Effect of Bridged Boron Nitride Coatings on the Flexure Behavior of a Unidirectional Ceramic-Fiber Ceramic-Matrix Composite

A series of 50 vol/% unidirectional Nicalon™ fiber/zirconium titanate matrix composites were fabricated by alkoxide infiltration of the fiber tows. The fibers had a thin, amorphous boron nitride coating that was either heavily or lightly bridged. The bridged coatings were a result of localized excess boron nitride deposits and had the effect of binding adjacent fibers at boron nitride nodules during the immersion process. The resultant composites contained matrix-rich ribbons, which exhibited laminate-like mechanical behavior reported previously. Highest strength was obtained when the composites were loaded parallel to the ribbon orientation, and highest toughness (WOF) was obtained when the composites were loaded perpendicular to the ribbons. The ribbon orientation had a more pronounced effect on composite behavior than the presence or lack of bridging boron nitride nodules. However, the bridging nodules altered the relative orientation of the matrix-rich ribbons during fabrication and, thus, the direction of optimal strength or toughness.     

Influence of hot compaction pressure on the interfacial properties in the amorphous metallic ribbon/thermoplastic matrix composite system

The application of rapidly solidified amorphous metal ribbons as continuous reinforcements for thermoplastic composites is examined. The metallic glass alloy Fe₄₀Ni₄₀B₂₀ (at. percent), with good stiffness, strength, and magnetic properties, was selected as the ribbon alloy. The mechanical properties of the ribbons (elastic modulus and fracture strength) were determined by tensile testing under plane-stress conditions. The continuous FE₄₀Ni₄₀B₂₀ amorphous ribbons were incorporated as reinforcements into a polypropylene (thermoplastic) matrix. To evaluate the quality of the composites formed, ribbon pullout tests were performed to measure the interfacial ribbon/matrix bond strength. It was noted that increasing the hot compaction pressure during fabrication and the surface texture of the ribbons by etching significantly improved the interfacial shear strength between the ribbon and thermoplastic matrix.     

Fabrication and Characterization of PZT/Thermoplastic Polymer Composites for High-Frequency Phased Linear Arrays

A new method of fabricating PZT/potymer composites with 2-2 connectivity is described. This fabrication technique offers significant advantages over conventional dice-and-fill fabrication methods, and the composites exhibit the high electromechanical coupling expected from conventional PZT/polymer composites. In this method, thin (≤20μm) sintered PZT plates and sheets of a thermoplastic polymer film (≤6 μm) are bonded together via thermal processing. A technique for sintering thin, flat PZT plates from tape cast materials was developed to provide the necessary PZT plates. The resulting composite blocks were cut to required dimensions, electroded, and poled. Electromechanical properties were measured to evaluate the composites.     

Physical interactions at carbon nanotube-polymer interface

Mechanical properties of carbon nanotube (CNT) reinforced polystyrene rod and CNT reinforced epoxy thin film were studied and the CNT-polymer interface in these composites was examined. Transmission and scanning electron microscopy examinations of CNT/polystyrene (PS) and CNT/epoxy composite showed that these polymers adhered well to CNT at the nanometer scale. Molecular mechanics simulations and elasticity calculations were used to quantify some of the important interfacial characteristics that critically control the performance of a composite material. In the absence of chemical bonding between CNT and the matrix, it is found that the non-bond interactions, consist of electrostatic and van der Waals forces, result in CNT-polymer interfacial shear stress (at 0 K) of about 138 and 186 MPa, respectively, for CNT/epoxy and CNT/PS. The high interfacial shear stress calculated, about an order of magnitude higher than micro fiber reinforced composites, is believed attributed to intimate contact between the two solid phases at the molecular scale. Simulations and calculations also showed that local non-uniformity of CNT and mismatch of the coefficients of thermal expansions between CNT and polymer matrix also promote the stress transfer ability between the two.     

New hybrid method for simultaneous improvement of tensile and impact properties of carbon fiber reinforced composites

For weight savings of automobiles to improve fuel efficiency, tensile and impact strengths of carbon fiber reinforced composites (CFRC) are important properties required for substitution of metallic or ceramic automotive parts by CFRC parts. Effect of surface treatments of carbon fiber (CF) such as plasma, nitric acid, and liquid nitrogen treatments on interfacial bonding and mechanical properties of CF reinforced thermoplastic composites was investigated and nitric acid treatment was the best method to improve the interfacial affinity between the used CF and thermoplastic polymer matrix since the treatment induced acidic functional groups on the surface and increased surface roughness simultaneously. A new hybrid fabrication method was suggested by applying a bi-component two-layer structure to the film insert molding to improve tensile and impact strengths of CFRC simultaneously. Compared with tensile and impact strengths of the base polymer, those of the new hybrid composites filled with rubber particles and CF were improved by about 41.3% and 105.7%, respectively. In particular, tensile and impact strengths of the composite specimen prepared by the hybrid fabrication method were improved by about 15.0% and 36.0%, respectively when compared with those of the composite specimen prepared by the conventional melt mixing.     

High performance carbon nanotube based composite film from layer-by-layer deposition

So far, preparation of strong carbon nanotube (CNT)/polymer composites still faces big challenges mainly due to the limited controls of CNT dispersion and alignment in polymers. Here, a new “layer-by-layer deposition” method is put forward to prepare CNT/polyvinyl alcohol (PVA) composite films. This is based on intermittent deposition of aligned CNT and PVA layers on a paper tape substrate. The in situ deposition allows PVA to infiltrate into the CNT film efficiently, and, as a result, the mechanical property of CNT/PVA composite film has been improved remarkably. For example, the composite film possesses a tensile strength of 1.7 GPa, which is almost one order of magnitude and 20 times higher than those of the pure CNT and PVA films, respectively. The high performance of the composite film could be ascribed to the role of PVA infiltration, which leads to not only the formation of strong interfacial bonding between CNTs and PVA matrix but also the reduction of film thickness. The novel process offers a new research direction for preparing CNT-based composites and future performance maximization.     

Stress analysis of ribbon reinforced composites

In this paper the results of both the theoretical and experimental stress analyses of composite materials reinforced with ribbons are presented. The reinforcing materials for such composites are characterized as two-dimensional elements which are isotropic in planes parallel to the faces. The theoretical work is based on the finite element method. Experimentally, the photoelastic technique is employed to determine the stresses around the glass ribbons embedded in an epoxy matrix. The specimens, containing a number of aligned ribbons, were loaded in tension parallel to the ribbon width. The variation of shearing stress at the ribbon-matrix interface as well as the pertubation effect on both the ribbon normal stress and interfacial shear due to the discontinuity of the neighboring ribbons are discussed.     

Effect of a Boron Nitride Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites

Typically, the debonding and sliding interface enabling fiber pullout for SiC-fiber-reinforced SiC-matrix composites with BN-based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress-rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner-rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress-state at the BN/matrix interface of melt-infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.     

On the strength and stiffness of planar reinforced plastic resins

The recent history of planar reinforced plastic resins, including glass flake, high modulus ceramic flake, and continuous vapor coated film composites, is reviewed. The theoretical mechanics of both continuous (film) and discontinuous (flake and ribbon) reinforcements are summarized in simple form. A novel set of design curves is presented from which the lower bound requirements for the flake composite constitutents may be read directly. At the same time, the dependence of the composite ultimate strength on the shear strength of the plastic resin matrix is demonstrated. The mechanical properties of experimental film and flake composites representative of recent work are reported and compared with the theoretical predictions. In conclusion, the potential of planar reinforced plastic resin composites is discussed and found to be significant for applications where low weight and high isotropic stiffness are required, for example in aero-structural, airfoil, or blade components.     

格构增强型复合材料夹层结构的制备与受力性能

真空导入成型工艺是一种新型的适合大型/异型复合材料结构件成型的技术.选用H-60 PVC泡沫、四轴向玻璃纤维布以及乙烯基酯树脂,通过在泡沫芯材上、下表面开槽,同时沿芯材厚度方向剖开,采用真空导入成型工艺制备出在结构上具有创新构型的格构增强型复合材料夹层结构.研究结果表明,真空导入成型工艺充模速度快、成型效益高;格构增强型复合材料夹层结构的剪切、平压与抗弯性能均较传统夹层结构得以提高;其格构腹板可有效抑制泡沫芯材剪切裂纹的扩展,避免面板与芯材的剥离破坏;阐明了格构增强型复合材料夹层结构的受弯极限承载能力.     

CHARACTERIZATION, PROPERTIES, AND PROCESSING OF LARC PETI-5® AS A HIGH-TEMPERATURE SIZING MATERIAL: III. ADHESION ENHANCEMENT OF CARBON/BMI COMPOSITES

A phenylethynyl-terminated imide oligomer (LaRC PETI-5®) with a number average molecular weight of 2500 g/mol has been applied onto the surfaces of PAN-based carbon fiber tows and woven carbon fabrics as a sizing material to introduce an interphase between the fiber and matrix in carbon/BMI composites. The adhesion between the fiber and matrix was enhanced by the presence of a properly processed LaRC PETI-5® interphase. The results showed that when LaRC PETI-5® was sized and processed at 150°C, the interfacial shear strength (IFSS) of unidirectional IM7/BMI composite measured by using a microindentation technique and the interlaminar shear strength (ILSS) of a carbon/BMI composite measured by short beam shear test were markedly improved by about 35% and 66%, respectively, in comparison with the unsized counterparts. The adhesion enhancement strongly depends not only on the presence or absence of LaRC PETI-5® sizing interphase but also on the temperature profile applied to the sizing before composite fabrication. Both of these factors critically influence the physical and chemical state of the sizing material. Scanning electron microscopic observations of the composite fracture surfaces support the improved interfacial property of carbon/BMI composites.     

Interfacial zone surrounding the diamond in reaction bonded diamond/SiC composites: Interphase structure and formation mechanism

Diamond/SiC composites have attracted considerable research interests due to their outstanding properties sought for a wide range of applications. Among a few techniques used for the fabrication of diamond/SiC composites, molten Si infiltration is an approach highly favored due to its cost-effectiveness and process flexibility. This study critically evaluated the interfacial zone surrounding the diamond in a reaction bonded (RB) diamond/SiC composite. XRD suggests that the composite consists of diamond, α-SiC, β-SiC, Si, and graphite. TEM reveals that a thin layer of graphite surrounds the diamond grain and it appears to form through a process of diamond graphitization and amorphous carbon transformation during the fabrication. In addition, a carbon dissolution and saturation process is proposed as a predominant mechanism for the formation of nano-crystalline SiC near the interface as well as the defects inside the SiC grits. A minor Al₄C₃ phase is occasionally detected near the interface region.     

Microstructure and fracture behavior of (co)injection‐molded polyamide 6 composites with short glass/carbon fiber hybrid reinforcement

The mechanical and fracture properties of injection molded short glass fiber)/short carbon fiber reinforced polyamide 6 (PA 6) hybrid composites were studied. The short fiber composites of PA 6 glass fiber, carbon fiber, and the hybrid blend were injection molded using a conventional machine whereas the two types of sandwich skin–core hybrids were coinjection molded. The fiber volume fraction for all formulations was fixed at 0.07. The overall composite density, volume, and weight fraction for each formulation was calculated after composite pyrolysis in a furnace at 600°C under nitrogen atmosphere. The tensile, flexural, and single‐edge notch‐bending tests were performed on all formulations. Microstructural characterizations involved the determination of thermal properties, skin–core thickness, and fiber length distributions. The carbon fiber/PA 6 (CF/PA 6) formulation exhibits the highest values for most tests. The sandwich skin‐core hybrid composites exhibit values lower than the CF/PA 6 and hybrid composite blends for the mechanical and fracture tests. The behaviors of all composite formulations are explained in terms of mechanical and fracture properties and its proportion to the composite strength, fiber orientation, interfacial bonding between fibers and matrix, nucleating ability of carbon fibers, and the effects of the skin and core structures. Failure mechanisms of both the matrix and the composites, assessed by fractographic studies in a scanning electron microscope, are discussed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 957–967, 2005     

Improvement of mechanical properties of structural thermoplastic composites using a reactive (low molecular weight) matrix component

An innovative manufacturing process for continuous fiber composites with the polymeric matrix made up of polypropylene and epoxy resin, as a model reactive low molecular weight component, was developed; variable process parameters give rise to different morphologies of matrix components surrounding the woven fabric reinforcement. Furthermore, the combination of both thermoplastic and thermosetting polymers permitted intimate fibers impregnation, typical of thermosetting matrix composites, with short process cycle time, which usually occurs in manufacturing process of thermoplastic matrix composites. Polypropylene (PP) films, glass fibers fabric, and epoxy resin film were used to produce flat composite through film‐stacking technique. The preparation process focused on control of both epoxy resin cure process and polypropylene melting. The process was able to induce the two matrix components to form either a planar (sandwich‐like) structure or a three‐dimensional (3D) network by means of controlling the process parameters such as pressure and heating rate. The strong enhancement of the mechanical properties (Young's modulus and tensile strength of the composites with the 3D structure were almost twice as high of those of the composites with sandwich‐like matrix structure) was due to the different microstructures produced by the interplanar flow of the thermoplastic polymer. POLYM. COMPOS., 31:1762–1769, 2010. © 2010 Society of Plastics Engineers.     

Measurement of Fiber/Matrix Interfacial Shear Stress at Elevated Temperatures

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic-matrix composites and one metal-matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differences.     

剑麻/含红土聚乙烯基复合材料制备与性能

针对废旧地膜资源化利用过程中出现的高成本和低性能问题,本文提出了废旧地膜免清洗和剑麻纤维边角料增强的废旧地膜资源化利用技术。采用挤出造粒和注塑成型工艺,制备了剑麻边角料填充含红土废旧聚乙烯复合材料,分析了红土和剑麻纤维边角料对废旧地膜的填充作用。结果表明,红土颗粒使废旧地膜注塑试样的拉伸模量、硬度和耐热温度分别提高了34.4%、41.3%、和33.1%。红土颗粒难以和塑料基体形成良好的界面粘结,导致含红土废旧地膜注塑试样的拉伸强度、弯曲性能和冲击强度轻微降低,表明红土颗粒不能对废旧地膜进行增韧增强,但可以提高模量和耐热温度。剑麻纤维边角料对含红土废旧地膜具有明显的增强增韧作用,随着剑麻纤维添加量的增加,剑麻纤维填充的含红土废旧地膜复合材料的力学性能增加。剑麻纤维填充量超过一定值后,会在复合材料中引入气孔,同时会降低剑麻纤维的分散程度,出现剑麻聚集体,导致复合材料的力学性能降低。     

The interfacial bond stress in steel fiber cement composites

In fiber cement composites most fibers are in a state of partial bond due to internal stresses arising from moisture migration during fabrication and subsequent volume changes in the matrix. A wide variation in the computed interfacial bond strength therefore occurs depending upon the type of test or when derived from phenomena such as crack spacing. In practice debonding of the fibers occurs in flexural tension in the presence of a strain gradient. This paper presents further data on steel fiber mortar and concrete to confirm the validity of the composite mechanics approach to predict the composite flexural strength. It is shown that the composition of the matrix and its strength properties influence the fiber-matrix interfacial bond stress and the relative contributions of the matrix and the fibers to the composite flexural strength.     

Interlaminar shear test by hybrid sandwich specimen under distributed load

A new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle-ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon-fiber-reinforced epoxy skins and a ±θ aramid-fiber-reinforced epoxy angle-ply core.     

Effects of Interfacial Films on Thermal Stresses in Whisker-Reinforced Ceramics

Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al₂O₃, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al₂O₃), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.     

界面聚合制备复合膜过程的数学模型

基于高分子物理化学、质量传递和相分离成膜理论,研究在复合膜制备过程中采用界面聚合反应成膜的机理,建立了非稳态条件下反应．扩散联合控制的数学模型;通过有针对性地简化,该模型可适用于反应控制和扩散控制。模型中无量纲参数有明确的物理意义,较好地反映了界面聚合反应成膜过程的机理。无量纲化处理使模型解析解形式更为简单、实用,模型与实验数据吻合良好,且优于现有模型。通过模拟计算,可得出单体(A组分)浓度、膜的厚度、膜厚增长率随时间的变化关系,并可考察聚合反应速率常数、单体(A组分)在复合层中的扩散系数、单体初始浓度等参数对成膜过程的影响．理论结果可用于指导界面聚合反应成膜实践。