that is directly linked to the PyC graphitic texture. Second, application of a self-aligning microtensile test enabled reliable extraction of the tensile strength and weakest link characteristics of the SiC/PyC/SiC interphase. Third, micropillar compression was used to evaluate 11 composite interphase conditions, defining a phenomenological equation for ultimate shear strength as a function of fiber roughness, PyC thickness, and residual compressive stress normal to the fiber surface. The effects of irradiation and fabrication-induced defects were also quantified. And fourth, a novel fiber fretting technique was developed for direct extraction of the cyclic degradation at the fiber/matrix interphase. Testing across four conditions revealed friction dependence on adhesive and abrasive mechanisms up to 1000 cycles. Post hoc characterization of the tribo-surface revealed a crystalline to amorphous transition of the PyC structure. The relatively accessible lab equipment and straightforward test configurations lend themselves to application in high temperature and oxidative environments, offering a robust toolkit for composite characterization and integrated design feedback on irradiated and next generation composite concepts. It is hoped that this research will open new doors to advanced optimization and modelling opportunities in the field of ceramic composites.