Thermal Stability of Polycarbosilane‐Derived Silicon Carbide Fibers under Reduced Pressures

Abstract

Three types of polycarbosilane-derived SiC fibers-Nicalon, Hi-Nicalon, and Hi-Nicalon S-were exposed at temperatures of 1573-1773 K under a reduced pressure of 1.3 Pa. The thermal stability of the fibers was investigated through examinations of the gas evolution, grain growth, specific resistivity, fiber morphology, and tensile strength. The thermal decomposition of the silicon oxycarbide phase began at 1523 K; then, active oxidation of the ␤-SiC crystallites occurred at >1673 K. The active oxidation caused serious damage to the fiber structure, which resulted in significant degradation of the fiber strength. Hi-Nicalon had a tensile strength of ϳ0.5 GPa after exposure at 1773 K, although Nicalon and Hi-Nicalon S fibers completely lost their strength, even after exposure at 1673 K. Hi-Nicalon fiber had relatively good thermal stability under reduced pressure.

Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

“…Several studies have investigated the thermal stability of SiC fibers in oxidizing environments and the effects of oxidation on SiC fibers and its relation to subcritical crack growth (delayed failure) [24,43,49,105,111,121,[123][124][125][126][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158]. LaraCurzio modeled the effects of oxidation on the stress-rupture behavior of fiber bundles and presented results for Nicalon T M fibers.…”

“…It has been shown that the partial pressure of the oxygen has a large effect on the oxidation of SiC fibers [144,[148][149][150]154]. Shimoo et al found that SiC fibers experience active oxidation at low oxygen partial pressure and passive oxidation at medium and higher partial pressures [144,150].…”

Hot Corrosion, Oxidation and Their Effects on the Tensile Strength of SiC Fiber in Alkaline Melts

“…The large bubbles likely formed when the smaller ones coalesce as they moved outward. Considering the Na 2 CO 3 case, the acidic SiO 2 is readily attacked by Na 2 O formed from the dissociation of Na 2 CO 3 by reaction (3).…”

“…SiC f /SiC composites reinforced with high performance continuous SiC fibers, retain exceptional strength and creep resistance to very high temperature and remarkably resistant to oxidizing environment (passive oxidation: oxygen partial pressure higher than 10 3 Pa at 1500 C). For these reasons, SiC f /SiC composites are being considered increasingly as promising candidate for the application of high temperature technologies, such as various engines and gas turbine structural components.…”

“…However, the mechanical properties and microstructure of reinforced fibers are very sensitive to the service environments. [3][4][5][6] The oxidation resistance of SiC f /SiC composites is strongly dependent on their passivation (formation of SiO 2 film) in high temperature environment. However, in combustion conditions, the production of alkaline (basic) melts can dissolve the SiO 2 film and subsequent oxidation can result in the degradation of the microstructure and mechanical properties of SiC f /SiC composites.…”