Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Effect of Vacuum Heat Treatment on Electron-Beam-Irradiation-Cured Polycarbosilane Fibers

Electron-beam-cured polycarbosilane fibers were heat-treated at 673–1773 K in a tube evacuated to 1.3 × 10⁻¹ Pa and then exposed at 1873 K in argon. The effect of vacuum heat treatment on improving the high-temperature stability of low-oxygen SiC fibers was investigated by examining gas evolution, grain growth, surface composition, tensile strength, and morphology. The fibers heat-treated at <1173 K lost strength, because of the vigorous generation of residual hydrogen. A minute amount of oxygen in the atmosphere caused the active oxidation of SiC during heat treatment at >1673 K, resulting in severe strength degradation for the as-heat-treated fibers. Vacuum heat treatment at 1573 K provided the best characteristics in low-oxygen SiC fibers.     

Application of radiation curing in the preparation of polycarbosilane-derived SiC fibers

Polycarbosilane fibers were irradiated by gamma-rays under vacuum and by electron beam in He gas flow or under vacuum at room temperature. Free radicals on Si and C atoms were produced. Most radicals reacted with each other, causing cross-links between polycarbosilane molecules. Some radicals, which did not contribute to cross-linking, were fairly stable under vacuum or in inert gas at room temperature but oxidized on exposure to air. The number of stable radicals under vacuum could be decreased by annealing. The remaining radical concentration was about 1% after annealing at 513 K. By a combination of radiation curing and annealing, SiC fibers with smaller quantities of oxygen were prepared. The mechanical properties of the SiC fibers showed a high tensile strength of 2.5 GPa after heat treatment at 1773 K. On the other hand, polycarbosilane fibers could be cured by radiation oxidation at room temperature, that is, gamma-ray or electron irradiation in oxygen, and the oxygen content could be well controlled by irradiation dose and dose rate. The SiC fibers obtained by the radiation oxidation had an oxygen gradient from the surface to the center which was dependent on the radiation oxidation conditions.     

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

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 β-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 ∼0.5 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.     

Microstructure and Oxidation Behavior of Silicon Carbide Fibers Derived from Polycarbosilane

Polycarbosilane-derived SiC fibers (CG Nicalon, Hi-Nicalon, and Hi-Nicalon type S) were exposed for 1–100 h at 1273–1673 K in air. Oxide layer growth and changes in tensile strength for these fibers were examined after exposure. The three types of SiC fibers decreased in strength as the oxide layer thickness increased. Fracture origins were located near the oxide layer–fiber interface. The Hi-Nicalon type S showed better oxidation resistance than the other polycarbosilane-derived SiC fibers after exposure in air at 1673 K for 10 h. This result was attributed to the nature of the silicon oxide layer on the surface of the SiC fibers.     

Preparation and characterization of SiC fibers with diverse electrical resistivity through pyrolysis under reactive atmospheres

Due to their fantastic mechanical and high-temperature resistant properties, SiC fibers with electrical resistivity of different orders of magnitude are of great interest for the fabrication of advanced composites with electromagnetic wave absorbent performance as both structural and functional materials. On the basis of well-developed route to prepare SiC fibers, in this work, we demonstrated a facile strategy to fabricate SiC fibers with electrical resistivity of 10⁻¹–10⁶ Ω cm by simply employing hydrogen and ammonia as the reactive atmospheres. The SiC fibers with different electrical resistivity had favorable morphologies, with uniform elemental distribution and stable C/Si ratio towards the fiber interior. Moreover, these fibers exhibited excellent mechanical strength and high temperature performance. Due to the innovative strategy, convenient operation and scalable preparation, the method in this work can be further extended to prepare SiC base fibers with adjustable electrical resistivity from other precursors.     

Microstructural,strength, and creep characterization of Sylramic™, Sylramic™-iBN and super Sylramic™-iBN SiC fibers

The chemical composition, microstructure, strength, and thermal stability of polymer-derived Sylramic? SiC fibers fabricated by Dow Corning and COI Ceramics, Inc., and nitrogen-treated Sylramic? SiC fibers, referred to as Sylramic?-iBN and Super Sylramic?-iBN SiC fibers, were investigated and compared. The baseline Sylramic? SiC fibers fabricated by both vendors as well as the nitrogen-treated Sylramic? SiC fibers are composed mostly of β-SiC (～97 wt%) with small amounts of TiB₂ (～2 wt%), amorphous carbon (～1 wt%) and trace amounts of B₄C. Most of the amorphous carbon is segregated at the core/interior of the fibers. Both baseline and nitrogen-treated Sylramic? SiC fibers have similar grain size and pore size distribution, except for a thin layer of in-situ grown crystalline BN (30–70 nm) on the surface of Sylramic?-iBN and Super Sylramic?-iBN fibers. Wide variation in strength within a batch as well as between batches is observed in both baseline and nitrogen-treated Sylramic? SiC fibers but both types of fibers are microstructurally stable at temperatures to 1800 °C in argon and nitrogen environments compared to Nicalon?-S and Tyranno®-SA SiC fibers. Under the same creep condition, Super Sylramic?-iBN fibers show better creep resistance compared to Sylramic?, Sylramic?-iBN, Hi-Nicalon?-S, and Tyranno®-SA fibers. Possible reasons for strength variability and the mechanism of in-situ BN formation on Sylramic? SiC fibers are discussed.     

Oxidation Behavior of Si-C-O Fibers (Nicalon) under Oxygen Partial Pressures from 102 to 105 Pa at 1773 k

Polycarbosilane-derived SiC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10² to 10⁵ Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 ×10² Pa and actively oxidized under an oxygen partial pressure of 10² Pa. Under oxygen partial pressures from 2.5 × 10² to 10³ Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer. Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was lost under the active-oxidation condition.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

Structural evolution and mechanical properties of Cansas-III SiC fibers after thermal treatment up to 1700 °C

The effects of thermal treatment on the Cansas-Ⅲ SiC fibers were investigated via heating at temperatures from 900 to 1700 ℃ for 1–5 h in argon atmosphere. The composition and morphology of the SiC fibers were characterized and the tensile strength of the SiC fiber bundles was analyzed via two-parameter Weibull distribution analysis. The results showed that the thermal treatment has negligible influence on the microstructure of the SiC fibers at temperatures ≤ 1100 ℃. At temperatures ≥ 1300 ℃, the surface of the fibers became rough with some visible particles. Particularly, at 1700 °C, numbers of holes appeared. With the increasing of heating temperature and holding time, the average tensile strength of the SiC fibers decreased gradually from 1.81 to 1.01 GPa. The decreasing of tensile strength can be attributed to the increase of critical defect sizes, grain growth and phase transformation (β→α) of SiC.     

Formation of carbon-rich layer on the surface of SiC fiber by sintering under vacuum for superior mechanical and thermal properties

SiC fibers have been intensively developed for application in advanced aerojet engines, stationary gas turbines and nuclear reactors of the future. In this work, SiC fibers with controllable carbon-rich layer were prepared by sintering under vacuum, which could be attributed to the release of gaseous silicon under vacuum at high temperature. The thickness of the carbon-rich layer on the fiber surface could be adjusted by changing the sintering temperature. Moreover, the carbon-rich layer was well distributed on the fiber surface, combining closely with the interior of the fiber. The as-prepared SiC fibers had high tensile strength and relatively low elastic modulus, which was favorable for weaving into different fabrics. Furthermore, the fibers also exhibited excellent ultra-high temperature resistance due to the presence of carbon-rich layer on the surface, which was better than that of the Hi-Nicalon and Hi-Nicalon S fibers.     

Preparation of Silicon Carbide Fiber from Activated Carbon Fiber and Gaseous Silicon Monoxide

Crystalline silicon carbide (SiC) fiber was produced by a new, simple procedure. Activated carbon fiber (ACF) was reacted with gaseous silicon monoxide and was converted to SiC fiber at elevated temperature and reduced pressure. The reaction was completed at temperatures as low as 1473 K. The reacted fiber consisted of submicrometer particles which were not observed in the original ACF. The SiC crystal size in the reacted fiber was approximately 30 nm. The microstructure of the fiber became dense after it was heat-treated in air at 1573 K or in nitrogen gas at 1873 K.     

先驱体制备SiC纤维的发展历程与研究进展

碳化硅(SiC)纤维具有高强度、高模量、耐高温、抗蠕变、抗氧化等优异性能,是增强耐高温陶瓷基复合材料的关键材料。介绍了先驱体法制备3代SiC纤维的发展历程:从第1代高氧碳含量SiC纤维发展到第2代低氧高碳含量SiC纤维,再到第3代近化学计量比SiC纤维,SiC纤维的微结构从非晶到微晶显著变化,纤维的耐热性能也显著提高。重点比较了第3代近化学计量比SiC纤维(Hi-Nicalon Type S纤维、Tyranno SA和Sylramic纤维等)的性质,结果表明:SiC纤维的热稳定性由近化学计量比SiC微晶的致密度和微结构决定,Sylramic和Tyranno SA纤维的组成和微结构可通过控制Si-C-O纤维的碳热还原反应来实现,烧结助剂的采用及陶瓷烧结工艺的有效应用可提高纤维的致密度。Hi-Nicalon Type S纤维的组成和微结构取决于聚碳硅烷分解过程中特定的气氛和温度。简介了SiC纤维的研究进展并讨论了其发展趋势。     

Thermal and Electrical Properties in Plasma-Activation-Sintered Silicon Carbide with Rare-Earth-Oxide Additives

This study investigates the thermal and electrical properties of SiC ceramics with a combination of Y₂O₃ and rare-earth-oxide additions as sintering additives, by comparing four types of SiC starting powders varying in particle size and chemical composition. The powder mixtures were plasma-activation sintered to full densities and then annealed at high temperatures for grain growth. The thermal conductivity and electrical resistivity of the SiC ceramics were measured at room temperature by a laser-flash technique and a current–voltage method, respectively. The results indicate that the thermal conductivity and electrical resistivity of the SiC ceramics are dependent on the chemical composition and particle size of the starting powders. The thermal conductivities observed for all of the annealed materials with a rare-earth La₂O₃ sintering additive were >160 W·(m·K)⁻¹, although low electrical resistivity was observed for all materials, in the range 3.4–450 Ω·cm. High thermal conductivity, up to 242 W·(m·K)⁻¹, was achieved in an annealed material using a commercial 270 nm SiC starting powder.     

Analysis of Size Dependence of Ceramic Fiber and Whisker Strength

The strengths of ceramic fibers and whiskers have been observed to increase with decreasing fiber diameter and length. Typically, both surface flaws and volume flaws exist in ceramic fibers and whiskers, which makes it impossible to characterize the strength dependence of both the diameter and the length with a single-modal Weibull distribution function. Our data also show that the single-modal Weibull distribution is inadequate to characterize the strength of fibers with varying diameters even in the case of a constant fiber length. In addition, experimental data also show that, for sapphire whiskers whose surface flaws were removed by chemical polishing, the whisker strength has a much stronger size dependence on diameter than predicted by the single-modal Weibull function, which indicates that factors other than those characterized by the Weibull function also play a role in the strength of sapphire whiskers. In this paper, the factors affecting the strengths of ceramic fibers and whiskers are analyzed in terms of Weibull statistics, fracture mechanics, and flaw size density variation with varying fiber diameters. A three-parameter modified Weibull distribution, which combines the above strength-affecting factors, is proposed to characterize both the diameter and the length dependence for ceramic fibers and whiskers with or without surface flaws. Characterization of the strength data of sapphire whiskers and Nicalon SiC fibers with varying diameters shows the validity of the modified Weibull distribution function.     

碳化硅/环氧树脂导热复合材料的制备与性能

采用浇铸成型法制备碳化硅/环氧树脂(SiC/EP)导热复合材料,研究了SiC种类、粒径、用量和表面改性方法对SiC/EP复合材料的导热性能、力学性能和热性能等影响。结果表明:SiC/EP复合材料的导热系数随纳米级SiC用量增加而增大,当φ(纳米级SiC)=17.80%时,导热系数为0.954 6 W/(m.K);SiC/EP复合材料的弯曲强度和冲击强度随纳米级SiC用量增加均呈先升后降态势,当φ(纳米级SiC)=3.50%时,两者均达到最大值。SiC经表面改性后可有效提高复合材料的导热性能和力学性能,并且改性SiC的加入可有效降低EP的玻璃化转变温度。     

电泳沉积时间对SiC_f/SiC复合材料性能的影响

针对电泳沉积结合先驱体浸渍裂解的方法制备SiC_f/SiC复合材料过程,探讨沉积时间对SiC纤维及SiC_f/SiC复合材料性能的影响规律。实验结果得出:随电泳沉积时间的延长,SiC纤维逐渐被腐蚀,致使其单丝强度下降;而在SiC纤维表面覆盖PyC涂层可以有效地保护SiC纤维,由于悬浮液中的通电作用,Py C涂层与SiC纤维的界面结合强度略有降低,纤维单丝强度随电泳时间的延长先增大后减小。5 min的电泳沉积结合9个周期的PIP得到了SiC_f/SiC复合材料最大的弯曲强度为731 MPa,随后其力学性能随着沉积时间的延长先降低后略微回升;SiC_f/SiC复合材料的热导率随沉积时间的延长先增大后减小,10 min电泳沉积得到了常温下SiC_f/SiC复合材料的最大热导率为4.658 W/(m·K)(25℃)。     

Rapid Preparation of SiC Fibers Using a Curing Route of Electron Irradiation in a Low Oxygen Concentration Atmosphere

Polycarbosilane (PCS) fiber was irradiated by electron beam at low dose in a flowing N₂/O₂ mixture with O₂ concentration of 1%. After the pyrolysis of the irradiated precursor fibers, SiC fibers with high strength of 2.4 GPa were obtained. Microstructural evolutions of the resultant fibers were explored. It was found that during the irradiation, free radicals were formed in the PCS and were oxidized by oxygen as Si–OH groups. The Si–OH groups then transformed into Si–O–Si linkage and resulted in further cross‐linking of the PCS during pyrolysis. A remarkable structure gradient along the fiber diameter was formed under the coupled effects of irradiation and oxidation. The content of oxygen decreased from the fiber surface to the core, whereas the crystallinity of β‐SiC increased in the same direction. The electrical resistivity of the as‐prepared ceramic fiber was 80.7 Ω cm, showing good potential for being as electromagnetic wave absorber.     

Additive manufacturing of high mechanical strength continuous Cf/SiC composites using a 3D extrusion technique and polycarbosilane-coated carbon fibers

A novel additive manufacturing approach is herein reported for manufacturing high mechanical strength continuous carbon fiber-reinforced silicon carbide (C_f/SiC) composite materials. Continuous carbon fibers were coated with polycarbosilane (PCS) using a colloidal evaporative deposition process and then coextruded with high solid content SiC ink. The zeta potential of the SiC ink was adjusted to optimize the printing ability of the suspension. During sintering, small SiC grains and whiskers were generated in the gaps in and around the PCS-coated carbon fibers, which led to the improved flexural strength and density of the composites. Meanwhile, the PCS coating on the surface of the carbon fibers prevented the carbon fibers from reacting with SiO gas generated by reactions between the SiC matrix and SiO₂ and sintering additives (Al₂O₃ and Y₂O₃), effectively preserving the structural integrity of the carbon fibers. Compared to the SiC specimens containing uncoated carbon fibers, the density of the specimens fabricated with coated carbon fibers was increased from 2.51 to 2.85 g/cm³, and the strength was increased from 190 to 232 MPa.     

空间望远镜用C/SiC镜面研制综述

综述了空间望远镜的主镜用高强度、高表面精度、低热膨胀系数的低温（约4K）用镜面的制备和检测过程.日本将Φ710mm的高强度反应烧结SiC材料已用于红外望远镜镜面.在短切炭纤维增强C/C复合材料毛坯的基础上进行液相硅渗透（LSI）而制备的C/SiC复合材料在光学镜面方面具有更大的优势.通过提高C/C复合材料毛坯中沥青基炭纤维体积分数及控制硅化速度,可有效地提高LSI-C/SiC复合材料的机械性能和表面光学精度;通过不同规格的炭纤维的混杂化,可使C/SiC复合材料热膨胀系数的各向异性降低至小于4％的差异.SiC、Si-SiC浆料涂层处理可有效地提高表面精度至2 nm rms的极高要求.