YANG Chenxi, HU Jiangbo, LIU Changgang, WANG Li, LIU Pengfei, HUANG Baoqing
(AECC Aviation Power Co., Ltd., Xi'an 710003, Shaanxi, China)
Extended Abstract:[Background and purpose] SiCf/SiC ceramic matrix composites exhibit various advantages, such as low density, high strength, excellent high-temperature resistance and superior oxidation resistance. Moreover, their toughness could be enhanced through mechanisms like fiber bridging, fiber pull-out and interfacial debonding, making them ideal candidate as high-temperature components in advanced aero-engines. The mechanical properties of SiC fibers at elevated temperatures significantly influence the service life of SiCf/SiC composites in aero-engines. Therefore, investigating the evolution of tensile strength of SiC fibers after exposure to high-temperature environments is crucial for assessing their suitability as reinforcing materials in these composites. In this study, the domestic second-generation SiC fibers with low-oxygen and high-carbon content were subjected to heat treatment at 1050 ℃ and 1350 ℃ for durations of 0.25 h, 1.00 h, 4.00 h, and 16.00 h, in a high-temperature argon environment. The objective was to investigate the effects of heat treatment temperature and durations on tensile strength of the fibers and analyze the associated strength degradation mechanisms, thereby elucidating the impact of high-temperature environments on fiber strength.[Methods] The fibers were first subjected to a one-hour air heat treatment at 500 ℃ to remove the polyvinyl alcohol sizing agent from the fiber surface. Subsequently, the fibers were sealed in a quartz glass tube filled with argon gas (pressure: 0.2×105 Pa) to establish the high-temperature argon environment. The sealed quartz tubes were then placed in a box-type electric furnace for heat treatment at 1050 ℃ and 1350 ℃, for durations of 0.25, 1.00, 4.00, and 16.00 h. Tensile tests on multiple groups of SiC fiber monofilaments in different states were conducted using a dynamic mechanical analyzer. The gauge length of the fiber specimens was 5 mm and the tensile rate was 1 mm·min−1. The phase composition of the SiC fibers before and after high-temperature argon gas treatment was analyzed using X-ray diffraction, in a scanning range of 10°–90°, at a scanning speed of 6 (°)·min−1. Tensile fracture morphology of the SiC fibers was observed and analyzed using a scanning electron microscope (SEM). Before the observation, the SiC fiber fractures were ultrasonically cleaned to ensure accuracy.[Results] The tensile stress-strain curves of the as-received SiC fibers exhibit dispersion and all show a linear relationship, indicative of typical brittle fracture characteristics. The strength ranges from 2.22 GPa to 4.34 GPa with a standard deviation of 0.57, while the strain is between 0.95% and 2.05%. Both the arithmetic mean strength and Weibull mean strength are 3.08 GPa. The fracture origin region of the as-received fibers is located on surface of the fibers, whereas the fracture morphology consists of the fracture origin region, mirror region, fog region and feather region. After exposure to argon at 1050 ℃ and 1350 ℃, the overall strength of the fibers decreases and the Weibull modulus is lower than that of the as-received fibers, indicating increased strength dispersion after the heat treatment at the temperatures. Specifically, the fibers treated at 1350 ℃ exhibit lower strength and higher strength dispersion than those treated at 1050 ℃. After heat treatment at 1350 ℃ for 16 h, the fiber strength reduces to 1.67 GPa, corresponding to a strength retention rate of 54%. Compared with the as-received state, the fracture morphology of the SiC fibers does not change significantly after high-temperature argon environment heat treatment. Before and after heat treatment, the main phase composition of the SiC fibers remains predominantly β-SiC, with the presence of an amorphous SiCxOy. The grain size of the SiC fibers was calculated using the Scherrer formula. Initially, the grain size is 3.4 nm. After heat treatment in an argon atmosphere at 1050 ℃ for 4 h and 16 h, the grain sizes increase to 3.7 nm (8.8%) and 3.9 nm (14.7%), respectively. After heat treatment at 1350 ℃ for 4 h and 16 h, the grain sizes increase to 4.2 nm (23.5%) and 4.5 nm (32.4%), respectively.[Conclusions] After SiC fibers were treated in an argon atmosphere at 1050 ℃ and 1350 ℃ for 0.25 h, 1.00 h, 4.00 h and 16.00 h, the fiber strength generally exhibited a downward trend with increasing heat treatment time at each temperature. For the same heat treatment duration, fibers treated at 1350 ℃ showed lower strength than those treated at 1050 ℃. Due to the phase scale of the fibers being much smaller than that of intrinsic defects such as surface cracks, surface pores and "tumor-like" adherents, the fracture morphology did not change significantly. It is concluded that temperature has a more significant effect on the grain growth of SiC. Additionally, the combined effects of SiCxOy phase decomposition, ordering of free carbon atoms and the release of SiO and CO gases in the high-temperature environment collectively contributed to the observed decrease in fiber strength.
Key words: SiC fiber; low-oxygen and high-carbon; tensile strength; heat treatment; argon atmosphere