Fugitive Interfacial Carbon Coatings for Oxide/Oxide Composites

The effectiveness of fugitive interfacial carbon coatings in Nextel™ 720-based composites was investigated. Dense (>90%) matrix (calcium aluminosilicate, 0° and ±45°) composites and porous matrix (mullite/alumina, eight-harness satin fabric) composites were fabricated and tensile tested in two control conditions (uncoated or carbon-coated) and with the carbon removed (fugitive interface). Results indicated that carbon removal in dense matrix composites did not significantly change unidirectional composite strength, even after long-term exposure at 1000°C. For porous matrix composites, composite strength was independent of the fiber/matrix interface, even after exposure at 1150°C for 500 h in air.     

Effectiveness of Monazite Coatings in Oxide/Oxide Composites after Long-Term Exposure at High Temperature

The effectiveness of monazite (LaPO₄) in providing an oxidation-resistant weak fiber/matrix interface was evaluated in a fiber roving/thin coating/ceramic-matrix composite with >20% fiber volume fraction. Nextel™ 610/monazite/alumina composites were fabricated and tensile tested after isothermal exposures of up to 1000 h. Some strength loss was seen after short-term exposures (1100°–1200°C/5–250 h); however, no further loss was observed after 1000 h at 1200°C. Conversely, control samples containing uncoated fiber displayed >70% strength losses after only 5 h at 1200°C. Fiber pullout was seen in monazite-containing samples even after 1000 h at 1200°C. Debonding was predominantly in the coating or at either the fiber/coating or coating/matrix interface. Push-out testing confirmed the weakness of the monazite coating interface.     

Continuous Coating of Oxide Fiber Tows Using Liquid Precursors: Monazite Coatings on Nextel 720™

Seven different aqueous or ethanolic precursors were used to continuously coat monazite (LaPO₄) on Nextel 720™ fiber tows. Immiscible liquid displacement was used to minimize bridging of coating between filaments. Precursor viscosities, densities, and concentrations were measured, and solids were characterized by DTA/TGA and X-ray. Coatings were cured in-line at 1200°-1400°C and characterized for thickness, microstructure, and composition by optical microscopy, SEM, and TEM. Tensile strengths of the coated fibers varied with the precursor used and were 25% to >50% lower than those of the as-received fiber. The coating stoichiometry and coating thickness of a particular precursor did not correlate with tensile strength. Median coating thicknesses were typically ∼50–100 nm for precursors with 40–80 g/L monazite, much larger than thicknesses predicted by theory for 12 mm diameter monofilaments. There were significant thickness variations from filament to filament, but usually little variation in composition or microstructure. Amorphous AlPO₄ layers formed from phosphate-rich precursors. Factors that could affect coating characteristics and tensile strength reduction were discussed.     

Evaluation of Oxide–Oxide Composites in a Novel Combustor Wall Application

Oxide–oxide composites were evaluated in a novel combustor design requiring higher wall temperatures than the conventional combustors. The evaluation was based on a combination of numerical modeling and experimental rig testing. The modeling included computational fluid dynamics (CFD) calculations whose results were used in a thermo-mechanical analysis using finite element modeling (FEM). The composites tested experimentally were obtained from a commercial vendor; they were reinforced using Nextel^™ 720 fibers. The rig tests showed that aluminosilicate matrix composites with higher room temperature strengths suffered cracking while the weaker alumina matrix composites performed satisfactorily. The results were consistent with numerical models that predicted residual stresses from creep during service. The models showed that in-plane gradients and their effects were more severe than those of through-thickness gradients and suggest that tailoring fiber architecture is important in transitioning these composites to applications.     

Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites

A porous oxide fiber coating was investigated for Nextel^™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only ∼20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating.     

Influence of Residual Stresses, Interface Roughness, and Fiber Coatings on Interfacial Properties in Ceramic Composites

Fiber pushout tests are performed on zircon-matrix composites especially fabricated with a variety of silicon carbide reinforcing fibers and fiber coatings in order to create samples with different interfacial properties, surface roughness, and possibly in different states of residual stress to demonstrate their role on the interfacial and mechanical properties of fiber-reinforced composites. The data obtained from fiber pushout tests are analyzed using linear, shear-lag, and progressive debonding models to extract important interfacial properties, residual stresses, and surface roughness. The nature and magnitude of residual stresses in composites are independently characterized by measuring the coefficient of thermal expansion of the fiber, the matrix, and the composite for comparison with similar values measured using the fiber pushout tests. These results are then compared for self-consistency among different ways of analyzing data and with independently measured and calculated values. The results have shown that independent and complementary methods of data acquisition and analysis are required to fully understand interfacial properties in ceramic composites. In particular, independent measures of the coefficient of thermal expansion, residual stresses, and surface roughness are required to confidently interpret interfacial properties obtained by different analytical approaches and then relate them to the overall mechanical response of composites. It is also shown that composites with optimum mechanical response can be created by suitably engineering the interface using multiple fiber coatings.     

Carbon-Coated-Glass-Fiber-Reinforced Cement Composites: I, Fiber Pushout and Interfacial Properties

Interfacial mechanical properties of carbon-coated-S-glass-fiber-reinforced cement were characterized by a fiber pushout technique. The pushout experiments were conducted on model composites, where the S-glass monofilaments with and without carbon coating were unidirectionally embedded in ordinary portland cement. Interfacial properties, including bonding strength, frictional stress, residual stress, and fracture energy, were extracted from the previously developed progressive debonding model. The composite with a carbon interface exhibited a weaker interfacial bonding strength and frictional stress than did the composite without a carbon interface. The interfacial fracture energy of the composite with a carbon interface was 7.9 J/m², as compared to 47.6 J/m² for the composite without a carbon interface. The composite with the carbon interface exhibited a smaller residual clamping stress (18 MPa), in comparison to that for the composite without a carbon interface (69 MPa). Scanning electron microscopy observations indicated that the filament without a carbon coating was significantly attacked by the alkaline environment and was strongly bonded onto the matrix, whereas the filament with a carbon coating remained intact under the same curing conditions. These studies suggest that carbon coating provides the glass fiber with significantly improved corrosion resistance to alkali in the cement environment.     

Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases

Room-temperature debonding and sliding of fibers coated with La-monazite is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite and sapphire) and directionally solidified eutectic (Al₂O₃/Y₃Al₅O₁₂ and Al₂O₃/Y-ZrO₂) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations, and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained areas (10 nm grain size) resembling recrystallized microstructures were also observed in the most heavily deformed regions of the coatings. Frictional heating during sliding is assessed. Potential mechanisms for forming such microstructures at low temperature are discussed, and a parallel is drawn with the known resistance of monazite to radiation damage. The ability of La-monazite to undergo both debonding and plastic deformation relatively easily at low temperatures may enable its use as a composite interface.     

Viability of Oxide Fiber Coatings in Ceramic Composites for Accommodation of Misfit Stresses

The C and BN fiber coatings used in most ceramic composites perform a less obvious but equally essential function, in addition to crack deflection; they accommodate misfit stresses due to interfacial fracture surface roughness. Coatings substituted for them must also perform that function to be effective. However, in general, oxides are much less compliant materials than C and BN, which raises the question of the feasibility of oxide substitutes. The viability of oxide coatings for accommodating misfit stresses in Nicalon fiber/SiC composites was investigated by calculating the maximum misfit stresses as functions of coating properties and geometries. Control of interfacial fracture path was also briefly considered. The implications regarding composite properties were examined by calculating properties for composites with mechanically viable oxide coatings.     

亚麻纤维增强复合材料湿热环境中界面性能研究

为了全面研究植物纤维增强复合材料界面的湿热行为,亚麻纤维增强环氧树脂复合材料以亚麻纤维作为增强材料,环氧树脂作为基体材料,采用真空辅助树脂传递模塑(RTM)成型工艺制备复合材料层合板,通过探究亚麻/环氧复合材料在湿热环境条件下的吸水率和界面剪切强度的变化,并借助扫描电子显微镜(SEM)、傅里叶红外光谱仪(FTIR)等材料表征设备,分析了植物纤维增强高分子基复合材料在湿热条件下微观结构的变化,揭示了植物纤维增强高分子复合材料在湿热环境下其界面性能下降的机理,为植物纤维增强复合材料的应用与界面改性提供了大量的理论依据。     

Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Commercially available SiC fibers were coated with monazite (LaPO₄) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La₂Si₂O₇) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.     

Developments in Oxide Fiber Composites

Prospects for revolutionary design of future power generation systems are contingent on the development of durable high-performance ceramic composites. With recent discoveries in materials and manufacturing concepts, composites with all-oxide constituents have emerged as leading candidates, especially for components requiring a long service life in oxidizing environments. Their insertion into engineering systems is imminent. The intent of this article is to present a synopsis of the current understanding of oxide composites as well as to identify outstanding issues that require resolution for successful implementation. Emphasis is directed toward material systems and microstructural concepts that lead to high toughness and long-term durability. These include: the emergence of La monazite and related compounds as fiber-coating materials, the introduction of the porous-matrix concept as an alternative to fiber coatings, and novel strategies for enabling damage tolerance while retaining long-term morphological stability. Additionally, materials and mechanics models that provide insights into material design, morphology evolution, and composite properties are reviewed.     

Some Properties of Glasses in the System Barium Oxide–Boric Oxide–Silica

Densities, refractive indices, liquidus temperatures, and primary phases of glass compositions in the glassforming region of the system BaO-B₂O₃-SiO₂ were determined. Anomalous changes in properties were observed for a continuous change in composition. The possibility that there is a relation between the compounds indicated by the phase diagram and the actual units in the glass is suggested.     

纳米粒子增强碳纤维复合材料界面性能的研究进展

详细介绍了纳米粒子改性碳纤维的方法、原理及其在增强碳纤维复合材料界面性能方面的研究进展,并指出了纳米粒子改性碳纤维复合材料界面性能存在的相应问题,为提高碳纤维复合材料界面性能的研究提供了参考。     

Regeneration of Interfacial Adhesion in Fiber Reinforced Composites

Our studies of the regeneration of interfacial adhesion in micro-composites have shown that fiber/thermoplastic (aramid/polycarbonate) bonds can be completely regenerated, the degree of regeneration depending on both time and temperature of heating. Complete regeneration requires high temperatures, suggesting that mechanical interlocking resulting from flow of heat-softened resin into fiber surface crevices may be the primary mechanism of bond strength regeneration. Only partial regeneration of fiber/thermosetting resin (epoxy with aramid and carbon fibers) bond strength has been achieved, and this appears to be independent of fiber and reheating time. Apparently, the viscoelastic behavior of the resin is a critical factor in bond strength regeneration.     

氧化铌/木质活性碳纤维复合材料的结构性能表征

以木质活性碳纤维(ACHF)为载体,通过浸渍草酸铌以及改变煅烧温度,制备出不同煅烧温度(400、600和800℃)下的氧化钯/木质活性碳纤维(Nb2O5/ACHF).采用扫描电镜(SEM)、X射线衍射(XRD)、X光电子能谱(XPS)和全自动比表面积与孔径分析(BET)仪对制备的氧化铌/木质活性碳纤维结构、表面孔径等进...     

Influence of Fiber and Interfacial Properties on Fracture Behavior of Fiber-Reinforced Ceramic Composites

The fracture and fracture resistance behaviors of zirconmatrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide fibers are studied by performing experiments in three-point flexure and by analyzing results analytically using a cohesive crack model that incorporates crack bridging and fiber pullout mechanisms. A comparison of experimental results with the model predictions demonstrates good agreement. This analytical approach is then used in a parametrical study to demonstrate the role of fiber and fiber-matrix interfacial properties on the mechanical behavior of fiber-reinforced ceramic-matrix composites. Material parameters that enhance ultimate strength and ductility or toughness are elucidated.