Adhesively bonded joints have many structural applications in the transport industry and particularly in the aeronautic field. Compared to mechanical joints, they allow distributing the load over a much wider area, reducing fabrication costs, increasing fatigue resistance of the joints, and improving aerodynamics over bonded areas, which results in considerable weight saving. These advantages are applicable to both metallic and composite aircraft structures. In fact over 15% of the structural weight of aluminium airplanes can be saved by using bonding techniques as opposed to using rivets and bolts [1
]. Also with bonded structures the maintenance cost is reduced due to the extension of the inspection interval, e.g., in Airbus A350, in comparison with the previous generation aircraft Airbus A330, direct maintenance costs are reduced by 15% [3
]. Moreover, the upward trend in using hybrid systems featuring fiber reinforced polymer composites (FRPC) as structural materials in aeronautic and commercial transport vehicles requires the ability to join dissimilar materials. In this context, the use of polymeric adhesives as an alternative solution to using rivets/bolts (which create stress concentration and/or corrosion initiation), and welding is of potential industrial interest. During the welding, the heating of adherends can lead to significant thermal stresses due to the difference in the coefficient of thermal expansion (CTE) between dissimilar materials (for instance the base metal, the welding material, and the composite). These induced stresses can result in the loss of dimensional stability (shrinkage), or delamination in these parts [4
]. However, the design of safe and cost-effective bonded joints is still a major challenge, and there is the need of a good understanding of the influence of material and geometric parameters on the joint’s strength [6
The quality of the adhesive bonded joint depends on different factors such as the suitable surface pretreatment of the adherends, the choice of the adhesive, the joint design, and the service conditions.
In metal-metal bonding, adhesives are active only on the molecular surface layer that forms the joint interface and on any surfaces contained in the porosity of the metal itself; consequently, adhesives for bonding metals are studied to bond metal–oxide. Aluminium (Al) is an almost ideal substrate for adhesives. However, the corrosion resistance of Al, as well as the durability of the joints made with epoxy adhesives, is very dependent on the type of Al alloy used. Bonds made with relatively corrosion-resistant 6061-T6 Al alloy will last about four times longer than equivalent joints made with the 2024-T3 Al alloy when exposed to marine environments. Fortunately, with the proper combination of surface treatment and adhesive, the adverse effect on durability to aggressive environments can be minimized [7
]. In addition, Al has a very reactive surface, and a cohesively strong oxide forms almost instantaneously when a freshly machined Al surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strength higher than that could be provided by most of available adhesives. The strength of the epoxy-bonded Al joint can be improved by cleaning the surface to remove contaminants or by converting the existing surface to a new surface that may be more consistent. In the case of the Al surface, chemical conversion can also protect the base metal from corrosion and enhance the durability of the bonded joint to various service environments. Between the different chemical surface treatments, chromate conversion coating on Al constitutes an effective way to enhance the surface bondability and also to improve the durability of the joint and the corrosion resistance of the interface.
Newly modified adhesives characterized by high adhesion strength, lower curing temperature, high fracture toughness, and other functionalities, such as electrical conductivity and heat/flame retardant resistance, are of practical interest [9
In order to take the benefits that nanotechnologies offer in the field of structural composite [10
], many researchers are working on the development of new multifunctional adhesives for structural applications. The strategy is based on appropriate epoxy resins nano-modified by carbon nano-forms, which enable it to increase the fracture toughness of the adhesive and hinder the insulator properties of the resin if employed beyond their Electrical Percolation Threshold (EPT) [11
]. This strategy allows us to simultaneously optimize the efficiency of the joints and preserve the conductivity of the lightweight materials that could be also able to provide self-sensing capabilities, as well as good lightning strike protection in the joints [21
]. Still, different types of hybrid nanofillers could be used in synergy with carbon nanostructured forms into the epoxy matrix, which not only improve the flame resistance and the thermal and photo-oxidative stability [23
], but also enhance the adhesive strength and toughness [27
]. Furthermore, the addition of hybrid nanofillers gives also a further benefit of balancing the increase in the viscosity of carbon nanofilled epoxy formulations [24
]. For this purpose, the recent reinforcement of polymer systems with nano-sized inorganic clusters, Polyhedrical Oligomeric SilSesquioxane (POSS), has been given considerable attention. POSS reagents combine a hybrid inorganic-organic structure, having dimensions comparable to those of most polymeric segments or coil that can be bound to the polymer, leading to the reinforcement of the system on molecular level [24
]. Recent studies have also shown that incorporated POSS in the polymer may have the potential to behave both like a filler particle and/or a plasticizing molecule, depending on the degree of dispersion in the polymer [29
]. In addition, the incorporation of an appropriate amount of POSS in an adhesive can effectively enhance its mechanical performance. In the study shown in Ref. [30
], the adhesive paste was applied to the aluminium bars adherends, which were preliminary degreased with trichloraethane and etched in Nochromix/sulfuric acid solution for 30 min, and then tested in single lap-bonded joint specimens. The authors observed that when using POSS with a concentration below 3 wt %, thermodynamically incompatible components form a loose transition region, which promotes a chain relaxation process and toughening of the adhesive. When POSS concentration was increased to 5 wt %, crystallization occurred over a large scale, leading to an increased glass transition temperature. This led to a decrease in adhesive bond line deformability and limited the further enhancement of adhesive performance. Also Jones et al. [31
] studied thermal and mechanical properties of nanocomposites containing epoxy resin and POSS. The tensile test results indicated that 5 wt % loading of POSS in epoxy resin constituted the system with the highest improvement in tensile strength and elastic modulus.
A different approach was adopted by Dodiuk et al. [32
] in studying the effect of the toughening of epoxy adhesives by POSS. They demonstrated that the design of new nanoscale-tailored adhesives with multifunctional properties required the appropriate selection and modification of the polymer matrix composition, the nanofiller type, and the control of the interactions at the interphase. In their studies, the shear and peel strengths of selected POSS epoxy formulations were characterized. In comparison with the neat epoxy formulation, an increase in shear strength of 20% was obtained for the selected POSS/epoxy formulations. Furthermore, a significant increase in peel strength was reported when the functionalities of the POSS were compatible with the epoxy system.
The aim of the current work is to study the effect of carbon nanotubes and POSS on the mechanical properties of the adhesive bonded aluminium joints, based on a TGMDA epoxy formulation containing neither toughening, such as elastomers, nor other additives typically used to provide a closer match in the coefficient of thermal expansion (CTE) between the adhesive and the adherends, such as aluminium or aluminium oxides powders. The reason why a blank formulation has been used was to discriminate only between the effects of the addition of the above-mentioned components. In addition, to establish the properties of nanofilled adhesives for aeronautic applications able to hinder both the insulating properties and poor flame resistance of epoxy resins, a different approach in preparing an adhesive was adopted: carbon nanotubes were dispersed above EPT [34
], together with POSS compounds at the concentration of 5 wt %, into the epoxy resin. DodecaPhenyl Oligomeric Silsesquioxanes (DPHPOSS) and Glycidyl Oligomeric Silsesquioxanes (GPOSS) were used for this purpose. The choice of these two types of POSS is the result of previous research activities [37
] where authors assessed the compatibility of these types of POSS molecules with a well-studied tetrafunctional epoxy resin, and also the effects of these nanofillers on the thermal stability and flame retardant properties of the resin [39
]. In particular, the epoxy resin employed in developing the adhesive paste has proven to be very effective for improving nanofiller dispersion due to a decrease in the viscosity [40
]. Bonded single lap joints (SLJs) were made using aluminium alloy T2024 adherends, whilst some of the adherends were treated with Chromic Acid Anodisation (Alodine 1200S). Then, the effect of different chemical functionalities of POSS and the synergetic effect between the MWCNT and POSS combination on adhesion strength were evaluated, taking also into account the benefits due to the surface treatment of the adherends. It has been found that treating the adherends with Alodine chromic anodization and modifying the epoxy adhesive with both GPOSS and MWCNTs enhance the adhesion shear strength substantially.
In this study, we investigated the effect of carbon nanotubes and two differently functionalized POSS compounds, Glycidyl Oligomeric Silsesquioxanes (GPOSS) and DodecaPhenyl Oligomeric Silsesquioxanes (DPHPOSS), on the adhesion properties of an epoxy resin based on the TGMA precursor, which is particularly suitable for aeronautical applications. The effects of the different chemical functionalities of POSS, as well as the synergetic effect of combination of the CNT and POSS combination on adhesion strength, were evaluated by viscosity measurement, tensile tests, DMA analysis, and morphological investigation. Bonded single-lap joints were made using both untreated and Chromic Acid Anodisation (CAA)-treated aluminium alloy T2024-T3 adherends. Single lap joint shear tests were conducted to measure the lap shear strength of the adhesive bond.
Evaluation of the viscosity at temperatures close to the curing temperature highlighted the beneficial effect that the GPOSS contributed to the formation of the less rigid cross-linked phase, which led to a lower viscous formulation counterbalancing the effect of CNTs in increasing the viscosity. On the contrary, the addition of the non-reactive and insoluble DPHPOSS to the epoxy formulation containing MWCNT caused a slight increment in the viscosity, similar to the case where inert solid micro/nano-fillers were added to polymer matrices, indicating lack of even weak interactions between DPHPOSS and the epoxy matrix and MWCNT.
Tensile tests of the nanocomposite resins showed that the incorporation of insoluble DPHPOSS in the neat epoxy system and in the formulation containing MWCNT did not cause any appreciable differences in the values of Young’s modulus and the tensile strength. On the contrary, the introduction of GPOSS increased all the mechanical characteristics of resin and this effect was ascribed to the formation of a less rigid continuous phase, which caused an improvement of the properties at break of the brittle epoxy matrix.
The dynamic mechanical analysis performed on cured samples confirmed this hypothesis. In fact, the results for GP-CNT/EP samples which contain both GPOSS and MWCNT showed the presence of an additional tan δ peak at a lower temperature, indicating the formation of a higher mobility phase. This additional tan δ peak was, instead, not observable in the DMA test performed on sample DPHP-CNT/EP containing both DPHPOSS and MWCNT.
Single lap joint shear strength tests on specimens made with untreated adherends demonstrated that DPHPOSS caused an improvement in the adhesion properties on the substrate of the adherends. This was attributed to a combined effect of the formation of the electron acceptor-free metallic ions Mg2+ on the surface of the adherends being able to interact with the more electronegative oxygen at the bridge between two Si in the POSS cage with the phenyl groups of the DPHPOSS. These interactions caused an improvement in the wettability and in the adhesion between the DPHPOSS-containing epoxy adhesive and the aluminium alloy.
Single lap joint shear strength tests also performed on joints made with CAA-treated adherends provided results with a completely different trend. The best results were obtained for the adhesive sample GP-CNT/EP, while no improvement, compared to the neat epoxy system, was observed for samples containing DPHPOSS. The Alodine chromic acid anodization of the adherends caused a substantial improvement of the adhesion properties of the GP-CNT-modified epoxy, whose adhesive composition included both GPOSS and MWCNTs. This important improvement was attributed to the combination of different phenomena: (i) the development of a stable surface of the adherends with a very homogeneous porosity leading to a strong Al/modified epoxy interphase; (ii) the improved dispersion of MWCNTs in the matrix due to the decrease in viscosity caused by the GPOSS incorporation; (iii) the formation of a phase with higher mobility governed by hydrogen bonds; and, most of all, (iv) a very effective micro bridging function of CNTs across the fracture surface. This last peculiar behavior of MWCNTs in the adhesive formulation provides a very convincing explanation of the relevant enhancement in Lap Shear Strength detected for the GP-CNT/EP specimen in comparison with the GP/EP specimen.
For the CAA-treated aluminium substrate joined with the epoxy DPHPOSS adhesive, instead, the beneficial effect due to the formation of a stable oxide layer was nullified by the complete removal of the magnesium ions from the surface of the adherends.