Crystallization behavior of polymer-derived Si-O-C for ceramic matrix composite processing

To facilitate the development of matrix processing protocols for SiC-based ceramic matrix composites (CMC) using the polymer infiltration and pyrolysis (PIP), the crystallization behavior of a commercial Si-(O)-C polymer derived ceramic (PDC) was studied using fiberless matrix monoliths replicating the microstructure features common in PIP CMC matrices. The study examined how the processing time and temperature impact the apparent bulk (average) crystallinity as well as how microstructure features lead to variations in the local crystallization rates. The first stage of crystallization involves rapid β-SiC precipitation throughout the bulk of the material. This stage appears to be insensitive to the local microstructure. Subsequent crystallization stages involve partial decomposition of PDC, producing CO and/or SiO. Because these processes involve gaseous exchange with the atmosphere, they are more directly influenced by the local microstructure. Therefore, although the bulk crystallinity quickly reaches an intermediate plateau corresponding to partial phase separation, the near-surface regions are fully crystallized more quickly than the interior regions. The implications of these effects are discussed in the context of the processing and performance of SiC-CMCs.