A.N. Rider

Boiling water and silane pre-treatment of aluminium alloys for durable adhesive bonding

Three series of aluminium alloys received a surface pre-treatment including immersion in boiling water followed by soaking in a 1% aqueous solution of 3-glycidoxytrimethoxysilane. When aluminium was pre-treated in this manner, adhesive joints formed with a range of epoxy resins produced notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pre-treatment improved joint durability to the level observed for the phosphoric acid anodise process. The development of a platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. Generally, immersion of the aluminium alloys at temperatures above 80°C for periods between 4 and 60 min provide the optimum conditions required to prepare a surface film, which will produce durable bonds with epoxy adhesive.

Hierarchical composite structures prepared by electrophoretic deposition of carbon nanotubes onto glass fibers.

Carbon nanotube/glass fiber hierarchical composite structures have been produced using an electrophoretic deposition (EPD) approach for integrating the carbon nanotubes (CNTs) into unidirectional E-glass fabric, followed by infusion of an epoxy polymer matrix. The resulting composites show a hierarchical structure, where the structural glass fibers, which have diameters in micrometer range, are coated with CNTs having diameters around 10-20 nm. The stable aqueous dispersions of CNTs were produced using a novel ozonolysis and ultrasonication technique that results in dispersion and functionalization in a single step. Ozone-oxidized CNTs were then chemically reacted with a polyethyleneimine (PEI) dendrimer to enable cathodic EPD and promote adhesion between the CNTs and the glass-fiber substrate. Deposition onto the fabric was accomplished by placing the fabric in front of the cathode and applying a direct current (DC) field. Microscopic characterization shows the integration of CNTs throughout the thickness of the glass fabric, where individual fibers are coated with CNTs and a thin film of CNTs also forms on the fabric surfaces. Within the composite, networks of CNTs span between adjacent fibers, and the resulting composites exhibit good electrical conductivity and considerable increases in the interlaminar shear strength, relative to fiber composites without integrated CNTs. Mechanical, chemical and morphological characterization of the coated fiber surfaces reveal interface/interphase modification resulting from the coating is responsible for the improved mechanical and electrical properties. The CNT-coated glass-fiber laminates also exhibited clear changes in electrical resistance as a function of applied shear strain and enables self-sensing of the transition between elastic and plastic load regions.

Tailoring Interfacial Properties by Controlling Carbon Nanotube Coating Thickness on Glass Fibers Using Electrophoretic Deposition

The electrophoretic deposition (EPD) method was used to deposit polyethylenimine (PEI) functionalized multiwall carbon nanotube (CNT) films onto the surface of individual S-2 glass fibers. By varying the processing parameters of EPD following Hamakers equation, the thickness of the CNT film was controlled over a wide range from 200 nm to 2 μm. The films exhibited low electrical resistance, providing evidence of coating uniformity and consolidation. The effect of the CNT coating on fiber matrix interfacial properties was investigated through microdroplet experiments. Changes in interfacial properties due to application of CNT coatings onto the fiber surface with and without a CNT-modified matrix were studied. A glass fiber with a 2 μm thick CNT coating and the unmodified epoxy matrix showed the highest increase (58%) in interfacial shear strength (IFSS) compared to the baseline. The increase in the IFSS was proportional to CNT film thickness. Failure analysis of the microdroplet specimens indicated higher IFSS was related to fracture morphologies with higher levels of surface roughness. EPD enables the thickness of the CNT coating to be adjusted, facilitating control of fiber/matrix interfacial resistivity. The electrical sensitivity provides the opportunity to fabricate a new class of sizing with tailored interfacial properties and the ability to detect damage initiation.

The influence of porosity and morphology of hydrated oxide films on epoxy-aluminium bond durability

Clad aluminium alloy was pretreated by immersion in boiling water for times ranging between 30 s and 4 h. The chemical and physical properties of the films produced in the 100°C water were characterized by techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). The durability of the bonds formed between the boiling water films and a rubber-toughened epoxy adhesive was assessed in terms of the film properties and fracture analysis of failed wedge specimens. In the early pretreatment stage, bond durability was limited by the fracture of the porous oxide film at the film-metal interface. For immersion times greater than 4 min, a decrease in film porosity and bond durability was observed.

Influence of adherend surface preparation on bond durability

Work presented in this paper focuses on the critical aspects of the surface preparation of aluminium employed for the manufacture of aluminium-epoxy joints. The surface preparation procedure examined is currently employed by the Royal Australian Airforce (RAAF) for repairs requiring metal-to-adhesive bonding. The influence of each step in the surface preparation on the bond durability performance of the adhesive joint is examined by a combination of methods. Double-cantilever wedge-style adhesive joints are loaded in mode 1 opening and exposed to a humid environment. Together with analysis of the adherend and joint failure surfaces, the results show conclusively that adhesive bond durability is sensitive to the presence of contaminant and the roughness of the adherend surface. The presence of contaminant can interfere with the effectiveness of an organosilane coupling agent as a durability improver. A two-step bond degradation model was developed to describe qualitatively the observed bond durability performance and fracture data.

Effect of Contaminant on the Durability of Epoxy Adhesive Bonds with Alclad 2024 Aluminium Alloy Adherends

Surface preparation of aluminium alloy adherends is designed to minimize the influence of organic contaminant on the durability of epoxy adhesive bonds. The organic contaminant molecules have the potential to inhibit the bonding of the adhesive with the surface oxide on the adherend. X-ray photoelectron spectroscopy (XPS) shows that a freshly exposed aluminium surface quickly adsorbs over ten atomic layers of organic contaminant. Models to fit XPS data measured at various specimen take-off angles indicate that this contaminant is not adsorbed uniformly. Experiments show that for mechanically prepared adherends treated with an organosilane coupling agent, the bond durability depends strongly on the contaminant concentration and the time of administration during the preparation sequence. The observed variations in bond durability can be explained in terms of inhomogeneities in the strength of adhesion between the adhesive and the metal oxide.

Studies of the degradation of metal-adhesive interfaces with surface analysis techniques

Abstract The application of bonded repairs and reinforcements to aircraft components places an emphasis on adherend surface treatment procedures because these treatments can significantly change bond strength and durability. A typical minimum treatment sequence for Alclad 2024-T3 aluminium alloy adherends includes a solvent degrease, an abrasion with a Scotchbrite ® pad, a clean with a methyl-ethyl-ketone (MEK) soaked tissue and a grit-blast with 45 μm alumina powder. The adherends are treated with γ-glycidoxy-propyl-trimethoxy-silane (γ-GPTS) coupling agent then bonded with an epoxy film adhesive. The composition of the adherent, the bond durability and the locus of fracture were examined at several stages of the adherent surface treatment. Boeing wedge tests show that grit-blasting the adherends creates a more durable adhesive bond than the Scotchbrite ® /MEK treatment and that the application of γ-GPTS improves bond durability in both cases. XPS has shown that the cleaning sequence decreases the concentration of hydrocarbon contaminant on the grit-blasted adherend to an average thickness of less than 1.5 nm. XPS analyses of the fracture surfaces indicates that for the grit-blast, grit-blast plus γ-GPTS and Scotchbrite ® /MEK plus γ-GPTS treatments, failure occurs primarily in the oxide film, whereas for the Scotchbrite ® /MEK treatment failure occurs at the adhesive/oxide interface possibly due to weakness induced by contaminant. XPS measurements show that a γ-GPTS overlayer retards the growth of the oxide on aluminium in humid air, until the γ-GPTS overlayer is desorbed. The improved bond durability with the coupling agent may be due to the inhibition of hydration sites on this oxide surface.

The influence of hydroxyl group concentration on epoxy–aluminium bond durability

The influence of hydroxyl group (OH) concentration on the durability of adhesive bonds formed between an epoxy resin and aluminium adherend has been examined. Initially, surface analysis in combination with chemical derivatisation was employed to characterise the OH and epoxy functional groups present in FM-73, a structural epoxy adhesive. Bulk FM-73 indicated a higher degree of cure than the surface of FM-73 present at the interface of an epoxy–aluminium adhesive joint. Plasma and water treatment of the aluminium adherend was employed to alter the metal oxides surface OH concentration. Despite a several-fold difference of aluminium surface OH concentrations for the different metal pre-treatments, there was no significant variation in the adhesive joint fracture toughness in a humid environment, G Iscc. In contrast, grit-blasting the aluminium prior to bonding increased G Iscc almost 15-fold. Simple calculations indicate that the aluminium surfaces used in the bonding experiments would have a large excess of OH groups available to react with a standard epoxy resin and that the influence of surface roughness on adhesion durability is not insignificant.

Ultrasonicated-ozone modification of exfoliated graphite for stable aqueous graphitic nanoplatelet dispersions

A novel ultrasonicated-ozonolysis (USO) processing method has been applied to commercially available exfoliated graphite (EG) with the aim of producing stable aqueous graphitic nanoplatelet (GNP) dispersions that are suitable for ink-jet printing and electrophoretic deposition. The processing has been compared to other low energy and environmentally friendly electrochemical exfoliation (EE) techniques. The results show USO can be used to prepare highly stable aqueous dispersions from both low and high surface area EG. The level of oxygen functionalization can be easily controlled with processing time as can the dispersion concentration. The degree of disorder in the GNP structure is similar to existing EE methods but offers higher yields without the need to remove any chemicals post-processing. Ink-jet printing onto heated quartz substrates produced films which reached electrical conductivities of 1400 s m(-1) after annealing. The films printed from USO-processed EGs had higher conductivity and significantly reduced thickness as compared to films printed from aqueous dispersions of reduced graphene oxide.

Characterisation of Films Grown on Aluminium Immersed in Aerated Saline Solutions Containing Nickel

Small additions of nickel chloride to salt solution significantly reduce the corrosion of aluminium alloys. Films produced in these solutions produce a microrough topography which may be suitable for adhesive bonding. Techniques such as XPS, SEM, FTIR and SIMS were used to investigate the properties of the films. The results established that nickel is incorporated in the outer layer of a hydrated aluminium oxide film and is responsible for altering the film growth mechanism. Boeing wedge tests were conducted to assess the relative durability of bonds with adherends pretreated in the salt solutions. The durability performance of the film as a surface treatment for adhesive bonding compared favourably with the Forest Products Laboratory chromate etch. A comparison of the nickel salt treatment with a sodium chloride solution treatment indicated that durability differences were related to their respective hydrated oxide structures.