High-temperature behavior and degradation mechanism of SiC fibers annealed in Ar and N2 atmospheres

• Published in: Journal of materials science 2016-05, Vol.51 (9), p.4650-4659
• Main Authors: Cao, Shiyi, Wang, Jun, Wang, Hao
• Format: Article
• Language: English
• Subjects: Ceramic fibers; Ceramic matrix composites; Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; Crystal defects; Crystal growth; Crystallography and Scattering Methods; Decomposition; Degradation; Fiber composites; Fiber strength; Heat; Heat treatment; High temperature; Materials Science; Original Paper; Oxygen content; Photoelectrons; Polymer Sciences; Porosity; Scanning electron microscopy; Silicon carbide; Solid Mechanics; Spectrum analysis; Stability; Temperature; Tensile tests; Thermal decomposition; X-ray diffraction
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-016-9780-3

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N₂ at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N₂. In particular, the fibers heat treated in N₂ at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC ₓ O y N z and SiC ₓ O y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N₂ on the decomposition of the SiC ₓ O y N z phase in the fiber surfaces and nitrided case on the decomposition of the SiC ₓ O y phase in the fiber cores, led to higher SiC fiber temperature stability in N₂ rather than Ar.