UC Santa Barbara

SiCf /SiC ceramic matrix composites (CMCs) have the potential to enable significant increases in thermal efficiency of aerospace engines. However, fabrication of dense CMCs that can operate for extended periods at targeted use temperatures (1500°C) remains an outstanding challenge. In this dissertation, fundamental studies on one promising fabrication approach – polymer impregnation and pyrolysis (PIP) – provide new insights on microstructure evolution during matrix processsing. X-ray computed tomography (XCT) is used to elucidate the key underlying phenomena, including fluid flow, fiber movement, bubble formation, and pyrolysis crack formation, during the first PIP cycle in unidirectional fiber beds. New analysis techniques are developed to enable qualitative observations and quantitative metrics of microstructure evolution. The results are used to elucidate coupled effects of capillary number, fiber movement and preferred flow channeling on axial permeability of fiber beds. Additionally, relationships between processing conditions, local fiber bed porosity, fiber movement, and void locations and sizes after both impregnation and curing are identified. Finally, a unified taxonomy of pyrolysis crack geometries and crack structures is developed, and the temporal sequence of their formation is revealed. Effects of local microstructural dimensions on crack spacing, initiation temperature and final hierarchical order are also quantified. Techniques employed in this work as well as the resulting insights on microstructure evolution could be used in development and validation of physics-based models for advancement of PIP processing.