Enhanced metal-composite interface via novel silver nanowire (AgNW) interleaves

The ever-increasing demand for high performance, damage tolerant materials with better mechanical properties and reduced weight compared to traditional monolithic metals led to the development of Fibre metal laminates (FMLs) at the Delft University of Technology (DUT), Netherlands in 1978. These hybrid materials are composed of alternating layers of metal alloy sheets and fibre reinforced polymer (FRP) composites. They have several advantages such as high strength and stiffness, high fatigue resistance, low density and, hence, low weight, high resistance to corrosion, excellent impact resistance etc. Despite their many desirable characteristics, FMLs are prone to debonding along the metal-composite interface due to the weak interfacial bond strength resulting from the adhesive joining of dissimilar materials. The low bonding strength between the metal and composite layers limits the application of FMLs in primary aircraft structures. Thus, many researchers focus on developing new strategies to overcome the weak metal-composite bonding, which can lead to enhancement in through-thickness properties of FMLs. Such improvement can, in turn, lead to the utilisation of FMLs in a broader range of applications. The main objective of this thesis is to overcome the weak metal-composite adhesion by developing a novel technique to incorporate multi-scale interleaves at the metal-composite interface along with physical bonding to the metal substrate. The effect of the novel interleaves on the through-thickness performance of FMLs are investigated under different loading conditions. <br><br> A comprehensive literature review is presented in this PhD thesis encompassing interfacial adhesion in FMLs, state-of-the-art techniques to overcome weak metal-composite bonding, the methods of interleaving and nano-bonding, associated processes and controlling parameters. Based on the reviewed literature, research gaps pertaining to metal-composite interfacial enhancement are identified and addressed via comprehensive experimental investigation. <br><br> In this research, a nano-bonding process which involves the bonding or interconnecting of nanomaterials at low temperatures is exploited to incorporate 3-D, interconnected silver nanowire (AgNW) interleaves at the metal-composite interface. The effects of different annealing temperatures and pressures on the bonding strength of FMLs are explored to establish optimised fabrication conditions. It is revealed that on subjecting AgNWs deposited on an aluminium substrate to the nano-bonding procedure, a portion of the nanowires coalesce and form micro-globules. Thus, the incorporated interleave has a combination of micro- (globules) and nano- (bonded AgNWs) features. In addition, a two-stage deposition strategy of AgNWs is investigated to evaluate its effect on the characteristics of the metal-composite interface. The novelty of this work is demonstrated by achieving bonding between the 3-D multi-scale interleave and the nano-features on the surface of the aluminium substrate.<br><br> The thesis presents an experimental study into the mode I (opening) performance of interleaved FMLs incorporating nano-bonded AgNW interleaves. Two different AgNW concentrations of 1 g/m2 and 3 g/m2 are used to create the interleave along with two different deposition strategies: one- and two- stage deposition. Also, different surface treatments of aluminium layer (acid-etching and combined sanding and acid etching) are investigated. Double cantilever beam (DCB) tests show that FMLs incorporating AgNW interleaves exhibit improved fracture toughness compared to control laminates. The most significant improvement in mode I fracture toughness - a 13-fold increase - is observed in the specimens containing 3 g/m2 AgNWs deposited in two stages. In this variant, the crack is pinned from propagating along the metal-composite interface and, thus, is diverted into the composite layer.<br><br> A further study explores the performance of interleaved FML variants under mode II (shearing) loading. Along with requiring higher loads for crack initiation and propagation, FML variants incorporating 3-D multi-scale AgNW interleaves show improved mode II fracture toughness compared to baseline sample. The best performance is observed with the FML variants incorporated with 3 g/m2 of AgNWs deposited in two-stages. Crack deflection into the composite layer is observed and subsequent crack propagation occurs entirely through the composite layer rather than the metal-composite interface, indicating the toughening effect of the 3-D multi-scale AgNW interleave. <br><br> The fracture surfaces of the tested samples are analysed using Scanning Electron Microscope (SEM) to ascertain the mechanisms behind the toughness enhancements. Extensive nano-bonding is observed along with 'micro-globules' on the aluminium substrate. Well-established mechanisms of nanowires debonding from the epoxy resin, pull-out and fracture of nanowires, and plastic void growth of epoxy are observed in the interleaved FML variants. In addition, due to the ductile nature of AgNWs, there is extensive plastic deformation and necking of nanowires. Remnants of ruptured AgNWs that had bonded to the aluminium are seen on the fractured aluminium surface. All these mechanisms lead to enhanced energy absorption during the fracture process, thereby, increasing the overall fracture toughness of the modified FMLs containing AgNW interleaves. <br><br> This thesis also presents a low-velocity impact study of interleaved FMLs at three different energy levels of 6 J, 10 J and 30 J. It is observed that the energy absorption increases with the presence of AgNWs at the metal-composite interface along with increased stiffness and impact resistance. The effect is more significant at low-energy impacts (6 J and 10 J) compared to high-energy (30 J) impacts due to the macroscopic nature of damage imparted at higher energy. Cross-sectional microscopy reveals that the degree and extent of debonding reduce with the incorporation of 3-D multi-scale AgNW interleave at the metal-composite interface. In lieu with the findings of mode I and mode II fracture toughness studies, FML variant with 3 g/m2 of AgNWs deposited in two stages is found to have the best performance in terms of the resistance against low-velocity impact damage.<br><br> A significant outcome of this PhD is that, for the first time, a novel technique to create 3-D multiscale interleaves with physical bonding to aluminium substrate has been established. The optimisation of the nano-bonding conditions has further increased the understanding of the newly developed technique. Experimental and SEM observations, relating to the effect of the incorporation of the 3-D multiscale AgNW interleaves and the toughening mechanisms activated by their presence, contributes substantially to the knowledge base in metal-composite interface enhancement. These findings can be utilised to incorporate more efficient metal-composite joints in a broader range of applications.