Development of Damage-Tolerant Ceramic Matrix Composites (SiC/SiC) using Si-BN/SiC/pyC Fiber Coatings and LSI Processing

Kurzfassung

Silicon-carbide-fiber-reinforced silicon carbide matrix composites (SiC/SiC) exhibit good thermal shock resistance, a low coefficient of thermal expansion and excellent physical properties as well as chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in heavily loaded turbine sections of jet engines. Liquid silicon infiltration was chosen as a technique characterized by short processing times to obtain composites with low porosity in a three-step process: infiltration of fiber preforms with a phenolic resin, pyrolysis and siliconization. Unfortunately, uncoated Tyranno SA3 fibers were cramped in the matrix, resulting in SiC/SiC with low strength and damage tolerance. In order to protect the fibers and to simultaneously provide a weak fiber matrix bonding, a CVD Si-BN/SiC/pyC fiber coating was chosen. The triple coating leads to a twofold higher bending strength of SiC/SiC as well as to more damage-tolerant fracture behavior compared to composites without fiber coating. Composites with various fiber volume contents are compared with regard to their mechanical properties. The microstructure of the composites was characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and computed tomography (CT), especially with regard to the functionality of the fiber coating.