The bonding performances of carbon nanotube (CNT)-reinforced epoxy adhesively bonded joints on steel substrates

Highlights

• Bonding performances of epoxy joints improved with the increase of CNTs up to 0.75%.

• Thinner bondlines showed better bonding performances than thicker ones.

• The influence of bondline thickness became less significant as the addition of CNTs.

Abstract

Carbon nanotubes (CNTs) are generally considered as a promising particle reinforcement of incorporating advanced properties and characteristics into epoxy nanocomposites. This paper investigated the bonding performances of CNT-reinforced epoxy adhesively bonded joints on steel substrates using the single lap shear (SLS) tests. The bonding performances (including bonding strength, fracture strain, toughness, and failure mode) were studied with three adhesive thicknesses (1 mm, 0.5 mm, and 0.25 mm) and three CNT weight fractions (0%, 0.375%, and 0.75%). The experimental results indicated that thinner bondlines and higher CNT additions could significantly improve the bonding performances and modify the failure mode of CNT-reinforced epoxy adhesively bonded joints. However, the effects of adhesive thickness became less significant with the increase of CNT weight fractions. In addition, the plastic behaviour of CNT-reinforced epoxy, CNTs pulling-out, and the aggregation of CNTs were observed by scanning electron microscopy (SEM) image analysis on the fracture surfaces of CNT-reinforced epoxy adhesively bonded joints, indicating the potential effectiveness of the CNT reinforcement.

Introduction

Epoxy resin is one of the most common structural adhesives due to its easy application, lightweight, high tensile strength, and good chemical and environmental resistance. Adhesively bonded joints using epoxy resin have been widely applied to bond different materials as laminar composite structures in various industries [1]. Compared to traditional joining methods, epoxy adhesively bonded joints yield lower joint weight, stronger corrosion resistance and more uniform stress distribution [[2], [3], [4]]. However, epoxy adhesively bonded joints also suffer from certain weaknesses such as the brittleness of epoxy resin along with the variation of material properties at the adhesive-substrate interface, which makes it difficult to maintain a solid adhesive bonding [5,6]. In fact, the adhesive bonding between the adhesive matrix and the substrate material is often the weakest part of the whole laminar composite structure, leading to the increasing demand of improving the bonding performances of epoxy adhesively bonded joints [7,8].

Nanocomposites consist of a polymer matrix embedded with at least one kind of inorganic particles in nano-dimension [9]. Introducing nanoparticles into the epoxy resin has become a promising method to reinforce the mechanical properties of epoxy and synthesizing epoxy adhesively bonded joints with enhanced bonding performances [[10], [11], [12]]. Since first discovered by Iijima in 1991 [13], carbon nanotubes (CNTs) have intrigued exclusive research attentions due to their outstanding mechanical, thermal and electrical properties, which are recognized as an ideal reinforcement for epoxy composites [[14], [15], [16]]. Extensive applications of the CNT-reinforced epoxy composites can be found in many different industries such as corrosion protection coatings [17], transparent heating films [18], and self-sensing components [19] in transportation, automotive, and aeronautics engineering. In civil engineering applications, since CNTs with extremely high tensile strength and elastic modulus are expected to overcome the weaknesses of neat epoxy and improve the adhesive bonding of the joints [23], the CNT-reinforced epoxy adhesively bonded joints have become more and more popular compared to the traditional joining methods [[20], [21], [22]]. The great potential of the CNT-reinforced epoxy adhesively bonded joints requires an effective bonding performance as a prerequisite [24,25]. For instance, if the CNT-reinforced epoxy adhesively bonded joints are applied as the protective coatings on steel structures in marine areas or corrosive environments and steel pipelines for corrosion mitigation and prevention, a better bonding performance means less coating delamination. Delamination is one of the major damaging modes for these coatings, which would induce hidden corrosion beneath epoxy coatings and is very challenging to detect and mitigate in field.

Previous studies reported that mixing CNTs into epoxy resin could improve the adhesion properties of neat epoxy adhesively bonded joints by increasing the fracture toughness of the epoxy [26,27] and restricting the crack propagation within the bondline layer [[28], [29], [30]]. Furthermore, the extraordinary high aspect ratio of CNTs could create an extraordinarily large contact area with the surrounding epoxy, resulting in a good load transfer to guarantee a sound cooperative work between CNTs and the epoxy adhesive matrix [23,24]. However, the reinforcing efficiency of CNTs varies from case to case [31,32], the effect of CNT fractions on the bonding performance of epoxy adhesively bonded joints is still an essential research topic.

In addition to the particle reinforcements, the bonding performances of epoxy adhesively bonded joints can also be influenced by many other factors, such as geometrical dimensions, substrate treatments, and curing conditions [[33], [34], [35]]. Among these influencing factors, the adhesive thickness of the epoxy layer is an of vital importance parameter which can greatly impact the bonding performances of epoxy adhesively bonded joints [36,37]. For adhesively bonded joints using neat epoxy resin, state of art and a previous study by the authors shared the same conclusion; The bonding strength of neat epoxy joints usually decreased with the increase of the adhesive thickness [[38], [39], [40]]. The neat epoxy resin is usually regarded as a brittle material because of the voids and micro-cracks generated within the epoxy matrix during the curing process, which could cause stress concentration and weaken the material properties. Epoxy adhesively bonded joints with thicker bondline layers tend to have more voids and micro-cracks, implying a higher porosity and higher possibility of a brittle failure [41,42].

However, for CNT-reinforced epoxy adhesively bonded joints, the addition of CNTs was found to lower the porosity of epoxy bondlines which could deviate and bridge the crack within the epoxy matrix [43,44]. Thus, the negative effect of thicker adhesive thickness could probably be reduced or even reversed with the CNT reinforcement. Because when the epoxy bondlines are free of any imperfections, the classic theoretical analysis showed that thicker bondlines positively influence the bonding performances of neat epoxy adhesively bonded joints as a result of a more uniform stress and strain distribution within the bondline layer. A number of researches have reported the investigations on the bonding performances of CNT-reinforced epoxy adhesively bonded joints, but they mostly focused on the impact of changing CNT weight fractions in the reinforced epoxy adhesively bonded joints. There is a severe lack of relevant studies on CNT- reinforced epoxy adhesively bonded joints with different adhesive thicknesses.

Thus, in this paper, the bonding performances of CNT-reinforced epoxy adhesively bonded joints on steel substrates with different CNT weight fractions and adhesive thicknesses were investigated using the single lap shear (SLS) tests. To the best of the authors' knowledge, for the first time, the effect of adhesive thickness is revealed systematically. The following bonding performances of epoxy adhesively bonded joints were evaluated included bonding strength, fracture strain, toughness, and failure mode. In addition, scanning electron microscopy (SEM) image analysis was also performed on the fracture surfaces of epoxy adhesively bonded joints to understand the reinforcing mechanisms of the CNTs in the epoxy adhesively bonded joints and reveal the potential effectiveness of the CNT reinforcement.