Yttrium Silicate Coatings for Oxidation Protection of Carbon–Silicon Carbide Composites

Yttrium silicate (Y₂SiO₅) coatings complement SiC coatings for protecting ceramic multilayer composite materials based on carbon-fiber-reinforced SiC composites (C-SiC). Thick (100 μm), dense Y₂SiO₅ coatings were prepared by dip coating, using concentrated aqueous slips. The resulting phases were studied by taking into account the simultaneous presence of oxide and non-oxide materials, which affected the chemical stability of the coatings. Thick, mechanically stable coatings were obtained by sintering in carbon crucibles and a SiC bed in an argon-flow furnace. Pure Y₂SiO₅ coatings completely separated from the SiC substrates. A high percentage of Y₂Si₂O₇ was necessary to fit the thermal expansion coefficients and ensure the stability of the coatings. Oxidation resistance of the coated substrates was investigated by isothermal and stepwise oxidation tests.     

Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.     

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si₃N₄/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si₃N₄/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si₃N₄/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.     

The effect of Al2O3 on the high‐temperature oxidation resistance of Si‐B‐C ceramic under air atmosphere

Si‐B‐C ceramics were prepared through reaction sintering, and the influence of Al₂O₃ addition on the high‐temperature (1100‐1300°C) oxidation behavior of the material under air atmosphere was studied. The erosion behavior and mechanism are determined from the measurement of weigh changes, microstructure observations, and characterization of the generated oxides on postexposure specimens. Results show that Al₂O₃ is enriched in the oxidized layer, inhibiting the volatilization of B₂O₃ and impeding the crystallization ability of oxide (cristobalite). Narrower erosion layer and less weigh change are observed with Al₂O₃. Low‐frequency Raman results reveals that with the increase in Al₂O₃, the bending vibrations of the BO₄ units and B‐O‐B stretching of the metaborate ring relative intensity are enhanced. Furthermore, high‐frequency Raman results shows that the relative proportion of high‐dimensional vibration modes Q³ and Q⁴ which result in a higher viscosity of melt and a greater resistance of oxygen diffusion are positively correlated with Al₂O₃.     

纳米碳颗粒/碳化硅陶瓷基复合材料的氧化行为

以微米硅(Si)和纳米碳黑(Cp)粉体为主要原料,采用经机械化学法合成的碳化硅(SiC)和15%和25%的纳米碳颗粒与碳化硅(Cp-SiC)的复合粉体,并经无压烧结得到了Cp/SiC陶瓷基复合材料,分析了在不同温度条件下Cp/SiC烧结体的氧化行为。结果表明:当温度小于700℃时,Cp/SiC复合陶瓷在空气中的氧化受C—O2反应控制,致使其为均匀氧化;700℃时,氧化后的复合材料显气孔率最大,弯曲强度达极小值;大于700℃,氧化过程受O2的气相扩散控制,呈非均匀氧化;700~900℃之间,O2通过微裂纹的扩散控制着Cp/SiC的氧化过程;900~1 100℃之间,O2通过SiC缺陷的扩散控制着Cp/SiC的氧化过程,并在1 000℃时的最初的2 h内,复合材料弯曲强度增大,且达到了极大值。同时表明,纳米碳含量是影响复合材料强度及氧化行为的关键因素,添加纳米碳质量分数为15%的Cp/SiC复合陶瓷可以作为一种抗氧化性能优良的玻璃夹具材料。     

SiC陶瓷基体及抗氧化涂层

介绍了5种主要SiC基体的成型方法,分别是化学气相渗透(CVI)、聚合物先驱体浸渍-裂解法(PIP)、液相硅渗透法(LSI)、反应烧结法、化学气相反应法(CVR)。阐述了各种基体的组织结构、致密效率及陶瓷基复合材料的性能,其中CVI+PIP/LSI的复合成型技术可达到优化的制备过程,提高基体的组织结构和致密化效率;C/C及C/SiC复合材料表面化学气相转换法SiC涂层及多层涂层技术是提高CMC抗氧化性能的有效途径,并已得到工程实际验证。     

反应烧结SiC材料的高温氧化行为研究

研究了反应烧结SiC材料在 110 0℃空气中的高温氧化行为。结果表明 :反应烧结SiC在110 0℃的氧化动力学曲线符合抛物线规律 ;材料的氧化受O2 和CO在玻璃态硅酸盐中的扩散所控制 ;材料中的杂质元素降低了SiO2 氧化膜的粘度 ,促进了O2 和CO在氧化膜中的扩散     

Dynamic oxidation and protection of the PAN pre-oxidized fiber C/C composites

Pre-oxidized fibers as reinforcement are candidates for reducing the overall cost of C/C composites with superior properties. This study investigated the dynamic oxidation and protection of the pre-oxidized fiber C/C composites (Pr-Ox-C-C). According to the Arrhenius equation, the oxidation kinetics of the Pr-Ox-C-C consisted of two different oxidation mechanism with the transition point was at about 700 °C. Scanning electron microscopy investigation showed that oxidation initiated from the fiber/matrix interface of composites, whereas the matrix carbon was easily oxidized. To improve the anti-oxidant properties of Pr-Ox-C-C, a ceramic powder-modified organic silicone resin/ZrB₂-SiC coating was prepared by the slurry method. The coated samples were subjected to isothermal oxidation for 320 h at 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C with incurred weight losses of ? 1.6%, 0.77%, ? 1.28%, 0.68% and 1.19%, respectively. After 110 cycles of thermal shock between 1100 °C and room temperature, a weight loss of 1.30% was obtained. The Arrhenius curve presented four different phases and mechanisms for coating oxidation kinetics. The excellent oxidation resistance properties of the prepared coating could be attributed to the inner layer which was able to form B₂O₃-Cr₂O₃-SiO₂ glass to cure cracks, and the ZrB₂-SiC outer layer that could provide protective oxides to reduce oxygen infiltration and to seal bubbles.     

碳纤维增强SiC陶瓷复合材料的研究进展

碳纤维增强SiC陶瓷基复合材料具有良好的高温力学性能,是航空航天和能源等领域新的高温结构材料研究的热点之一．本文回顾了增强体碳纤维的发展,对材料的成型制备工艺,材料的抗氧化涂层研究进展和现有的一些应用做了综述,并展望了碳纤维增强SiC陶瓷基复合材料以后的研究重点及发展前景．     

国产连续碳化硅纤维的进展及应用

介绍了国产连续碳化硅（SiC）纤维的研究进展及国外连续碳化硅纤维产品的主要应用,展望了国内连续SiC纤维及其产品的发展趋势.表明SiC纤维具有高比强度、高比模量、耐高温、抗氧化、抗蠕变、耐化学腐蚀、耐盐雾和优良的电磁吸收特性,是耐高温陶瓷基复合材料的关键材料.     

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB₂ ultra‐high temperature ceramic (UHTC) particulate filled C_f/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc‐jet testing of the UHTC composites highlighted the excellent ultra‐high temperature oxidation performance of these materials.     

Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite

A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC composite was fabricated via simple in situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification; the purpose of the SiC nanowires was to markedly improve strength and toughness. The nanowires consisted of single-crystal β-phase SiC with a uniform ∼5 nm carbon shell; the nanowires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was ∼5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nanowires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite.     

Bimodal Microstructure in Silicon Nitride–Barium Aluminum Silicate Ceramic-Matrix Composites by Pressureless Sintering

A distinct bimodal microstructure has been obtained in a Si₃N₄–BAS (barium aluminum silicate) ceramic-matrix composite by pressureless sintering. It is shown that the addition of coarse β-Si₃N₄ seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si₃N₄α-to-β phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering.     

Thermal Response and Oxidation of Tyranno™-Fiber-Reinforced Si-Ti-C-O Matrix Composites for a Thermal Protection System in High-Enthalpy Dissociated Air

The thermal response and oxidation of Tyranno™ Lox-M fiber-reinforced Si-Ti-C-O matrix composites in high-enthalpy dissociated air was investigated in an arc jet facility (an arc wind tunnel). The maximum surface temperature reached 1310–1670°C. Catalytic recombination of oxygen and nitrogen on the composite surface under dissociated air was not significant. Surface recession was insignificant below 1600°C surface temperatures and above 5 kPa of oxygen partial pressure at the stagnation point. Passive-to-active oxidation transition of the composite agreed with Balat's theory for monolithic silicon carbide. A glass sealant prevented active oxidation of the composite for short-time exposures.     

Electrophoretic Deposition Forming of Nickel-Coated-Carbon-Fiber-Reinforced Borosilicate-Glass-Matrix Composites

The present paper introduces a novel processing technique that involves in situ electrophoretic deposition (EPD), followed by pressureless sintering, to produce dense, defect-minimized, carbon-fiber-reinforced borosilicate-glass-matrix composites with a nickel interface. The process relies on the deposition of submicrometer-sized, colloidal charged particles onto unidirectionally aligned nickel-coated carbon fibers. The preparation and characterization of a kinetically stable nanosized borosilicate sol suitable for EPD are described. The most-important EPD processing parameters in the formation of dense, fully infiltrated, green-body compacts are described, and issues that concern the infiltration of very tight carbon fiber preforms are discussed and effectively solved. Using the crack-path-propagation test, the metallic nickel interface is determined to be very effective to improve the composite mechanical performance, in terms of the nonbrittle fracture behavior. Catastrophic crack growth is prevented by such mechanisms as constrained plastic deformation of the interface and fiber debonding and pullout. The proposed processing technique has great potential to fabricate defect-minimized and damage-tolerant fiber-reinforced brittle-matrix composites with a ductile interface. Overall, this new approach offers a cost-effective and short-time processing route for the fabrication of continuous-fiber-reinforced ceramic-matrix composites.     

Intermediate temperature oxidative strength degradation of a SiC/SiNC composite with a polymer‐derived matrix

The article describes an experimental investigation of oxidative degradation in mechanical performance of a SiC fiber‐reinforced composite with a SiCN matrix produced by polymer infiltration and pyrolysis. Tensile stress rupture and retained strength tests were performed at 800°C in dry air and in water vapor. Fracture surfaces were examined to determine the degree of fiber pull‐out and constituent oxidation and to measure radii of representative fiber fracture mirrors. The results indicate that degradation in tows adjacent to cut surfaces occurs equally rapidly in water vapor with or without application of stress; regions in the composite interior and near as‐processed (uncut) surfaces appear far less affected. Similar effects are evident but less pronounced in dry air. Although oxidation of fiber coatings is observed in some cases, collectively the results suggest that fiber degradation is the main mechanism leading to reduced composite strength.     

Oxidation mechanisms of SiC-fiber–reinforced Si eutectic alloy matrix composites at elevated temperatures

SiC-fiber–reinforced binary Si eutectic alloy composites have been developed for aerospace applications using the melt infiltration method. In this study, the oxidation mechanisms of various binary Si eutectic alloys were evaluated at elevated temperatures. We suggest that the oxidation resistance of eutectic alloys could be predicted using the Gibbs energy change for the oxidation reaction. Based on these calculations, eutectic alloys of Si-16at%Ti, Si-17at%Cr, Si-22at%Co, Si-38at%Co, and Si-27at%Fe were prepared. These alloys produced uniform SiO₂ layers and showed the same oxidation resistance as Si at 1000°C under humid conditions. Therefore, SiC composites using Si alloys with excellent oxidation resistance can be predicted using thermodynamic calculations.     

Thermo-mechanical characterization of carbon fiber reinforced silicon carbide composites having boron nitride as an interphase material

The carbon fiber reinforced silicon carbide composites were prepared by an isothermal chemical vapour infiltration process. In order to achieve the required density, the carbon fiber preforms in the form of rectangular panels were infiltrated by silicon carbide (SiC) matrix. Prior to the matrix infiltration, a thin coating of boron nitride, as an interphase, was applied on the fiber preform. The test samples were subjected to seal coating of silicon carbide by chemical vapour deposition process. The effect of protective SiC seal coating was examined by testing (3-point bend test) the uncoated and the seal coated samples at different temperatures. Higher value of the flexural strength was observed for the seal coated samples as compared to the uncoated samples, when got tested at high temperature (up to 1400?°C). The detailed analysis of the fractured surfaces of the tested samples was carried out.