Effects of surface modification and graphene nanoplatelet reinforcement on adhesive joint of aluminium alloys
Introduction
Adhesive bonding is a widely used technique in the fabrication of hybrid aluminium-composites structures, especially in the aerospace and wind energy industries. Adhesive bonding offers many advantages over conventional mechanical fastening such as lower structural weight, lower stress concentration in the structure and better airtight capabilities; but this type of joints may suffer from weak adhesion between epoxy and aluminium substrate resulting in reduced shear strength. Joint strength depends on many factors, including types of adherends and adhesive, thickness of adhesive, and surface treatments applied [1]. Generally, five broad categories of aluminium surface treatments exist including mechanical, chemical, electrochemical, coupling agent and dry surface treatment. Critchlow and Brewis [1] stated that optimal surface treatment produces a surface which is free from contamination, wettable by the adhesive, mechanically and hydrolytically stable with good corrosion resistance. Many studies have investigated the effects of surface treatment on the bonding strength for various types of epoxy [[2], [3], [4]]. Boutar et al. [2] investigated how abrading influenced the lap shear strength and found that aluminium with a surface roughness of 0.6 μm produced the highest adhesive strength. Prolongo et al. [3] compared several mechanical and chemical surface treatments and different adhesives to optimise the joint strength. It was found that joint strength depends not only on the properties of the oxide layer, such as its composition, pore aspect ratio and porosity, but also on the viscosity of the adhesive during the application. Xu et al. [4] reported that the joint’s bonding strength could be improved by adjusting phosphoric acid anodising conditions. Anodising parameters such as voltage, time and acid concentration have significant effects on the surface energy, surface roughness and the ability of epoxy to penetrate the aluminium nanostructures [4]. So far, all these studies used pure epoxy resins as bonding adhesive.
More recently, carbonaceous nanofillers such as graphene nanoplatelets (GNPs) or carbon nanotubes (CNTs) have been increasingly used in epoxy adhesive due to their superior mechanical and electrical properties [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. Salom et al. [12] reported 20% increase in Young’s modulus of GNPs reinforced epoxy with 2 wt% of GNPs. Moriche et al. [11] observed 206% and 306% enhancement in thermal conductivity with 8 and 10 wt% GNPs reinforcement in the epoxy adhesive. Moreover, graphene increases adhesive thermal stability [6] and mechanical performance at elevated temperatures [10]. The improvement in mechanical, thermal properties and competitive price of GNPs make them an excellent candidate to reinforce epoxy adhesive.
Most of the work on adhesive bonding focuses on the effects of the volume fraction of nanofillers on mechanical properties [5,6,11,12]. Akpinar et al. [5] reported 276% increase in lap shear strength for epoxy reinforced with 2 wt% of GNPs. The same research group also observed that the method of mixing nanofillers with epoxy has a significant influence on the adhesive properties. On the other hand, some studies reported no improvement [11] or even a decrease (~20%) in adhesive properties caused by the introduction of GNPs [12,13]. The decrease in properties of nanocomposite was usually attributed to poor nanofiller dispersion, the weak interface between nanofillers and epoxy and the agglomeration effects caused by Van der Waals force. In adhesive bonded joints, the interface between aluminium and epoxy plays an important role in the adhesion properties. So far, most studies on GNPs reinforced adhesives focus only on single surface treatment, usually grit blasting or chemical etching. To the best of authors’ knowledge, only one study compared the effects of two selected surface treatments on aluminium, i.e. alkaline cleaning and P2 etch [12]. However, no correlation between the surface treatments and GNPs epoxy adhesive strength was observed.
The present work aims to carry out a systematic study on the effects of different aluminium surface treatment and GNPs reinforcement on the strength of adhesive joints of aluminium alloy. The surface energy of aluminium with mechanical, chemical and electrochemical surface treatment was measured using sessile drop method. The surface morphology and wetting behaviour of epoxy on the aluminium surface were studied by scanning electron microscopy (SEM) and contact angle measurements. The bonding strength of surface-treated aluminium plates with epoxy or GNPs reinforced epoxy was investigated by using single lap joints and correlated with surface morphology and energy measurements.
Section snippets
Materials
Aluminium alloy grade 1050 A is widely used in applications such as tanks, boilers, fan blades, automotive trim and many more [16]. Sheets (RS Components, UK) with a thickness of 2 mm were used as adherends in this study. Low viscosity epoxy resin IN2 (Easy Composite, UK), composed of epoxy resin and slow hardener AT30, was used as adhesive due to its excellent flowability and compatibility with carbonaceous materials. This mixture has a pot life around 80–100 min and low viscosity in the range
Surface topography
The surface morphology of the treated aluminium is presented in Fig. 2. Aluminium alloy sample cleaned with acetone presents a rather smooth surface with some grooves arising from prior thermo-mechanical processing of the alloy (Fig. 2a). On the other hand, after grit blasting (see Fig. 2b) the sample surface shows a significant increase in roughness. During the grit blasting process, abrasive particles impinge the aluminium surface at high speeds, forming craters with irregular sizes. This
Conclusions
This work investigated the combinative effects of aluminium surface treatments and GNPs reinforced adhesive on the joint’s strength. Mechanical, chemical and electrochemical surface treatments were applied to aluminium, and resultant surface morphologies were discussed. Surface energy of aluminium was doubled due to the applied electrochemical surface treatment. The effects of phosphoric acid anodisation, hot water sealing process and GNPs to the adhesive on the joints’ strength were for the
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
Marzena Pawlik is supported by graduate teaching assistant and postdoctoral fellowship funding from the College of Engineering and Technology at the University of Derby. The authors want to thank Dr Graham Souch for his contribution in SEM study.
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