Poster Spotlight: In-situ High-Temperature Fracture Behavior of Polymer-Derived SiC Fibers for Extreme Environment

HyukJun Lee ^{2, 1} , YoungJin Shim^{2, 1}, YoungKeun Jeong², KwangYoun Cho¹, YoungJun Joo¹

1. Korea Institute of Ceramic Engineering and Technology, Jinju-si, Korea (the Republic of).
2. Department of Applied Hybrid Materials, Pusan National University, Pusan, Pusan, Korea (the Republic of).

HyukJun Lee ^{2, 1} , YoungJin Shim^{2, 1}, YoungKeun Jeong², KwangYoun Cho¹, YoungJun Joo¹

1. Korea Institute of Ceramic Engineering and Technology, Jinju-si, Korea (the Republic of).
2. Department of Applied Hybrid Materials, Pusan National University, Pusan, Pusan, Korea (the Republic of).

Silicon carbide (SiC) fibers and its woven fabrics have been proposed as reinforcing materials of ceramic matrix composites (CMCs) for aerospace materials due to their excellent mechanical properties such as tensile strength (≥2.45 GPa) and tensile modulus (220 GPa) up to high temperatures above 1573K. In general, the high-temperature evaluation of polymer-derived SiC fibers proceeded at room temperature after heat treatment under inert gases or air gas, which does not reflect environments of space reentry or hypersonic impact. Therefore, in order to simulate and evaluate the extreme environment, the in-situ tensile strength of the SiC fiber was measured while a thermal-shock of Δ1773K in the air. The in-situ fracture behavior of polycrystalline SiC fibers was analyzed through the customized measurement equipment to investigate the effects of impurity phase or microstructure at high temperature. And the in-situ tensile strength was measured while thermal-shock to temperatures of 1273K, 1573K and 1773K in air without the heating rate. In addition, thermal-exposure time at a high temperature controlled from 5 to 120 min. The tensile strength of SiC single-filament measured by the in-situ measurement method was immediately decreased from 3.0 GPa to 2.45 GPa, 1.9 GPa, and 1.5 GPa after thermal-shock at 1273K, 1573K and 1773K, respectively. After thermal-exposure at 1773K for 120 minutes, the tensile strength of SiC single-filament was continuously reduced to 0.87 GPa. In the in-situ measurement method, the stress-strain curve of SiC fibers showed three-step behavior such as thermal expansion, elastic deformation, and ductile deformation compared to the general measurement method. In particular, the fracture surface in SEM images showed a sharp morphology like the tip of a pencil, indicating ductile deformation. In XRD and TEM, the crystal size of β-SiC was maintained up to 1773K, but the proportion of SiO₂ phase ratio increased gradually. In addition, the peak of SiO₂ was easily observed after thermal-shock at 1773K, but it began to form when thermal-shock at 1573K for more than 120 min. The formation of an oxide layer on the fiber surface did not have a significant effect on the high-temperature tensile strength, but the excess carbon present on the surface was a source of destruction because it formed a defect at the interface between the oxide layer and the fiber surface when exposed thermal-shock to high temperature. Consequently, it is expected that the in-situ high-temperature measurement method more closely simulates the environment in which polycrystalline SiC fibers are actually used in extreme environments and suggests the microstructural solution for next-generation polycrystalline SiC fibers.