Microstructure characterization of CVI-densified carbon/carbon composites with various fiber distributions

Mechanical behavior of multi-phase composites is crucially influenced by volume fractions, orientation distributions and geometries of microconstituents. In the case of carbon–carbon composites manufactured by chemical vapor infiltration, the microconstituents are carbon fibers, pyrolytic carbon matrix, and pores. The local variable thickness of the pyrolytic carbon coating, distribution of the fibers and porosity are the main factors influencing the properties of these materials. Two types of fiber arrangements are considered in this paper: 2D laminated preform and random felt. The materials are characterized by determining their densities and their fiber distribution functions, by establishing types of pyrolytic carbon matrix present in the composites, and by studying the porosity. A technique utilizing X-ray computed tomography for estimation of the orientation distribution of the fibers and pores with arbitrary shapes is developed. A methodology based on the processing of microstructure images with subsequent numerical simulation of the coating growth around the fibers is proposed for estimation of the local thickness of the coating. The obtained information is appropriate for micromechanical modeling and prediction of the overall thermo-mechanical properties of the studied composites.     

Fabrication of a NicalonTM fiber/Si3N4-based ceramic-matrix composite by the polymer pyrolysis method

A processing route for ceramic matrix composites is developed based uponpolymer pyrolysis. Three types of Nicalon^TM fiber woven fabrics,—i.e., uncoated, carbon-coated, and carbon/SiC-coated—are impregnated with apolysilazane solution. Thus-formed prepregs are then cut, laminated,pressed and fired to 1000 °C in a nitrogen atmosphere. Upon pyrolysis,polysilazane converts to a Si₃N₄-based ceramic matrix with 60 wt% yield. The composites made with uncoated Nicalon^TM fibers have poor flexural andtensile strength (103 and 19 MPa, respectively) and show brittle fracturebehavior. That is due not only to the poor fiber-matrix interface but alsoto processing-induced fiber damage. For carbon and carbon/SiC-coatedNicalon^TM fiber composites, the coating layers on the fiber surfacemanipulate the appropriate fiber-matrix interface and also protect thefibers from damage during polymer pyrolysis, so these composites exhibithigher flexural (250 and 274 MPa, respectively) and tensile (138 and 196 MPa, respectively) strength. Also, the load stress-deflection behavior ofcomposites with two types of coated fibers cause noncatastrophic fracture.     

Interface engineering of fiber-reinforced all-oxide composites fabricated by the sol–gel method with fugitive pyrolytic carbon coatings

This paper describes the interface engineering of three–dimensional (3D) Nextel™440 fiber-reinforced aluminosilicate composites fabricated by the sol–gel method with fugitive pyrolytic carbon (PyC) coatings. The coating thickness on the fiber strength, interfacial characteristics and there corresponding effects on mechanical properties of the composites were investigated. The study shows that the fiber strength was influenced by the coating thickness and optimized with the thickness of 0.15 μm. The composites with uncoated fibers showed brittle fracture behavior without fiber pullout because of strong interactions between the fiber and the matrix. However, higher strengths and pseudo-ductile fracture behaviors were obtained in the composites with PyC interphases, where different deflections and branches of propagating cracks and fiber pullout patterns were observed. Moreover, induced fugitive PyC interface conditions have great effects on the density, microstructure and mechanical properties of the resultant composites.     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

Interfacial failure behavior of PyC-Cf/AZ91D composite fabricated by LSEVI

Carbon fiber reinforced AZ91D matrix composites with pyrolytic (PyC) coating deposited on fiber surface (PyC-C_f/AZ91D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI). Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg₁₇Al₁₂ were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-C_f/AZ91D composites, which was the failure between PyC coating and AZ91D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-C_f/AZ91D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.     

From random chopped to oriented continuous SiC fibers–ZrB2 composites

ZrB₂–continuous SiC fiber composites were prepared by vacuum-bag infiltration and hot pressing, using homemade 1D fabric preforms of Tyranno SA3 SiC fibers. Sintering behavior and microstructural features such as secondary phases and matrix/fiber interface were compared to those of chopped SiC fibers-reinforced composites. The infiltration process allowed the overall fiber content to be increased up to 40 vol%, because of the ordered arrangement of fibers. When the fiber preforms were properly infiltrated, the composites were nearly fully dense and the densification mechanisms were the same as those of unreinforced matrices. Different from composites containing short discontinuous fibers, the degree of chemical interaction at the fiber/matrix interface was very limited and this resulted in an easier pull out in the fractured surfaces, even in absence of fiber protective coating.     

Oxide Fiber Reinforced Glass: a Challenge to New Composites

The brittle nature and low fracture resistance of glass and ceramic materials limit their use in engineering applications. To achieve a tougher glass material, oxide fiber reinforced glass composites were synthesized by slurry infiltration and hot‐pressing. Significant increases in bending strength and work‐of‐fracture have been noticed for fiber reinforced glass matrix composites with carbon coated fibers. These effects are related to the low bonding between fiber and matrix, caused by the fiber coating.     

Microstructure and mechanical properties of Al–2Mg alloy base short steel fiber reinforced composites prepared by vortex method

Fabrication and characterization of cast Al–2Mg alloy matrix composites reinforced with short steel fibers are dealt with in the present study. Three types of steel fiber were used: uncoated, copper coated and nickel coated. All the composites were prepared by the liquid metal route using vortex methods. When tested in tension, all composites exhibited improvement in strength due to high relative strength of steel fibers. The ductility was lowered except for the composite with copper coated fibers. Copper coated fiber reinforced composites gave the highest strength. Higher strength accompanied with appreciable ductility demonstrated by composites with copper coated fibers is attributed to the solid solution and fiber strengthening as well as good bonding at the interface. Composites reinforced with uncoated and Ni coated steel fibers did not exhibit strengthening to the level exhibited with copper coated fibers because brittle intermetallic phases are formed at the interface. These phases promote initiation and facilitate propagation of cracks. The observed fracture mechanism of composites was dimple formation, fiber breakage and pullout of fibers. Fracture surface of uncoated and Ni coated composites showed extensive pull out of fibers as well as fiber breakage confirming the above inference. In case of the copper coated composites dimple formation and coalescence was more extensive. EDX analysis showed a build up Cu, Ni, and Fe at the interface.     

Effect of copper and nickel coating on short steel fiber reinforcement on microstructure and mechanical properties of aluminium matrix composites

Commercially pure Al base short steel fiber reinforced composites were prepared by stir casting method and poured into a cast iron mould. Steel fibers were coated with copper and nickel by electroless deposition method. The density, hardness and strength of composites increased as compared to matrix alloy. The mechanical properties of these composites were measured and the results were correlated with the microstructure observation. It was found that copper-coated short steel fiber reinforced composites show considerable improvement in strength with good ductility because copper form a good interface between Al matrix and short steel fiber. Nickel-coated steel fiber reinforced composites showed improvement in strength to a lower extent possibly because of formation of intermetallic compound at the interface. The improvement in strength with uncoated fibers and nickel-coated fibers is on the lower side because of formation of brittle intermetallic compounds like Fe₂Al₅ and FeAl₃. Fracture surface of tensile specimen was examined under SEM, which revealed a ductile fracture. Copper coating on steel fiber improved the strength properties while retaining a high level of ductility due to better interface bonding.     

Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases

In order to tailor the fiber–matrix interface of continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites for improved fracture toughness, alternating pyrolytic carbon/silicon carbide (PyC/SiC) multilayer coatings were applied to the KD-I SiC fibers using chemical vapor deposition (CVD) method. Three dimensional (3D) KD-I SiC_f/SiC composites reinforced by these coated fibers were fabricated using a precursor infiltration and pyrolysis (PIP) process. The interfacial characteristics were determined by the fiber push-out test and microstructural examination using scanning electron microscopy (SEM). The effect of interface coatings on composite mechanical properties was evaluated by single-edge notched beam (SENB) test and three-point bending test. The results indicate that the PyC/SiC multilayer coatings led to an optimum interfacial bonding between fibers and matrix and greatly improved the fracture toughness of the composites.     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

Effect of fiber modulus of elasticity on the long term properties of micro-fiber reinforced cementitious composites

Fibers for reinforcing cementitious composites are typically short and randomly dispersed in the matrix. Consequently, most of the fibers are inclined to the cracks that develop in the cement matrix and suffer from bending stress as these cracks open. For brittle fibers, such as carbon fibers, the bending stress may lead to flexural fiber rupture before the fiber attains its full capacity in direct tension. As a result, the efficiency of these fibers may be reduced. This phenomenon is not expected to occur in ductile fibers, which can yield locally rather than rupture. Predictions of a theoretical model show that the bending stress increases as the matrix becomes denser and suffer (an event which occurs as the matrix ages or due to the addition of silica-fume) and decreases for fibers of lower modulus of elasticity. Therefore, a reduction in strength with time in composites with dense matrices is expected for very brittle fibers of high modulus, moderate or no reduction for low modulus brittle fibers, and no reduction in strength is expected for ductile fibers. The long term properties of cementitious composites reinforced with various microfibers was studied to validate the model; PAN and Pitch type carbon fibers represented brittle fibers of high and low modulus, respectively; polypropylene and polyacrylonitrile fibers represented ductile fibers. The results showed good agreement with the theoretical model.     

Continuous C fibre composites with a porous ZrB2 Matrix

ZrB₂–continuous C fibre composites were prepared by vacuum-bag infiltration and hot pressing, using 1D fabric carbon preforms from different commercial products. Sintering behaviour and microstructural features such as secondary phases and matrix/fibre interface were analysed. The infiltration process allowed the overall fibre content to be increased up to 70 vol.%, whilst the matrix densification was enabled by sintering aids. Due to presence of residual porosity, the degree of chemical interaction at the fibre/matrix interface was limited and this resulted in an easier pull out in the fractured surfaces, even in absence of fibre protective coating. The composites exhibited a damage tolerant behaviour.     

C/Al复合材料中镍涂层的界面行为

为改善纤维与基体的润湿性,在碳纤维表面涂覆Ni、SiC-Ni。经铝液浸渗实验表明Ni涂层可以使纤维很好地分布于基体中。但Ni单涂层导致碳纤维损伤,Ni、SiC-Ni两状态下界面与基体中均产生大量脆性相,急剧降低复合材料的强度。      

Ceramic composites: roles of fiber and interface

Results are presented that elucidate: (a) the effects of fiber coating on retained fiber strength and mechanical properties of Nicalon-fiber-reinforced SiC matrix composites; and (b) the role of residual stresses in the interfacial bond strength of SiC-fiber-reinforced reaction-bonded Si₃N₄ matrix composites. For Nicalon-fiber-reinforced SiC matrix composites that were fractured in a flexural mode, retained in-situ fiber strength, ultimate strength and work-of-fracture (WOF) of the composites increased with increasing thickness of the fiber coating and reached maximum values at a coating thickness of ≈0.3 μm. A direct correlation between the variation of in-situ fiber strength and the variation of ultimate strength and WOF of the composites clearly indicates the critical role of the retained in-situ strength of reinforcing fibers in composites. Fiber pushout tests performed on SiC-fiber-reinforced reaction-bonded Si₃N₄ matrix composites indicate that both debonding and frictional shear stresses decreased with increasing fiber content. These variations are consistent with the variation of residual radial stress on fibers, as measured by neutron diffraction, i.e. residual stresses decreased with increasing fiber content. Because fracture behavior is strongly controlled by interfacial bond strength, which is proportional to the residual radial stress, appropriate control of residual stresses is critical in the design of composites with desired fracture properties.     

Faserverstärkte oxidkeramische Werkstoffe

Oxide ceramic matrix composites Oxide ceramics display excellent thermal and chemical stability up to high temperatures. A suitable way to overcome the inherent brittleness of oxide ceramics is the reinforcement with oxide fibers. Although both constituents, fibers and matrices are brittle, the composites display quasi‐ductile deformation behavior, due to mechanisms such as crack deflection, crack bridging or fiber pull‐out. A premise for these mechanisms to work is the relatively weak bonding between fibers and the matrix. To achieve a weak fiber/matrix bonding either suitable fiber coatings are employed or, in an alternative approach, highly porous matrices are used. WHIPOX, developed at DLR is a ceramic matrix composite belonging to the porous matrix group. These materials have a high potential for thermal protection systems and liners in gas turbine engines and beyond.     

C/Al复合材料中镍涂层的界面行为

为改善纤维与基体的润湿性,在碳纤维表面涂覆Ni、SiC-Ni。经铝液浸渗实验表明Ni涂层可以使纤维很好地分布于基体中。但Ni单涂层导致碳纤维损伤,Ni、SiC-Ni两状态下界面与基体中均产生大量脆性相,急剧降低复合材料的强度。     

Enhancing both strength and toughness of carbon/carbon composites by heat-treated interface modification

A simple method to increase both strength and toughness of carbon/carbon (C/C) composites is presented. This method is based on the heat treatment of the pre-deposited thin carbon coating, leading to the formation of more orderly pyrolytic carbon (PyC) as a functional interlayer between fiber and matrix that could optimize the interfacial sliding strength in C/C composites. Effects of such a heat-treated PyC layers on the microstructure, tensile strength and fracture behavior of unidirectional C/C composites were investigated. Results showed that although the in-situ fiber strength was deteriorated after the introduction of interfacial layer, tensile strength of the specimen was greatly improved by 38.5% compared with pure C/C composites without any treatment. The interfacial sliding stress sharply decreased, which was interpreted from finite element analysis and verified by Raman spectra. Therefore, the fracture behavior was changed from brittle fracture to multiple-matrix cracking induced non-linear mechanical behavior. Finally, the ultimate strength can be predicted by different models according to the interfacial sliding stress. Our research would provide a meaningful way to improve both strength and toughness of C/C composites.     

Effect of nano-SiO2 modified emulsion sizing on the interfacial adhesion of carbon fibers reinforced composites

The influence of nano-SiO₂ modified epoxy emulsion sizing on the interfacial adhesion properties of carbon fibers reinforced composites was investigated. The interfacial interaction between carbon fibers and the matrix was characterized by X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM) and three-point short-beam shear testing. The results showed that the amount of hydroxyl groups was slightly increased on the carbon fibers surface after treatment with nano-SiO₂ modified sizing. Compared to the unsized composites, the interlaminar shear strength (ILSS) values for the composites with unmodified sizing and nano-SiO₂ modified sizing were increased by 9% and 14%, respectively. The holes and carbon fibers pullout were not observed in their fracture sections. Surprisingly, the fracture section of the composites with nano-SiO₂ modified sizing was more compact and the fiber debonding was more difficult.     

电子束固化复合材料界面

电子束固化复合材料界面粘结性能较低是急待解决的问题。利用阳极氧化技术和偶联剂涂层对碳纤维表面进行处理。处理前后的碳纤维表面性能利用SEM、XPS和接触角测试方法进行分析,通过层间剪切强度表征电子束固化复合材料界面粘结性能,并且与热固化复合材料进行对比。结果表明: 当碳纤维在酸性电解液中进行阳极氧化时,有利于提高电子束固化复合材料界面粘合性能,在碱性电解液中进行阳极氧化时, 则导致较低界面粘接性能。阳极氧化与偶联剂双重增效作用能够提高电子束固化复合材料界面粘合性能。