Properties of B4C/Al–B4C composite with a two-layer structure

In order to obtain a material with a promising bulletproof performance, a two-layer structure composite consisting of B₄C/Al-B₄C was obtained using a two-step method for both hot pressing and infiltration aluminum in vacuum. Before aluminum infiltration the B₄C porous layer of the two-layer preform looked like a three-dimensional network of interconnected capillaries. For the B₄C ceramics layer the microstructure showed no apparent change before and/or after aluminum infiltration. The two-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C-Al material.     

Influence of atmosphere and carbon contamination on activated pressureless infiltration of alumina–steel composites

Al₂O₃/steel metal–matrix composites (MMCs) were fabricated by activated pressureless infiltration at atmospheric pressure in different gas atmospheres; N₂, Ar and Ar–5%H₂. The infiltration quality was evaluated with examination of the microstructure, infiltrated area and remaining porosity. The atmosphere with the best infiltration quality was chosen for improvement of infiltration by varying infiltration parameters such as temperature, holding time and heating and cooling rates. Further improvement was achieved by addition of Si or SiO₂ powder to the preform in order to reduce the effect of the residual carbon. The results show that the activated melt infiltration can be successfully done at atmospheric pressure in inert gas.     

Preparation and R-curve properties of laminated Si/SiC ceramics from paper

Laminated Si/SiC ceramics were synthesized from paper via impregnation with phenolic resin, followed by lamination and carbonization of the paper–resin laminates and subsequent infiltration and reaction with liquid silicon at a temperature of 1550 °C for 10–90 min. Due to the capillarity infiltration and in situ reaction with liquid silicon, intrinsic micro- and macrostructure in the carbon preform was retained within the final ceramics. The XRD, TGA, and microscopy analysis indicated that the final material exhibited a distinguished laminar structure with alternating arrangement of SiC and silicon layers. The thick SiC layer was composed of beta-SiC and a little of free silicon and un-reacted carbon. Studies on the evaluation of R-curves behavior by the indentation-strength method indicated a strong R-curve behavior for the Si/SiC composites.     

Squeeze-casting conditions of Al/Al2O3 metal matrix composites with variations of the preform drying process

The characteristics of the preform play a role in determining the final properties of MMCs. Effects of organic binder and microwave drying on preform microstructure have been examined by SEM. In the preform with organic binder, flocking processes are observed during drying. The preform has a uniform distribution of binder and dries quickly with microwave drying owing to its internal and volumetric heating patterns. The fundamental manufacturing process and controlling parameters of squeeze casting, including preform temperature, mould temperature, applied pressure and molten metal temperature, have been studied in Al/Al₂O₃ composites. MMCs have poor mechanical properties with too high temperatures of preform and molten metal due to thermal shocking of the preform, oxidation of the matrix and thermal damage to the fibers. Mould temperature barely affects the tensile strength of MMCs. High applied pressure reduces voids and solidifies the matrix faster. Conditions for squeeze casting to achieve optimal processing, are suggested. The tensile strength of MMCs can be improved by up to about 20% compared with the unreinforced matrix alloy.     

Microstructure and mechanical properties of a three-layer B4C/Al–B4C/TiB2–B4C composite

A three-layer structure material, consisting of B₄C/Al, B₄C/TiB₂ and B₄C composites, was obtained using a two-step method for both hot pressing and aluminum infiltration in vacuum. The three-layer B₄C/Al–B₄C/TiB₂–B₄C composite showed good interfacial bonding. Before aluminum infiltration the B₄C porous layer in the three-layer preform looked like a three-dimensional network of interconnected capillaries. The microstructures of both B₄C/TiB₂ and B₄C layers showed no apparent changes before and/or after aluminum infiltration. The three-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C/Al material.     

Controlled replication of butterfly wings for achieving tunable photonic properties

The fine structure of the wing scale of a Morpho Peleides butterfly was examined carefully, and the entire configuration was completely replicated by a uniform Al(2)O(3) coating through a low-temperature ALD process. An inverted structure was achieved by removing the butterfly wing template at high temperature, forming a polycrystalline Al(2)O(3) shell structure with precisely controlled thickness. Other than the copy of the morphology of the structure, the optical property, such as the existence of PBG, was also inherited by the alumina replica. Reflection peaks at the violet/blue range were detected on both original wings and their replica, while a simple alumina coating shifted the reflection peak to longer wavelength because of the change of periodicity and refraction index. The alumina replicas also exhibited similar functional structures as waveguide and beam splitter, which may be used as the building blocks for photonic ICs with high reproducibility and lower fabrication cost compared to traditional lithography techniques.     

飞机刹车用长寿命高摩擦特性炭/炭复合材料制备技术(英文)

以针刺炭纤维准三向结构整体毡为预制体,经丙烯气体狭缝定向流的"外热内冷"、"内热外冷"径向热梯度CVI工艺致密技术,优化组合的热解炭/树脂炭双元炭基体技术,通过调控高温处理技术等三大关键技术制备了A320系列飞机炭刹车盘材料。与现用的A320系列飞机进口炭刹车盘进行了地面台架对比试验和装机应用。结果表明:自主开发的炭刹车盘其设计着陆能量和超载着陆能量的摩擦特性与国外相当,但在高能载(RTO)刹车时,其摩擦系数提高了21%～48%,静摩擦系数提高了28%;装机应用寿命平均达到2700次以上,比国外产品寿命提高了23%,凸现出长使用寿命和高摩擦特性的特色。     

Synthesis of an Al2O3/Al co-continuous composite by reactive melt infiltration

The route for the fabrication of an Al₂O₃/Al co-continuous composite by reactive melt infiltration was investigated using scanning electron microscopy, energy dispersive X-ray microanalysis and X-ray diffraction analysis. It was found that in the process of molten aluminium infiltration into the SiO₂ preform, the chemical reaction of 3SiO₂ + 4Al → 2Al₂O₃ + 3Si occurred at the infiltration front, and generated a transition zone containing a new type of continuous porosity about 100 μm in width. The reaction continued with further infiltration of molten aluminium alloy into this porosity which reacted with the residual SiO₂ until all the SiO₂ was transformed into Al₂O₃. A comparison was made between this route and that by direct infiltration of molten aluminium alloy into the open porosity of an Al₂O₃ preform. As a result of the increased wetting ability of the molten aluminium alloy by the chemical reaction, reactive melt infiltration took place at a higher rate for the SiO₂ preform than that for the direct infiltration of the Al₂O₃ preform. A fracture surface examination demonstrated a toughening effect provided by the continuous aluminium alloy in the composite.     

Pulse chemical vapour infiltration of SiC in porous carbon or SiC participate preform using an r.f. heating system

SiC was infiltrated into a porous carbon or an SiC particulate preform from a gaseous system of 6% CH₃SiCl₃-H₂ using a pulse chemical vapour infiltration apparatus and r.f. heating at 1273 to 1423 K. At 1273 K, the SiC matrix infiltrated the porous carbon initially to half the thickness of the substrate and finally over the full thickness. After 10000 pulses, three-point flexural strength saturated at about 120MPa. SiC particulate preform made from an average particle size of 4m was infiltrated by SiC. After 30000 pulses at 1273 K, the flexural strength of the composite increased to 200 to 220 M Pa.     

Inverse Mopho butterfly: a new approach to photonic crystal

In this paper we showed a new approach for the fabrication of a photonic crystal with a three-dimensional structure. By replicating biomaterials such as the wing of Mopho butterfly with TiO2 nanoparticles using the nanoparticles infiltration method, we can derive photonic crystals with unique structures, which is difficult to fabricate by other approaches. New optical properties are anticipated.     

A wax-mold route to fabricate polymer-derived SiC scaffold

A wax-mold route was developed to fabricate phenolic resin-derived SiC scaffold. Firstly, a wax-mold with the desired structure of the scaffold was obtained. Then resin mixtures were poured into the mold. After curing and pyrolyzing, porous carbon preform was obtained. The wax-mold was removed through being melted during curing, and the pattern material could be recycled. Finally, the SiC scaffold was fabricated by infiltrating liquid silicon into the preform. The dimension shrinkage of SiC scaffold was between 1.3 and 3 % before/after infiltration, which was affected by the infiltrating temperature and the starting components in resin mixtures, and there is no distortion. When the preform with apparent porosity of 45.5% was employed, the flexural strength and the density of SiC scaffold were 445 MPa and 3.07 g/cm³, respectively.     

Monitoring Density and Temperature in C/C Composites Processing by CVI with Induction Heating

Carbon/carbon composites are processed by chemical vapor infiltration (CVI) with radio-frequency inductive heating, which leads to inside-out temperature gradients, suitable for the production of homogeneously densified pieces if properly controlled throughout the whole processing. We present here a 2D axisymmetrical case where a comprehensive numerical model is tested against experimental runs. The numerical thermal model takes into account induction heating, radiative, conductive, and convective effects, intermediate regime diffusion and densification reactions in the pores, and the evolution of the porous medium. The results are the time evolution of the temperature, concentration, and composite material density field, as well as the input power necessary to ensure a given maximal temperature in the preform. Experimental data are measurements of the temperature and density fields at various infiltration stages. Comparison between experience and simulation, yielding an useful agreement, shows that porosity becomes trapped inside the preform as densification proceeds, because of the progressive lowering of the temperature gradient steepness. The discrepancies between computations and experimental data rely on the only approximate knowledge of some quantities, principally the reaction kinetics, which are currently under investigation.     

工艺参数对聚脲甲醛包覆环氧树脂微胶囊物性的影响

以聚脲甲醛为囊壁、双酚A 型环氧树脂( E-51) 和正丁基缩水甘油醚(501 # ) 混合物为囊芯, 采用原位聚合法合成聚脲甲醛包覆环氧树脂体系微胶囊。通过L16 (45 ) 正交试验研究了原料配比、终点p H 值、搅拌速率、酸化时间、平均升温速率等工艺参数对微胶囊的合成状态、粒径大小和分布的影响规律, 并利用扫描电子显微镜(SEM) 、光学2摄影显微镜(OM) 进行表征。结果表明: 原料配比和终点p H 值对微胶囊的合成状态有显著影响;适当延长酸化时间和增加升温速率, 有利于增加微胶囊表面的粗糙度; 随着搅拌速率的增加, 微胶囊的粒径减小、分布变窄。优化的工艺条件为: 原料配比为0. 8∶1 , 终点p H = 2. 0～4. 0 , 酸化时间为2. 0～3. 0 h , 升温速率为0. 17～0. 25 ℃·min -1 , 搅拌速率为325～350 r·min -1 。      

The microstructures of aluminium alloy metal-matrix composites manufactured by squeeze casting

Squeeze-cast metal-matrix composite ingots have been manufactured by recasting A356/10% SiC_p and by melt infiltrating A357/Al₂O_3f. The resulting microstructures have been examined by optical and transmission electron microscopy. The primary aluminium in squeeze-cast A356/10% SiC_p exhibits a dendritic structure with eutectic silicon particles and SiC particulate distributed throughout the interdendritic regions. Unlike squeeze-cast monolithic A357, the primary aluminium dendrite arm spacing and eutectic-silicon particle size in squeeze-cast A356/10% SiC_p are not strongly affected by an increased cooling rate at high applied pressure. With low die and preform temperature, a high applied pressure is essential to obtain complete melt infiltration in A357/Al₂O_3f. During melt infiltration, the ceramic preform acts as a filter, preferentially allowing silicon rich liquid to pass through.     

Subtractive Structural Modification of Morpho Butterfly Wings

Different from studies of butterfly wings through additive modification, this work for the first time studies the property change of butterfly wings through subtractive modification using oxygen plasma etching. The controlled modification of butterfly wings through such subtractive process results in gradual change of the optical properties, and helps the further understanding of structural optimization through natural evolution. The brilliant color of Morpho butterfly wings is originated from the hierarchical nanostructure on the wing scales. Such nanoarchitecture has attracted a lot of research effort, including the study of its optical properties, its potential use in sensing and infrared imaging, and also the use of such structure as template for the fabrication of high‐performance photocatalytic materials. The controlled subtractive processes provide a new path to modify such nanoarchitecture and its optical property. Distinct from previous studies on the optical property of the Morpho wing structure, this study provides additional experimental evidence for the origination of the optical property of the natural butterfly wing scales. The study also offers a facile approach to generate new 3D nanostructures using butterfly wings as the templates and may lead to simpler structure models for large‐scale man‐made structures than those offered by original butterfly wings.     

Study of the infiltration and mechanical behavior of C/C composites prepared by ECVI

Three kinds of preforms, chopped fibers/resin carbon, spreading layers of carbon cloth, and needle-pricked long fiber felt, were used in this study. The preforms were densified by using the electrified preform heating CVI method (ECVI), and infiltrated using natural gas. Initial thermal gradients were determined. Resistivity and density evolutions with infiltration time have been recorded. A tensile test was applied to investigate the influence of preform architecture on the tensile properties of the C/C composites. Results show that the architecture of preform strongly influences the uniformity of infiltration and fibers/matrix bonding. The samples prepared from using 1K plain carbon cloth have the smallest density variations with position (about 0.011 g/cm³), and possess the highest tensile strength and modulus, while the samples produced from chopped fibers/resin carbon possess the lowest tensile strength due to their strong interfacial bonding between resin carbon and carbon fibers and poor microstructure.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

以淀粉为填充剂的碳坯渗硅制备反应烧结碳化硅陶瓷

探索了一条高性能RBSC低成本制造的新途径,本研究以石油焦粉为碳质原料制坯,玉米淀粉为填充剂调整碳坯的密度,纯碳素坯经高温渗硅得到密度为3.12g/cm³,强度为580MPa的反应烧结碳化硅陶瓷.研究结果表明掺加淀粉后素坯中含有更多的微孔,烧结体晶粒平均尺寸为2-4μm,晶粒细化是材料性能比传统RBSC材料高的原因.     

Casting particulate and fibrous metal-matrix composites by vacuum infiltration of a liquid metal under an inert gas pressure

A new method of making metal-matrix composites is reported. This method combines the essentials of three liquid-phase fabrication methods: (i) vacuum infiltration, (ii) infiltration under an inert gas pressure, and (iii) squeeze casting. In this method, the particulate or fibrous preform is placed in a mould and the matrix alloy is placed above the preform. The matrix alloy is heated to the liquidus temperature together with the mould and the preform under vacuum. Then an inert gas like argon is compressed on to the top surface of the matrix-alloy melt, forcing the melt to infiltrate the preform. The pressure is 1000 to 2500 psi. As the melt is just at liquidus temperature, it is much lower than that used in squeeze casting. Moreover, the pressure is an order of magnitude lower than that used in squeeze casting. The low temperature lessens the interfacial reaction between the matrix and the filler, while the low pressure essentially eliminates preform compression. This method has been successfully used to fabricate aluminium-matrix composites reinforced by short ceramic fibres, continuous ceramic fibres, SiC particles, Al₂O₃ particles, graphite flakes and SiC whiskers.     

Finite element analysis of textile composite preform stamping

The forming or draping of a textile composite preform may result in large changes in the fibrous microstructure of the preform. This change in the local fiber orientation leads to significant changes in the fabric permeability as well as the mechanical properties of the ensuing composite structure. Therefore, this change in orientation of the tows of the preform needs to be known accurately to calculate the various effective properties of the composite. A new finite element approach for stamping analysis of a plain-weave textile composite preform has been developed. This model is simple, efficient and can be used in the existing finite element codes. The model represents the preform as a mesh of 3-D truss elements and 3-D shell elements. The truss elements model the tows, which are allowed to both scissor and slide relative to one another. The shell elements represent a fictitious material that accounts for inter-tow friction and fiber angle jamming. The model takes into account large strains and large deformations. In-plane uniaxial tension tests have been performed on plain-weave specimens for determining the constitutive law of the transforming medium and to show the inter-tow sliding. Application of the model is demonstrated by simulating the stamping of a preform by a spherical punch. The results from the simulation show good correlation with results from the experiments.