Prospect of nanoscale interphase evaluation to predict composite properties

For meeting the requirements of lightweight and improved mechanical properties, composites could be tailor-made for specific applications if the adhesion strength which plays a key role for improved properties can be predicted. The relationship between wettability and adhesion strength has been discussed. The microstructure of interphases and adhesion strength can be significantly altered by different surface modifications of the reinforcing fibers, since the specific properties of the interphase result from nucleation, thermal and/or intrinsic stresses, sizing used, interdiffusion, and roughness. The experimental results could not confirm a simple and direct correlation between wettability and adhesion strength for different model systems. The main objective of the work was to identify the interphases for different fiber/polymer matrix systems. By using phase imaging and nanoindentation tests based on atomic force microscopy (AFM), a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites was conducted. As model sizings for PP composites, γ-aminopropyltriethoxysilane (APS) and either polyurethane (PU) or polypropylene (PP) film former on glass fibers were investigated. The EP-matrix was combined with either unsized glass fibers or glass fibers treated with APS/PU sizing. It was found that phase imaging AFM was a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficiently small indentation force was found to be sufficient for measuring actual interphase properties within a 100-nm region close to the fiber surface. Subsequently, it also indicated a different gradient in the modulus across the interphase region due to different sizings. The possibilities of controlling bond strength between fiber surface and polymer matrix are discussed in terms of elastic moduli of the interphases compared with surface stiffness of sized glass fibers, micromechanical results, and the mechanical properties of real composites.     

Fracture behavior of vinylester resin matrix composites reinforced with alkali‐treated jute fibers

Jute fibers were treated with 5% NaOH solution for 4 and 8 h, respectively, to study the mechanical and impact fatigue properties of jute‐reinforced vinylester resin matrix composites. Mechanical properties were enhanced in case of fiber composites treated for 4 h, where improved interfacial bonding (as evident from scanning electron microscopy [SEM]) and increased fiber strength properties contributed effectively in load transfer from the matrix to the fiber; but their superior mechanical property was not retained with fatigue, as they showed poor impact fatigue behavior. The fracture surfaces produced under a three‐point bend test and repeated impact loading were examined under SEM to study the nature of failure in the composites. In case of untreated fiber composites, interfacial debonding and extensive fiber pullout were observed, which lowered the mechanical property of the composites but improved their impact fatigue behavior. In composites treated for 4 h under repeated impact loading, interfacial debonding occurred, followed by fiber breakage, producing a sawlike structure at the fracture surface, which lowered the fatigue resistance property of the composites. The composites with fibers treated with alkali for 8 h showed maximum impact fatigue resistance. Here, interfacial debonding was at a minimum, and the fibers, being much stronger and stiffer owing to their increased crystallinity, suffered catastrophic fracture along with some microfibrillar pullout (as evident from the SEM micrographs), absorbing a lot of energy in the process, which increased the fatigue resistance property of the composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2588–2593, 2002     

Influence of different glass fiber sizings on selected mechanical properties of PET/glass composites

The properties of fiber-reinforced plastics are considerably influenced by fiber-matrix interaction. The aim of this study was to investigate the influence of glass fiber surface treatments on the morphology of poly(ethylene terephthalate) (PET) and on selected mechanical properties of unidirectional PET/glass fiber composites. The materials used here were E-glass fibers treated with model sizings including aminosilane as a coupling agent and polyurethane and epoxy resin dispersions as film formers and PET as the matrix. For identification of the degree of crystallinity of the PET matrix, differential scanning calorimetry (DSC) was used. To study the influence of the different sizings on the mechanical properties, the following tests were performed: interlaminar and intralaminar shear tests and a transverse tensile test. Dynamic-mechanical analysis (DMA) was used to characterize the behavior of the composites under dynamical load. The DSC results show that the overall crystallinity and the melting behavior of the PET matrix were hardly influenced by the glass fiber surface treatments used. The various strength properties of the composites are influenced not only by the silane coupling agent, but also by the type of film former. With an epoxy resin dispersion, the mechanical properties were enhanced compared with a polyurethane dispersion. These results were confirmed by characterization of the composites by DMA.     

Mechanical characterization and fractography of glass fiber/polyamide (PA6) composites

The mechanical properties of the glass fiber reinforced Polyamide (PA6) composites made by prepreg tapes and commingled yarns were studied by in‐plane compression, short‐beam shear, and flexural tests. The composites were fabricated with different fiber volume contents (prepregs—47%, 55%, 60%, and commingled—48%, 48%, 49%, respectively) by using vacuum consolidation technique. To evaluate laminate quality in terms of fiber wet‐out at filament level, homogeneity of fiber/matrix distribution, and matrix/fiber bonding standard microscopic methods like optical microscopy and scanning electron microscopy (SEM) were used. Both commingled and prepreg glass fiber/PA6 composites (with V_f ∼ 48%) give mechanical properties such as compression strength (530–570 MPa), inter‐laminar shear strength (70–80 MPa), and transverse strength (80–90 MPa). By increasing small percentage in the fiber content show significant rise in compression strength, slight decrease in the ILSS and transverse strengths, whereas semipreg give very poor properties with the slight increase in fiber content. Overall comparison of mechanical properties indicates commingled glass fiber/PA6 composite shows much better performance compared with prepregs due to uniform distribution of fiber and matrix, better melt‐impregnation while processing, perfect alignment of glass fibers in the composite. This study proves again that the presence of voids and poor interface bonding between matrix/fiber leads to decrease in the mechanical properties. Fractographic characterization of post‐failure surfaces reveals information about the cause and sequence of failure. POLYM. COMPOS., 36:834–853, 2015. © 2014 Society of Plastics Engineers     

The effect of pyrolysis on the mechanical properties and microstructure of carbon fiber-reinforced and stabilized fiber-reinforced phenolic resins for carbon/carbon composites

Carbon/carbon composites were prepared with phenol-formaldehyde resin, one kind of commercial carbon fiber, and a stabilized fiber that was developed in our laboratory. The effect of pyrolysis on the microstructure, fracture behavior, and flexural strength of the composites during the carbonization process was studied. During the pyrolysis of the composites a chemical reaction at the fiber/resin interface apparently took place. A thermogravimetry (TG) study indicated that the use of stabilized fiber reinforced composites inhibited decomposition reactions and thermal fragmentation in the matrix resin, and reduced the weight loss of the final composites. The X-ray reflection of the resin and the two composites showed a reflection appearing at 2θ ≈ 12° when the samples were carbonized above 600°C. The intensity of this reflection in the composites made with stabilized fiber was higher than that of the composite made with carbon fiber. Because of the formation of strong bonding in the fiber-matrix interface, the composites made with stabilized fiber showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with an increase in the carbonization temperatures, even if the fracture behaviors showed catastrophic failure. The flexural strength of the composites made with carbon fiber showed pseudo-plastic patterns and debonding with very little fiber pullout. Above 800°C, these composites showed a catastrophic failure and smooth failure surfaces. During pyrolysis the flexural strength decreased with an increase in the carbonization temperature.     

玻璃纤维增强PPBES基复合材料性能

以共聚型二氮杂萘联苯结构聚醚砜(PPBES)树脂为基体,连续玻璃纤维(GF)为增强体,通过溶液预浸,热压成型工艺制备单向复合材料。通过对树脂溶液黏度、复合材料纤维体积含量测试,并对复合材料样条进行三点弯曲、层间剪切试验,研究了纤维体积含量对复合材料力学性能的影响,借助断面形貌分析了复合材料受力破坏模式。结果表明,PPBES/GF复合材料的弯曲强度随纤维体积含量的增加呈现先增大后减小的趋势,极值出现在纤维体积含量为57%时,弯曲弹性模量和层间剪切强度随纤维体积含量的增加呈现逐渐增大的趋势,复合材料的受力破坏模式为界面脱粘破坏和树脂基体内部破坏同时存在。     

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.     

Boron Nitride Nanotubes-Reinforced Glass Composites

Boron nitride nanotubes (BNNT) of significant lengths were synthesized by reaction of boron with nitrogen. Barium calcium aluminosilicate glass composites reinforced with∼4 wt% of BNNT were fabricated by hot pressing. Ambient-temperature flexure strength and fracture toughness of the glass-BNNT composites were determined. The strength and fracture toughness of the composite were higher by as much as 90% and 35%, respectively, than those of the unreinforced glass. Microscopic examination of the composite fracture surfaces showed pullout of the BNNT. The preliminary results on the processing and improvement in mechanical properties of BNNT-reinforced glass matrix composites are being reported here for the first time.     

Relation of interfacial adhesion in Kevlar®/epoxy systems to surface characterization and composite performance

A central problem in composite materials is the poorly understood relation between the nature of the surfaces at the fiber/matrix interface, the actual interfacial bond strength, and interface-sensitive composite properties, in this study on the Kevlar®/epoxy composite system, the interface was varied chemically by fiber sizings. The sized and unsized fiber surfaces and the cured matrix surface were characterized by contact angle measurements. The interfacial shear strength was directly measured by single-filament pull-out tests of sized and unsized fibers in epoxy matrix. The shear strengths of the composites made with sized and unsized fibers were measured. The results from surface analysis, interfacial shear tests, and composite shear tests were consitent. This suggests that surface-contact-angle analysis and single-filament pull-out tests may be helpful in screening strength of the composite.     

LaPO4 coating on alumina-based fiber: Strength retention of fiber and improvement of interfacial performances

The characteristics of the interface are the key factors that determine the mechanical properties and fracture behavior of fiber-reinforced ceramic matrix composites. Design and preparation of coatings which can preserve fiber strength and maintain appropriate interfacial bonding strength are of great challenges. LaPO₄ coating is a promising weak interface coating for oxide fiber reinforced oxide ceramic matrix composites. Through this coating, the toughening mechanism of the composite such as fiber pulling out and fiber debonding is triggered. The LaPO₄ coating was deposited on the surface of alumina-based fibers by a solution precursor heterogeneous precipitation method. The effects of different precursors and different deposition temperatures on fiber strength were studied, and the mechanism of the strength degradation of the coated fiber was analyzed. It was found that the fibers coated with phytic acid precursor and deposited at 90 °C had the highest tensile strength compared to other coated fibers. The retention of strength is attributed to its loosely stacked coating. Besides, a single fiber pullout test was carried out to evaluate the effect of the coating on the interface of the composites. The results show that the composites coated by depositing citric acid precursor and phytic acid precursor at 90 °C can reduce the interfacial bonding strength by 32.5% and 46.7%, respectively compared to uncoated composites. This study has potential application value in the preparation of ceramic matrix composites used in oxidation and high temperature environments.     

Effect of macromolecular coupling agent on the property of PP/GF composites

Silane‐grafted polypropylene manufactured by a reactive grafting process was used as the coupling agent in polypropylene/glass‐fiber composites to improve the interaction of the interfacial regions. Polypropylene reinforced with 30% by weight of short glass fibers was injection‐molded and the mechanical behaviors were investigated. The results indicate that the mechanical properties (tensile strength, tensile modulus, flexural strength, flexural modulus, and Izod impact strength) of the composite increased remarkably as compared with the noncoupled glass fiber/polypropylene. SEM of the fracture surfaces of the coupled composites shows a good adhesion at the fiber/matrix interface: The fibers are coated with matrix polymer, and a matrix transition region exists near the fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1537–1542, 1999     

Evaluation of Porous ZrO2-SiO2 and Monazite Coatings Using NextelTM 720-Fiber-Reinforced Blackglas™ Minicomposites

A procedure was developed to fabricate oxide-fiber-reinforced minicomposites with a dense matrix and evaluate two oxidation-resistant interface coatings, porous oxide (zirconia-silica mixture) and monazite. The coatings were evaluated using Nextel^TM 720-fiber-reinforced Blackglas^TM-matrix minicomposites. Boron nitride (BN) coated and uncoated fibers were used as controls for comparison. The evaluation was based on ultimate failure strengths, fractography, and fiber pushin tests. All the composites that used fiber coatings had ultimate strengths significantly better than the control that used uncoated fibers. In addition, porous-oxide-coated fibers were found to be similar to BN-coated fibers in strength, fractography, and fiber pushin behavior. Monazite-coated fibers resulted in similar ultimate strengths but showed no appreciable fiber pullout. Fiber pushin tests showed that monazite debonds readily but frictional resistance is higher than for BN or porous oxide fiber coatings.     

Swelling and dissolution rates of glass fiber sizings in matrix resin via micro-dielectrometry

Complex fiber-matrix interactions occur in the processing of glass fiber reinforced polymeric composites, because of the proprietary, complex composition of commercial sizings applied to the glass fiber surface. Research involving a vinyl ester resin system and three model commercial glass fiber sizings, having varying levels of solubility in the resin, has shown that micro-dielectrometry can provide important information about interactions and may be useful as a tool in optimizing sizing-matrix resin interactions. Two distinct types of interactions may be monitored by micro-dielectrometry: The initial resin swelling of the sizing, as well as the dissolution of the sizing into the resin. An estimate of the times associated with swelling and dissolution of the sizing into the matrix resin can be made from micro-dielectric measurements to optimize composite processing.     

Green Composites from Natural Rubber and Oil Palm Fiber: Physical and Mechanical Properties

Natural rubber (NR) composites were prepared by incorporating short oil palm fibers of different lengths (viz., 2, 6, 10, and 14 mm) into natural rubber matrix in a mixing mill according to a base formulation. The curing characteristics of the mixes were studied and the samples were vulcanized at 150°C. The vulcanization parameters, processability characteristics, and tensile properties of these composites were analyzed. The effects of fiber length, orientation, loading, and fiber-matrix interaction on the mechanical properties of the green composites were studied. The reinforcement property of the alkali-treated fiber was compared with that of the untreated one. The extent of fiber orientation was studied from green strength measurements. From anisotropic swelling studies, the extent of fiber alignment and the strength of fiber–rubber interface adhesion were analyzed. Scanning electron microscopic (SEM) studies were carried out to analyze the fiber surface morphology, fiber pullout, and fiber–rubber interface.     

Effects of polyhedral oligomeric silsesquioxane coatings on the interface and impact properties of carbon‐fiber/polyarylacetylene composites

Two kinds of polyhedral oligomeric silsesquioxane (POSS) coatings were used for the modification of the interface in carbon fiber (CF) reinforced polyarylacetylene (PAA) matrix composites. The effects of the organic–inorganic hybrid POSS coatings on the properties of the composites were studied with short‐beam‐bending, microdebonding, and impact tests. The interlaminar shear strength and interfacial shear strength showed that the POSS coatings resulted in an interfacial property improvement for the CF/PAA composites in comparison with the untreated ones. The impact‐test results implied that the impact properties of the POSS‐coating‐treated composites were improved. The stiffness of the interface created by the POSS coatings was larger than that of the fiber and matrix in the CF/PAA composites according to the force‐modulation‐mode atomic force microscopy test results. The rigid POSS interlayer in the composites enhanced the interfacial mechanical properties with a simultaneous improvement of the impact properties; this was an interesting phenomenon in the composite‐interface modification. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5202–5211, 2006     

Strength and Toughness of Continuous-Alumina-Fiber-Reinforced Glass-Matrix Composites

Unidirectional, continuous-fiber composites were fabricated using polycrystalline alumina fibers and four different silicate glass matrices of differing thermal expansion. Fracture toughness measurements, strength measurements, and fractographic analysis of failed specimens are used to identify the failure mechanism. Results show that the elastic modulus mismatch between the matrix and the fibers shields the reinforcing fibers from matrix crack extension, thereby increasing composite toughness without fiber pullout. Fractographic analysis shows that fiber shielding leads to fiber failure ahead of matrix crack. Composite toughness increases linearly with increases in the residual compressive stress in the matrix phase. Ultimate composite strengths are dependent upon thermal-expansion-induced residual stresses and fiber strength.     

Physico-Chemical and micromechanical analysis of the interface in a poly(phenylene sulfide)/glass fiber composite—a microbond study

The glass fiber/PPS composite has excellent thermal and chemical properties. The main disadvantage of the composite is its poor mechanical resistance to impact. To improve this property, the fibers were coated with a new type of sizing. The equired characteristics for this sizing is to create strong interactions between the PPS matrix and the glass fiber surface. The ability of the sizings to improve the glass/PPS adhesion has been assessed by the microbond technique. An inconvenience of this technique is the difficulty in defining a parameter that is characteristic of the interfacial adhesion. The objective of this publication is to demonstrate that a plastic flow of the PPS matrix around the fiber leads to a uniform shear strength. The adhesion between these two materials can therefore be obtained by the mean interfacial shear strength.     

A Method for Coating Fibers in Oxide Composites

A method for applying monazite coatings onto oxide fibers after matrix infiltration and sintering is devised and demonstrated. It uses an intermediate fugitive coating, followed by impregnation and pyrolysis of a monazite precursor solution. Composites with monazite coatings produced in this manner as well as those with only a fugitive coating exhibit superior mechanical performance to composites without any coating, as manifested in higher notched strength and fracture energy as well as significantly greater amounts of fiber pullout.     

Effects of reactive and non-reactive fiber coatings upon performance of graphite/epoxy composites

Interlayers of controlled composition and thickness were applied to commercial graphite fiber bundles by electrochemical copolymerization, and the impact and interlaminar shear strength of composites from these coated fibers were examined. Glycidyl acrylate/methyl acrylate copolymers represented coatings that were reactive to the epoxy matrix during curing; acrylonitrile/methyl acrylate copolymers represented non-reactive systems. The reactive systems showed 10 to 30 percent simultaneous improvement in impact and interlaminar shear strengths, while the non-reactive system failed at the interlayer-epoxy interface and showed no improvement. There is an optimum interlayer thickness of 0.1 to 0.15 micron; the possible reasons are discussed. A detailed scanning electron microscope study illustrates how the structure of the composite fracture surface varies with the systematic changes in interlayer reactivity, composition, and thickness. Determination of the locus of failure is discussed. The observations are consistent with the mechanical property measurements.     

Microstructure and properties of CNTs reinforced CVI-pyrocarbon matrix

Pyrocarbon (PyC) matrices were prepared in two kinds of quartz fiber preforms by chemical vapor infiltration (CVI), and then the fibers were leached by HF. Effects of CNTs on the microstructures and mechanical properties of the quartz fiber reinforced carbon composites and PyC matrices, as well as the interface behaviors of the fiber reinforced composites, were discussed. Randomly oriented CNTs reinforced PyC micro-composites account for the pseudo ISO structure and contribute to the mechanical properties of the PyC matrix. Relative strength between reinforcement and matrix and interface bonding significantly affect the mechanical behaviors of the quartz fiber reinforced pyrocarbon composites: Quartz fiber with low strength and strong interface bonding result in limited strengthening effect on flexural strength of the fiber reinforced composite; low strength unidirectional quartz fiber and weak interface bonding in a much stronger matrix result in limited strengthening effect on tensile strength of the composite.