Synergy of matrix and fibre modification on adhesion between carbon fibres and poly(vinylidene fluoride)

Interfacial properties between carbon fibres and poly(vinylidene fluoride) (PVDF) were tuned by modifying both constituents. Atmospheric plasma fluorination (APF) was utilised to tailor the surface composition of carbon fibres, which resulted in an incorporation of up to 3.7 at.% of fluorine functionalities in to the fibre surfaces. The PVDF matrix was modified by blending pure PVDF with maleic anhydride (MAH) grafted PVDF. Both fibre and matrix modifications act in synergy with improvements of up to 50% in the apparent interfacial shear strength (IFSS) above the level of pure fibre or matrix modification. Modification of both constituents led to the formation of various interactions at the fibre/matrix interface namely dispersive and polar (H-bonds) between (modified) PVDF and the fluorine as well as oxygen functionalities on the fibre surfaces. The apparent IFSS between the modified fibres and matrix reaches a maximum of 42 MPa, which is almost the tensile strength of the pure PVDF. The improvements in apparent IFSS in single fibre model composites for both fibre and matrix modifications translated to a seven times improvement in the interlaminar shear strength of unidirectional composites.     

Impact of bleaching pine fibre on the fibre/cement interface

The goal of this article was to evaluate the surface characteristics of the pine fibres and its impact on the performance of fibre–cement composites. Lower polar contribution of the surface energy indicates that unbleached fibres have less hydrophilic nature than the bleached fibres. Bleaching the pulp makes the fibres less stronger, more fibrillated and permeable to liquids due to removal the amorphous lignin and its extraction from the fibre surface. Atomic force microscopy reveals these changes occurring on the fibre surface and contributes to understanding the mechanism of adhesion of the resulting fibre to cement interface. Scanning electron microscopy shows that pulp bleaching increased fibre/cement interfacial bonding, whilst unbleached fibres were less susceptible to cement precipitation into the fibre cavities (lumens) in the prepared composites. Consequently, bleached fibre-reinforced composites had lower ductility due to the high interfacial adhesion between the fibre and the cement and elevated rates of fibre mineralization.     

Photoelastic study of the durability of interfacial bonding of carbon fibre-epoxy resin composites

The physical techniques of polarizing microscopy, including the quantitative measurements of small optical retardations, have been used to investigate elastic fields adjacent to short carbon fibres in epoxy resin composites. The elastic fields associated with shear stress distribution along the fibre-matrix interface have been employed to monitor the initiation of interface debonding during hot (100 °C) water uptake. By examining the development of stress birefringence during resin swelling in the resin adjacent to individual fibres, the differences in the durability of interfacial bonding and the fibre failure modes for differently coated fibres have been obtained. The results show that the state of self-stress in model composites, comprising a single carbon fibre in a film of epoxy resin, can, by immersion in hot distilled water, be enhanced to such an extent that the axial tension in the fibre can be sufficient to initiate fibre fracture. The results also show that, for fibres that have been given certain proprietary surface treatments, the fibre fractures by different failure modes.     

A study of the influence of fibre/resin adhesion on the mechanical behaviour of ultra-high-modulus polyethylene fibre composites

The interlaminar shear strength of unidirectional ultra-high-modulus polyethylene composites was measured as a means of accessing the level of fibre/epoxy resin adhesion for a number of different reinforcing yarns, produced by melt and gel-spinning. The fibres were shown to possess poor adhesive properties due partly to inadequate wetting associated with the inert polyolefine surface and also because of a weak boundary layer, formed by the segregation of low molecular weight impurities to the surface during fibre formation. The interlaminar shear strength was significantly increased by pretreating the reinforcement with an oxygen plasma. This improved wetting by producing oxygen-containing groups on the fibre surface and removed the weak boundary layer by the formation of a cross-linked skin. For a fixed fibre volume fraction, the interlaminar shear strength was found to be inversely proportional to the filament diameter. The other mechanical properties were shown to be largely independent of fibre/resin adhesion, with plasma treatment having little or no effect.     

Surface modification of superdrawn polyoxymethylene fibres

A new surface modification of superdrawn polyoxymethylene (POM) fibres, curing with resorcinol at mild temperatures, was developed to apply to rubber composites and the adhesion to the rubber matrix behaviour of modified fibres is discussed in terms of the interfacial miscibility between the fibre and adhesive surface layers. The modified fibre reached the maximum pull-out adhesion level in which a cohesive failure of the fibre occurs, resulting from the fact that the modified POM layer is spectroscopically close to the standard resorcinol-formaldehyde (RF) resin, thoroughly miscible and thermodynamically compatible with the resorcinol-formaldehyde-latex (RFL) adhesive.     

Equilibrium segregation of Ti to Au–sapphire interfaces

Equilibrium segregation of Ti to Au–sapphire interfaces was measured from dewetted Au(Ti) films on the (0001) surface of sapphire. Quantitative energy dispersive spectroscopy was used to determine a Ti excess at the Au–sapphire interface of 2.2 Ti atoms/nm², which together with an excess of 4.6 Ti atoms/nm² at the (0001) sapphire surface, is associated with a decrease in the solid–solid Au–sapphire interface energy. Quantitative high resolution transmission electron microscopy showed that the segregated Ti is distributed within a 1.54-nm thick intergranular film at the Au–sapphire interface, which is not a bulk phase but rather an equilibrium interface state. As a result, Ti segregation without the formation of a bulk reaction at the interface is associated with a decreased interface energy, improved wetting, and may be an important part of the total complex mechanism responsible for improved wetting and spreading in “reactive” braze systems.     

Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites

A novel method of biomimetic surface modification was used for aramid fibres aiming to enhance the interface properties between epoxy resin and the modified aramid fibre. Inspired by the composition of adhesive proteins in mussels, a thin layer of poly(dopamine) (PDA) was self-polymerized onto the surface of the aramid fibre. The graphene oxide (GO) was then grafted on the surface of PDA-coated aramid fibres. The microstructure and chemical characteristics of the pristine and modified fibres were characterised using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), indicating successful grafting of GO on the PDA-coated aramid fibres. Single fibre tensile test and microbond test were carried out to evaluate the mechanical properties of the modified fibres. It was found that the fibre surface modification improved the interfacial shear strength by 210% and the fibre tensile strength was protected by GO-PDA coating.     

Some aspects of interface adhesion of electrolytically oxidized carbon fibres in an epoxy-resin matrix

A comprehensive investigation of the adhesion at the interface of a carbon fibre in an epoxy resin was made. The fibre surfaces were modified, to increase their adhesion to resin, by an electrolytic surface treatment which was applied at various current densities. Subsequent changes in the fibre properties relating to possible mechanical, physical and chemical contributions to adhesion were monitored. Tensile tests on single fibres indicated that the treatment altered the strengths of the fibres, which were found to have their highest values and to be least variable at an optimum adhesion level. A method was developed to estimate the strength of the fibres in the resin, this confirmed the single-fibre data. A novel method of labelling the acidic sites by producing adsorption isotherms was developed to identify surface functionality. Surface acidity correlated well with adhesion levels. Single-fibre pull-out tests, modelled using a new combination failure criterion and fragmentation tests, indicated that the optimum adhesion level for this fibre/resin system was achieved with an electrolytic treatment at 25 C m^–2. The principal effects of this treatment were considered to be due to chemical modification of the fibre surface coupled with the removal of a loosely adherent surface layer.     

Application of Interlaminar Tests to Marine Composites. Relation between Glass Fibre/Polymer Interfaces and Interlaminar Properties of Marine Composites

The need for improved performance and the development of new composite manufacturing methods require a better understanding of the role of interface phenomena in the mechanical behaviour of these materials. The influence of the cure cycle on the bulk and surface properties of the matrix resin, and of composites based on polyester and epoxy resins reinforced with glass fibres has been studied. While the mechanical properties of the epoxy vary with cure temperature the surface tension is not affected. The increase in interfacial shear strength and interlaminar shear strength with increased cure temperature cannot be simply explained by the wetting of the fibres by the matrix. The importance of thermal stresses, generated at the interface by resin shrinkage and differences in thermal expansion, for the mechanical behaviour of the composite are demonstrated.     

Influence of surface treatment on adhesion of polyethylene fibres

Abstract

A detailed examination has been undertaken of the influence of surface treatment on the adhesion of polyethylene fibres to epoxy resin. The pull-out adhesion has been determined for untreated, chromic acid treated, and plasma etched monofilaments with different draw ratios and thermal annealing treatments. In a few cases, additional chemical treatments were applied to plasma treated fibres before the pull-out test. The polyethylene surface energy also has been determined by measurement of contact angle. The results, taken together, suggest that the adhesion depends on three factors: (i) the wettability (or physicochemical interactions), which is affected by the extent and nature of the surface treatment as well as the fibre draw ratio; (ii) the surface roughness, after plasma etching only, where a honeycomb structure of pits permits mechanical keying between the fibre and the resin (this structure has been examined by scanning electron microscopy); and (iii) the number of chemical bonds per unit area between the fibre and the resin. It is concluded that these three factors can be regarded as additive and that optimum results are obtained when their respective pull-out strengths reach their maximum values, ~2, ~3, and ~1·7 MN m^?2.

MST/640     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Calcia–alumina fibre-reinforced Al 7075 alloy composites produced by melt-infiltration: Interfacial wetting and reaction

Inviscid melt-spun calcia–alumina (CA) fibre-reinforced aluminium 7075 alloy matrix composites were produced at 700 and 927°C by using a melt-infiltration method. Interfacial wetting and chemical reaction of the composites were investigated by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The composites processed at 700°C showed interfacial wetting and magnesium accumulation at the interfacial region. The composites processed at 927°C showed the formation of a 15 μm thick interphase region as well as excellent interfacial wetting. EDS analysis gave averaged compositions of this interface region at 63.5 at% Al and 31.5 at% Mg, which corresponds to the composition of spinel, MgAl2O4. The formation of spinel at the interface was confirmed by XRD analysis on the CA fibres separated from the composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Interfacial characterization of Nicalon SiC fibre reinforced oxynitride glass matrix composites

Nicalon SiC and Hi-Nicalon SiC fibre oxynitride glass and glass–ceramic composites were prepared and the interface between the fibres and matrix characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that the formation and thicknesses of interfacial layers were primarily determined by the type of fibre reinforcement, but the role of these interfaces in influencing composite properties was dependent on the thermal properties of the matrix. For Nicalon SiC composites, the carbon-rich layer did not promote fibre debonding and toughening unless the matrix had a smaller thermal expansion coefficient than the fibres. For Hi-Nicalon SiC composites, the absence of oxygen in the fibre significantly encouraged chemical reaction between fibre and matrix, resulting in no strengthening or toughening. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture criterion for kinking cracks in a tri-material adhesively bonded joint under mixed mode loading

The fracture behavior of a composite/adhesive/steel bonded joint was investigated by using double cantilever beam specimens. A starter crack is embedded at the steel/adhesive interface by inserting Teflon tape. The composite adherend is a random carbon fiber reinforced vinyl ester resin composite while the other adherend is cold rolled steel. The adhesive is a one-part epoxy that is heat cured. The Fernlund-Spelt mixed mode loading fixture was employed to generate five different mode mixities. Due to the dissimilar adherends, crack turning into the adhesive (or crack kinking) associated with joint failure, was observed. The bulk fracture toughness of the adhesive was measured separately by using standard compact tension specimens. The strain energy release rates for kinking cracks at the critical loads were calculated by a commercial finite element analysis software ABAQUS in conjunction with the virtual crack closure technique. Two fracture criteria related to strain energy release rates were examined. These are (1) maximum energy release rate criterion (G_max) and, (2) mode I facture criterion (G_II = 0). They are shown to be equivalent in this study. That is, crack kinking takes place at the angle close to maximum G or G_I (also minimum G_II, with a value that is approximately zero). The average value of G_IC obtained from bulk adhesive tests using compact tension specimens is shown to be an accurate indicator of the mode I fracture toughness of the kinking cracks within the adhesive layer. It is concluded that the crack in tri-material adhesively bonded joint tends to initiate into the adhesive along a path that promotes failure in pure mode I, locally.     

Characterization of thermally oxidized carbon fibre surfaces by ESCA,wetting techniques and scanning probe microscopy,and the interaction with polyethylmethacrylate. Development of a biocompatible composite material

The development of biocompatible weft knitted carbon fibre-reinforced thermoplastics needs optimization of each composite component: fibre, matrix and interface. The material investigated was a composite of polyethylmethacrylate reinforced with a knitted and sized T300 carbon fibre. After chemical removal of the fibre sizing, the fibres were thermally oxidized at temperatures between 400 and 600°C. Angle-resolved photoelectron spectroscopy (ESCA) and Wilhelmy surface energy measurements have been used to describe the modification of the surface chemistry by thermal oxidation. The surface morphology, visualized by scanning probe microscopy and scanning electron microscopy, indicates an increased surface roughness. The interaction between fibre and matrix was investigated by observing the microscopic wetting behaviour of the thermoplastic at sinter temperature by the solid-body wetting technique. It is found that the strength and failure mechanisms of the knitted fibre-reinforced composite are determined by the interface properties.     

Strength variability of single flax fibres

Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress–strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress–strain curves tend to show a higher tensile strength, a higher Young’s modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal residual microstress generation during the processing of unidirectional carbon fibre/epoxy resin composites: regular fibre arrays

The thermal residual microstresses generated during the processing of unidirectional carbon fibre/epoxy resin composites are predicted, assuming regular fibre arrays. Stresses are determined in unit cells covering a range of fibre coordinations, fibre diameters, minimum interfibre distances and fibre volume fractions. The method of calculation involves the finite element method. Elastic material behaviour with temperature-dependent epoxy resin properties are assumed, together with transversely isotropic carbon fibres. Of the parameters studied, the greatest effect on the maximum principal stress was produced by the minimum thickness of epoxy resin between the fibres and the ratio of this thickness to the fibre radius. Values of the maximum principal stress were found in some cases to exceed the tensile strength of the epoxy resin. However, there was little experimental evidence to support this prediction. Cracks occurred only to a limited extent and occurred around the fibre/epoxy interface, rather than between the fibres, as predicted by the model. Reasons suggested for this discrepancy include relatively weak fibre/epoxy resin bonds and limitations on the accuracy of the stress generation model. Methods by which the model may be improved are discussed.     

Study on the interface of phenolic resin/expanded graphite composites prepared via in situ polymerization

Phenolic resin/expanded graphite (EG) composites were synthesized via in situ condensation polymerization of the monomers in the presence of foliated graphite. SEM observation showed that the graphite flakes were well dispersed in the phenolic resin matrix. The electrical conductivity of the composites was investigated as a function of the foliated graphite fraction. The composites containing graphite sheets exhibited an electrical conductivity percolation threshold with 3.2 wt% graphite content in polymer matrix. Inverse gas chromatography (IGC) measurements were carried out to characterize the surface of the foliated graphite before and after condensation polymerization of phenolic resin using a series of both non-polar and polar acid–base probe gases. The data obtained indicated that the character of graphite surface changed after the polymerization of phenolic resin. The dispersive component of surface free energy decreased greatly. Before polymerization the graphite surface is predominantly acidic while the surface turns to basic after polymerization. The increased polarity of surface contributed to the stronger interactions between graphite and phenolic resin and the fine dispersion of expanded graphite in the matrix, and resulted in the low conductivity percolation threshold.     

Interfacial polycondensation of nylon-6,6 at the glass fibre surface and its effect on fibre–matrix adhesion

The surface of glass fibres was modified using chemical treatments to improve fibre–matrix interface properties. Interfacial polycondensation was performed with the fibre acting as the interface, and nylon-6,6 chains were grafted on the free hydroxyl groups located at the fibre surface. Grafted nylon was observed through the scanning electron microscope. The effect of the treatment on the fibre-matrix adhesion was investigated by measuring the interfacial shear strength in fragmentation micromechanical tests. The two-parameter Weibull distribution was used to analyse the experimental results. Polarized optical microscopy showed the existence of a transcrystalline layer in treated samples, indicating better fibre wettability by the matrix. Scanning electron microscopy confirmed the presence of an excellent bonding between fibre and matrix in treated samples, whereas in untreated samples, fibre pull-out was predominant, indicating poorer fibre–matrix adhesion. This revised version was published online in November 2006 with corrections to the Cover Date.