X-ray photoelectron spectroscopic studies of carbon fibre surfaces

Carbon fibres were anodically surface treated by passing them continuously through an electrochemical bath, thus simulating a possible industrial surface-treatment process. Composites were made from these fibres with an epoxy resin and their interlaminar shear strength (ILSS) tested. The surfaces of the fibres were examined by X-ray photoelectron spectroscopy after treatment. Both golvanostatic and potentiostatic cell control led to treated fibres that produced composites with high interlaminar shear strengths (80 to 90 MPa). The effect of potential, reaction time, electrolyte and subsequent heating of the fibres in a vacuum on the ILSS of the composites is reported. The rise in ILSS with surface treatment is not dependent upon the O-1 s: C-1 s ratios or the amount of carboxyl functionality present on the surface. This suppors the view that mechanical keying of the resin to the fibre surface plays an important role in forming the fibre-resin bond.     

X-ray photoelectron spectroscopic analysis of barium-labelled carbon fibre surfaces

A technique for labelling acidic functional groups on carbon fibre surfaces with barium has been developed for X-ray photoelectron spectroscopy analysis of the surface acidity of commercial Type II fibres, with differing degrees of oxidation. As the dibasic barium is believed to label adjacent monobasic groups, the results show that up to 58% of the surface oxygen, on the commercially treated fibres, is present in the acidic form.     

Developing improved models of oxidatively treated carbon fibre surfaces,using molecular simulation

In this work computer modelling investigations are reported into the chemical functional groups that are believed to contribute to composite fibre/matrix adhesion. The CERIUS [Compos A: Appl Sci Manuf 29 (1998) 1291] and MOPAC computational packages are employed to simulate the sorption interactions of small molecules at a range of functionalised surfaces. The results of these calculations are compared with fibre surface energetic data reported previously, and models of the fibre surface chemistry with varying degrees of surface treatment are developed. There is agreement that surface treatment of carbon fibres improves adhesion and adsorption characteristics, yet the factors controlling this have still to be determined fully. The empirical study using the computational model developed in this work attempts to resolve the changes in fibre adsorption phenomena into contributions arising from specific surface oxygen and nitrogen functional groups. The general trend in surface concentration of these atoms is similar to XPS analyses of the surface treated carbon fibres and is in agreement with previously published work.     

The crystallization and interfacial bond strength of nylon 6 at carbon and glass fibre surfaces

The nucleation and crystallization of nylon at the interface in glass-fibre and carbon-fibre reinforced nylon 6 composites has been investigated by electron microscope studies of sectioned and etched bulk specimens and solution cast and melt crystallized thin films. The fracture energies of the composites were obtained from tensile strength tests and the interfacial bond strengths were calculated from fibre pullout measurements. The fibres are shown to nucleate a columnar structure at the interface with marked differences between the structures nucleated by glass fibres and by carbon fibres and also between that nucleated by type I and type II carbon fibres. The structure around glass fibres was non-uniform and influenced to some extent by the presence of the size coating on the fibre surface. In the carbon-fibre composites the columnar structure was due primarily to physical matching of the graphite crystallites. Surface treatment of the carbon fibres to improve chemical bonding is shown to have a significant effect on bond strength which cannot be explained in terms of the columnar structure at the fibre surface. The treated fibres gave rise to only small amounts of fibre pull-out and low fracture energies whereas the untreated fibres showed extensive pull-out which was reflected in high fracture energies.     

Electrochemical doping of carbon fibre

Preliminary results are reported for the electrochemical synthesis of compounds formed by carbon fibre which are analogous to the ‘synthetic metals’ based on graphite and polyacetylene.     

A new glassy carbon fibre

It is shown that phenol-hexamine polymers may be extruded from the melt to produce fibres which may be carbonised to form fine high-strength glassy carbon fibres with a tensile strength of up to 2 GNm^–2 (300 000 Ib in^–2) after 900° C heat-treatment. The fibres have a specific modulus of 5 Mm compared with 14 Mm for carbonised polyacrylonitrile fibres and 3 Mm for silica glass fibres. Both strength and modulus increase rapidly with decrease in diameter. The fibres are subjected to no special surface treatment after extrusion but electron microscopy indicates the presence of a thin textured sheath surrounding a true glassy carbon core in the final fibre. The fibres have the advantages of glassy carbon (inertness to chemical attack, resistance to abrasion) and give promise of a new range of cheap high-strength carbon fibres derived from coal tar fractions.     

Crystallization of carbon fibre reinforced polypropylene

Polypropylene (PP) was compounded with carbon fibre of various contents (0, 5, 10, 15, 20 vol%) using a single and a twin screw extruder. The influence of both the carbon fibre content and the compounding method on the thermal behaviour and characteristics of crystallization was studied using differential scanning calorimetry (DSC), polarizing optical microscopy and scanning electron microscopy (SEM). Melting and crystallization temperatures increased with the amount of carbon fibre. Isothermal crystallization was observed using DSC and it was found that crystallization was accelerated by the presence of carbon fibres. Using polarizing optical microscopy, it was found that the nucleation of polypropylene started at the crossing point of two or more fibres.     

Enhancement of optical contrast on carbon/carbon fibre composites

Journal of Materials Science Letters -     

Fracture toughness of 2-D carbon fibre reinforced carbon composites

The fracture toughness of 2-D woven carbon fibre reinforced carbon laminate has been evaluated by linear elastic fracture mechanics (LEFM),R-curve andJ-integral analysis using the single edge-notched bending (SENB) specimen of edge and flatwise geometries. The edgewise specimens failed by a small extension of the self similar crack whereas the flatwise specimens failed by delamination. The surface damage developing from the tip of the initial crack was revealed by the brittle lacquer coating technique and the zone shape varied with the specimen geometry, i.e. the loading axis relative to the woven layers. Acoustic emission (AE) was also used to monitor crack growth, and the total ring down count of AE was observed to increase as the initial crack length was decreased. Both the damage zone size and total AE counts were found to increase in two linear stages as a function of the square of the stress intensity factor,K.     

Time-dependent resistivity in carbon fibre sheets

The electrical properties of sheets of short carbon fibres in resin, glass-fibre and wood-pulp materials have been investigated. For carbon fibre in wood-pulp, a conductor-to-insulator transition was observed at 3 wt % (0.6 vol %) carbon fibre above which conductivity varied linearly with weight fraction. This result is interpreted in terms of a percolation threshold in a system of high aspect ratio. The data agree well with previous measurements on carbon-fibre in polymer composites, and satisfactorily with two-dimensional Monte Carlo calculations. At high concentrations of carbon fibre in all materials, the in-plane resistivity was found to be strongly time-dependent, the fractional change being proportional to Int. A theoretical model is presented which assumes a continuous increase in the number of interconnecting pathways as fibres physically move together under electrostatic attractive forces. Thermal activation over a continous spectrum of energy barriers leads to logarithmic time dependence as observed experimentally. Studies of the effect of external compression support the model for the time dependence. Shell (UK) Ltd Research Fellow in Materials Science.     

Acoustic emission in carbon fibre composites

Acoustic emission analyses have been performed on carbon fibre-epoxy composites in order to correlate acoustic activity with well defined fracture processes. Different types of laminates were studied to discriminate the basic processes of fracture within the laminates. Test specimens were subjected to three-point flexural tests so as to separate fracture initiation and propagation. Acoustic emission signals were recorded by digital techniques for off-line analysis.

Results show that during fracture initiation, high amplitude signals are correlated with fracture processes controlled by fibre rupture, and low amplitude signals with matrix fracture. For laminates characterized by major fracture propagation, acoustic emission is generated by the processes induced by the friction of the fracture surfaces created. This acoustic activity, associated with abnormally long duration and high amplitude signals, exceeds the emission generated by the fracture processes themselves.     

Oxidation protection for carbon fibre composites

Carbon fibre-reinforced ceramic matrix composites are promising candidate materials for high-temperature structural applications such as gas turbine blades. In oxidizing environments at temperatures above 400°C, however, carbon fibres are rapidly oxidized. There is, therefore, a need to coat the composite in order to protect it against oxidation. This review identifies the requirements of an effective oxidation protection system for carbon fibre-reinforced ceramics and summarizes the work which has been carried out towards this goal over the last 50 years. The most promising coatings are those composed of several ceramic layers designed to protect against erosion, spallation and corrosion, in addition to possessing a self-healing capability by the formation of glassy phases on exposure to oxygen.     

Hygrothermal characteristics of carbon fibre reinforced plastics

Journal of Materials Science - The following account describes investigations of the moisture absorption characteristics of specimens prepared from a unidirentionally reinfonced sheet of carbon...     

A method for the chemical anchoring of carbon nanotubes onto carbon fibre and its impact on the strength of carbon fibre composites

This article proposes an alternative way to use carbon nanotubes to improve the performance of carbon fibre-reinforced composites. A chemical process, based on esterification of surface groups, is used to anchor nanotubes onto carbon fibre surface. Anchored nanotubes form a network surrounding the carbon fibres. After CNT anchoring, the tow is impregnated with an epoxy resin and tensile tests are performed on this minicomposite sample. By enhancing matrix properties and fibre/matrix interface, the CNT network has a significant influence on the composite strength.     

Pull-through performance of carbon fibre epoxy composites

An investigation of the through-thickness properties of carbon fibre prepreg laminates, Non-Crimp Fabric laminates and non-crimp 3D orthogonal woven composites by pull-through testing was performed. Influence of matrix system and curing temperature on the performance of the 3D woven composites was investigated.     

Testing of single fibre bundles of carbon/carbon composite materials

A method has been developed for excising single fibre bundles of carbon/carbon material from large billets, and of testing them in axial tension to failure. The fibre bundles are strain gauged so that the tensile modulus and complete stress/strain curves to failure can be obtained. Experimental results are presented and discussed.     

Effect of fibre length on fatigue of short carbon fibre/epoxy composite

The effect of fibre length on the fatigue of a random short carbon fibre/epoxy composite containing 1, 5 or 15 mm length fibres has been studied. All laminates gave a sloping S-N curve with longer fatigue lives obtained at decreasing peak stresses. The fatigue life was independent of fibre length at any peak strain, within experimental variation. Damage accumulation during fatigue cycling is studied in terms of residual strength and modulus reduction. Both techniques suggest that fatigue failure is the result of a ‘sudden death’ mode of failure. Finally, the effect of matrix type on the fatigue life of laminates containing 5 mm length fibres was investigated by adding a greater quantity of flexibilizer to the epoxy matrix. Shorter fatigue lives were obtained for laminates having the more flexible matrix.