Chemical Interactions in Diboride-Reinforced Oxide-Matrix Composites

Thermochemical analyses of interfacial reactions in titanium, zirconium, and hafnium diboride reinforced oxidematrix composites have been carried out to evaluate the chemical compatibility. The chemical reactivity of these diborides with oxygen and the high volatility of B₂O₃( l ) at reduced oxygen pressures are concerns during processing and operating conditions. The thermochemical stability and the vaporization behavior of B₂O₃( l ) are discussed in terms of partial pressures of dominant gaseous species of the boron-oxygen system at 1700 and 2300 K. The TiB₂/ZrO₂ and TiB₂/HfO₂ systems are thermodynamically stable in a limited oxygen pressure range. The TiB₂/Al₂O₃ system is stable, but the reactions in this system may apparently be accompanied by formation of gaseous products (B₂O₃, AlO, Al₂O, and lower boron oxides) in the presence of elemental oxygen. These thermochemical considerations are very useful in evaluating the effectiveness of oxides as diffusion barrier coatings on diboride reinforcements.     

Improvement in the Oxidation Resistance of Oxide-Matrix Silicon Carbide-Particulate Composites by Mullite Infiltration

A SiC–∼50 vol% mullite-particulate composite fabricated by melt infiltration was found to exhibit excellent oxidation resistance at temperatures >1500°C in air. Cristobalite was found on the surface of samples after 100 h oxidation at temperatures of 1515°, 1620°, and 1650°C. The oxidation rate constant at 1515°C was almost comparable to hot-pressed bulk SiC, and at least 1 order of magnitude lower than the lowest value for the oxide-matrix SiC-particulate composites made by conventional processes, as reported in the literature and made by melt infiltration in the present study.     

Oxidation Behavior from Room Temperature to 1500°C of 3D-C/SiC Composites with Different Coatings

A carbon-fiber-reinforced silicon carbide composite (3D-C/SiC) was prepared by chemical vapor infiltration. A SiC and SiC/Si-Zr coating were deposited on the composite to investigate the effect of different coatings on the oxidation behavior of 3D-C/SiC composites. The 3D-C/SiC(SiC/Si-Zr) composite decreased in weight below 1000°C and increased in weight above 1000°C. With an increasing oxidation time, the weight loss increased greatly and the weight gain increased little. The 3D-C/SiC(SiC) composite always decreased in weight over the full temperature range. With an increasing oxidation time, the weight loss increased rapidly below 1000°C and reached its minimum value at 1400°C. The 3D-C/SiC(SiC/Si-Zr) composite had a higher oxidation resistance above 1000°C, and the 3D-C/SiC(SiC) composite had a higher oxidation resistance below 1000°C. The wider the coating cracks, the larger the maximum weight loss and the lower the temperature corresponding to the maximum weight loss. With an increasing oxidation time, the activation energy of the 3D-C/SiC(SiC/Si-Zr) composite increased from 96 to 138 kJ/mol, and the 3D-C/SiC(SiC) composite increased from 130 to 180 kJ/mol.     

Oxidation Behavior of Silicon-Infiltrated Carbon/Carbon Composites in High-Enthalpy Convective Environment

Thermal response and oxidation behavior of commercial metal-silicon-infiltrated carbon/carbon composites (MICMAT^TM; Si-CC) were evaluated in a high-enthalpy convective environment using an arc jet facility (an arc wind tunnel). Composite specimens were put into a supersonic plasma air stream having a gas enthalpy of 12.7–18.8 MJ/kg for 50–600 s. Cold-wall heat fluxes measured by a Gardon-type calorimeter ranged from 1.0 to 1.8 MW/m², and the maximum surface temperature reached 1300°–1660°C. After the arc jet testing, no surface recession was observed in the samples, and the mass loss rate of the composites was far less than that of graphite. The excellent oxidation resistance was caused by formation of a porous SiC layer at the surface of the composite. Oxidation behavior of the composites is discussed based on a simplified airflow blocking model of the porous SiC layer. The composites exhibited excellent oxidation resistance for short-term exposure in high-enthalpy airflow.     

Oxidation of Silicon Carbide-Reinforced Oxide-Matrix Composites at 1375° to 1575°C

Oxidation studies were conducted on Al₂O₃-SiC and mullite-SiC composites at 1375° to 1575°C in O₂ and in Ar-1% O₂. The composites were prepared by hot-pressing mixtures of Al₂O₃ or mullite and SiC powders. The reaction products contained alumina, mullite, an aluminosilicate liquid, and gas bubbles. The parabolic rate constants were about 3 orders of magnitude higher than those expected for the oxidation of SiC. Higher rates are caused by higher oxygen permeabilities through the reaction products than through pure silica. Our results suggest that oxygen permeabilities are comparable in the three condensed phases observed in the reaction products.     

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.     

Borate treatment of carbon fibers and carbon/carbon composites for improved oxidation resistance

Carbon fibers and carbon/carbon composites have been treated with borate additives and then cured at 500–600°C to produce a continuous film of boron oxide on all exposed surfaces.This treatment has been found to be highly effective in retarding oxidation of the carbonaceous substrate for extended periods in flowing air at temperatures up to 1000°C. At higher temperatures, and in the presence of water vapor, borate species were appreciably volatile and the oxidation protection provided by the coatings was less effective.     

短切碳纤维增强LAS玻璃—陶瓷的研究

研究了短切碳纤维增强ＬＡＳ玻璃－陶瓷基复合材料的制备工艺及纤维含量，热压工艺对其强韧性的影响，获得了短切碳纤维均匀分散并单向排列的复合材料，当纤维体积分数为１％左右时，材料强度和断裂韧性分别达到４３０ＭＰａ和８．８ＭＰａ．ｍ＾１／２。用光学显微镜和扫描电子显微镜观察了复合材料中短切碳纤维的分布状态和断口形貌。     

麻纤维/热塑性树脂复合材料的应用与发展

介绍了麻纤维/热塑性树脂复合材料的性能和应用,并从增强体/树脂改性、成型制备工艺和界面相容性等方面综述了麻纤维/热塑性树脂复合材料研究的最新进展。     

A Review on Wear and Friction Performance of Carbon–Carbon Composites at High Temperature

Carbon–carbon (C/C) composite is one of the best ceramic matrix composite due to its high mechanical properties and applications at control environments in various sectors. Carbon–carbon composite is made of woven carbon fibers; carbonaceous polymers and hydrocarbons are used as matrix precursors. These composites generally have densities <2.0 g/cm³ even after densification. C/C composites have good frictional properties and thermal conductivity at high temperature. Also C/C composite can be used as brake pads in high‐speed vehicles. In spite of various applications, C/C composites are very much prone to oxidation at high temperature. Therefore, C/C composites must be protected from oxidation for the use at high temperature.     

Oxidation Inhibition Mechanisms in Coated Carbon–Carbon Composites

The mechanism of oxidation resistance in SiC-coated carbon-carbon composites containing boron-based inhibitors has been investigated using thermodynamic calculations, thermogravimetric analysis, and electron microscopy. A model is developed based on the formation of a volatile B₂O₂ sub-oxide in the interior of the composite which condenses to B₂O₃ upon encountering a locally high oxygen partial pressure in coating cracks. The active-to-passive transition for the oxidation of elemental boron has been determined.     

Processing and Tribological Properties of Si3N4/Carbon Short Fiber Composites

Si₃N₄/carbon fiber composites were fabricated using several types of fiber. All the composites had higher fracture toughness compared with monolithic Si₃N₄ ceramics. Tribological properties were investigated by a ball-on-disk method under unlubricated conditions. The composite containing fibers with a high orientation of graphite layers and high graphite content indicated a low friction coefficient. It was identified, by Raman spectroscopy, that graphite was transferred from the composite to the Si₃N₄ ball of the counterbody during the wear test. This transferred layer was effective for producing the low friction behavior of the composite.     

表面处理对碳/酚醛材料层间性能影响的研究

本文分别研究了在有氧与无氧状态下表面处理对碳／酚醛材料界面特性的改善，尤其是在高温环境下的变化。通过扫描电镜(SEM)、电子能谱(XPS)研究了不同状态处理下的碳布，测试了碳／酚醛复合材料的剪切强度。研究表明，在500～600％有氧状态下处理的碳／酚醛材料层间性能最佳。     

Developing Interfacial Carbon-Boron-Silicon Coatings for Silicon Nitride-Fiber-Reinforced Composites for Improved Oxidation Resistance

C-B-Si coatings were formed on a Si₃N₄ fiber using chemical vapor deposition and embedded in a Si-N-C matrix using polymer impregnation and pyrolysis. The boron-containing layer was anticipated to form borosilicate glass and seal oxygen-diffusion passes. Two types of C-B-Si coatings were tested on the fiber–matrix interface, and they improved the oxidation resistance of the composite. The first coating was multilayered: a crystalline sublayer composed of B-Si-C was sandwiched between two graphitelike carbon sublayers. The second coating was a graphitelike carbon layer containing a small amount of boron and silicon. The carbon (sub)layer of both coatings weakened the fiber–matrix bonding, giving the composites a high flexural strength (1.1 GPa). The composites retained 60%–70% of their initial strength, even after oxidation at 1523 K for 100 h. The mechanism for improved oxidation resistance was discussed through the microstructure of the interface, morphology of the fracture surface, and oxygen distribution on a cross section of the oxidized composite.     

The Mechanical Properties and Chemical Resistance of Short Glass-Fiber-Reinforced Epoxy Composites

ABSTRACT

Epoxy–short glass fiber composites were prepared by directly blending two-pack system of Araldite (CY-230) and hardner (HY-951) with short glass fibers. The short glass fiber content was varied from 2% to 10% by weight of the total matrix. These composites were then characterized for morphology using scanning electron microscopy, mechanical properties, that is, tensile and flexural properties and resistance toward various chemicals. The epoxy-glass fiber composites showed improved tensile and flexural properties but increased dispersion among the properties with increasing fiber content. Several reasons to explain these effects in terms of reinforcing mechanisms were discussed. These composites were stable in most chemicals but were completely destroyed in concentrated sulfuric acid, nitric acid, and pyridine.     

Overlap Model for Chemical Vapor Infiltration of Fibrous Yarns

Attention is focused on fabrication of fiber-reinforced ceramic composites by chemical vapor infiltration. A model is proposed for infiltration of a cylindrical fiber bundle, or yarn strand, comprised in turn of small cylindrical fibers. Along any cross section perpendicular to the yarn axis, the centers of the fibers are assumed to be randomly distributed throughout the cross-sectional area without fiber overlap. As infiltration/densification proceeds, growth is assumed to occur via sequential deposition of uniform layers such that the fiber-matrix composite consists of growing cylinders whose edges eventually overlap. Based on the random overlap model, expressions for key time-dependent properties are developed for both the reaction-limited case and for cases with significant diffusional limitations. An analytical solution to the resulting equations is obtained for the reactionlimited case whereas numerical solutions are required for the diffusion-limited case. The effects of geometric, kinetic, and diffusional parameters on the infiltration dynamics are explored.     

Interface Design for Oxidation-Resistant Ceramic Composites

Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coatings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified.     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Surface Modification of Carbon Fibers by Free Radical Graft‐Polymerization of 2‐Hydroxyethyl Methacrylate for High Mechanical Strength Fiber–Matrix Composites

Free radical polymerization of vinylic monomers in the presence of carbon fibers results in the grafting of polymers onto the carbon fiber surface. Graft polymers cannot be removed by intense washing with good polymer solvents. The density and size of these structures are successfully controlled by reaction conditions. Grafting of the carbon fiber surface with hydroxyethyl methacrylate allows for introducing functional groups suitable for the reaction with an epoxy‐based resin. The resulting fiber‐reinforced composites show enhanced mechanical properties compared to samples prepared from carbon fibers equipped with a standard sizing for epoxy resins. Thus, tensile strength increases by 10%, while interlaminar shear strength improves by 20%.     

Modeling the Ultimate Tensile Strength of Unidirectional Glass-Matrix Composites

The ultimate tensile strengths of a unidirectional glass-matrix composite were measured as a function of fiber volume fraction. The results were compared with predictions, using a refined solution of the stress field generated by an axisymmetric damage model, which incorporated the effect of stress concentration in the fiber caused by the presence of a matrix crack both before and after deflection at the fiber/matrix interface. Two possible locations for the fiber failure were considered: (1) at a transverse matrix crack, near a bonded fiber/coating interface and (2) at the tip of a debond, at the fiber/coating interface. At low fiber volume fractions, the measured ultimate tensile strength matched the prediction calculated, assuming no crack deflection. For higher volume fractions, the predictions calculated for a debonded crack matched the observed values. The model results were relatively insensitive to debond length and interfacial shear stress for the range of values in this study. In comparison, the global load-sharing model, which does not account for the stress singularity at the fiber/matrix interface, was found to overpredict the values of the ultimate tensile strength for all fiber volume fractions. An important contribution of the present work was to introduce the use of fiber volume fraction as a parameter for testing theoretical predictions of the mode of fiber failure.