Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiCf/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated. Comparing the microstructures of SiCf/C carbon preform by a one-step pyrolysis process at 600 degrees C and twostep pyrolysis process at 600 and 1600 degrees C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiCf/SiC composites manufactured by the LSI process at 1450-1550 degrees C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiCf/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process. In addition, among the SiCf/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 degrees C showed 30% higher flexural strength on average than those at 1450 or 1550 degrees C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiCf/SiC specimens in which the LSI process was performed at 1500 degrees C was higher than that of the 1550 degrees C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.

Automated summary

SiCf/SiC composites fabricated through a two-step pyrolysis process and LSI process exhibit increased density, reduced porosity, and higher flexural strength compared to those fabricated through a one-step pyrolysis process. Among specimens processed with the same two-step pyrolysis, those subjected to LSI at 1500 degrees Celsius show significantly higher flexural strength than those at 1450 or 1550 degrees. Additionally, mechanical properties of SiCf/SiC specimens processed at 1500 degrees Celsius are superior to those at 1550 degrees Celsius despite similar porosity and density due to the rapid degradation of Tyranno-S grade SiC fibers at higher LSI process temperatures.