Optimising the Fibre-Matrix Interphase in Thermoplastic Composites Used in Aerospace Application

Carbon fibre reinforced plastics (CFRP) have been widely used in automotive, aerospace and marine applications because of its high strength, non-corrosive and lightweight properties. On curing, thermoset polymers like epoxy, vinyl ester can form intermolecular covalent chemical bonding both within them and also with the functional groups present on the fibre surface, producing stronger interfacial bonding between the fibre and matrix. On the downside, the cross-linked covalent bond makes the thermoset composites brittle, less tough, non-meltable, and non-recyclable. To overcome the drawbacks of thermosets, thermoplastics like Polyamide 6 (PA 6), Polycarbonates (PCs), Polyetherimide (PEI), Polyphenylene sulfide (PPS), and Poly-ether-ether-ketone (PEEK) are being used to produce more tougher, meltable, weldable, and recyclable composite parts which can be manufactured at higher production rates. However, the lack of cross-linked chemical bonding on solidification in thermoplastic composites is causing weaker adhesive bonding between the fibre and thermoplastics leading to premature failure of the material. Hence, nano/micro particles are used to modify the fibre surface and increase its bonding with the thermoplastic matrices through mechanical interlocking mechanism. Among different nanoparticles, carbon-based nanoparticles like carbon nanotubes (CNT)[1], graphene nanoplatelets (GNP), graphene oxide (GO), and reduced graphene oxide [2] have been found to be more suited for the high-performance thermoplastics used in aerospace applications as the carbon based nanoparticles can withstand the high temperature and pressure produced during the fabrication cycle. Even though one-dimensional nanoparticle like CNTs with high particle aspect ratio, can increase the mechanical interlocking between fibre and thermoplastics, they are very expensive and cannot be used for fabricating large-scale composite parts. Two-dimensional nanoparticles like GNPs with high surface area, are also effective in improving the fibre-matrix adhesion but they are prone to agglomeration, and they are also not a cost-effective material suited for continuous production. Meanwhile, a zero-dimensional nanoparticle called carbon black nano powders has been used to strengthen the fibre matrix bonding in thermoset, and thermoplastic composites. However, the carbon nano onions (CNOs) from the combustion of ghee lamp have never been used before. The concentric graphite layers and the porous structure of CNOs can produce capillary action by absorbing the molten thermoplastic during hot pressing and enhance the interfacial strength better than the baseline samples. It can reduce the crack propagation through toughening mechanisms like crack deflection, fibre bridging, fibre pullout, particle rupture and particle pull out. CNOs were first reported as a by-product material during the synthesis of carbon nanotubes by Dr Iijima in 1980. Since then, several techniques like electric arc discharge, laser ablation, and electron beam irradiation have been used to synthesize CNOs. However, in all these methods catalyst contamination, requirement of sophisticated instruments and high vacuum working conditions were the drawbacks. Additionally, the limited quantity of CNOs produced from the aforementioned techniques is insufficient for fabricating large scale composite structures. Hence, a recently developed simple flame synthesis process using easily available ghee oil can be used for continuous production of CNOs and subsequently use them in the manufacturing of carbon fibre reinforced thermoplastic composites (CFRTPs). Through this research work, ghee derived CNOs were prepared, characterized, and used to modify carbon fibre reinforced polyamide 6 (CF/PA6) composite film. Then, the CF/PA6 samples were tested using tongue tear test to determine its mode III fracture toughness. The characterization of ghee derived CNOs confirmed the presence of concentric graphite rings in it. The results also showed that CNOs are amorphous, rich in carbon content, and possess negligible amount of surface functional groups. After characterization, the untreated carbon fibre surface was modified using ghee derived CNOs through three kinds of fibre processing techniques namely: soot deposition process, direct immersion sizing (DIZ) process, and the spray coating process. The processed fibres were then coated with PA6 powder and heated at 260°C for 5 minutes under argon atmosphere to form a CF/PA6 composite film. After fabricating the CF/PA6 samples, they were subjected to tongue tear test. The test result shows that the soot deposition, DIZ, and spray coating process can increase the tear toughness of CNO modified CF/PA6 by nearly 143%, 322%, and 395% respectively. However, in all three fibre modification techniques, processing fibres beyond its optimal working parameters can cause agglomeration of CNOs which lowers the bonding between fibre and matrix. The tested samples were observed under SEM and stereomicroscope to study the damage mechanisms in the baseline and modified samples. The fracture images show that fibre-matrix debonding is more prevalent in baseline samples. However, for CNO modified samples, toughening mechanisms like crack bridging, crack pinning & deflection, fibre breakages, and matrix cracking are majorly occurring indicating an improvement in fibre-matrix bonding after CNO incorporation in CF/PA6 samples. The improvement of interfacial bonding in CF/PA6 samples after CNO modification can be attributed to the presence of concentric graphite rings in CNO particles and its role in developing anchoring points on the fibre surface to mechanically interlock it with nylon 6 matrix material. However, the dispersion and agglomeration of CNO nanoparticles on fibre surface can play a crucial role in forming a strong interfacial adhesion in CF/PA6 samples. The CNO dispersion is high, and its agglomeration is low in the spray coating process. Hence, the spray coating achieved the maximum tear toughness value than the other two fibre processing techniques. However, the soot deposition process is very cost-effective, and the DIZ process is highly suitable and efficient for processing a large fibre surface area. Thus, this research work which studied the effect of ghee-derived CNOs on the fibre-matrix bonding, can open the way for the next generation of high-performance thermoplastic composites, unlocking their potential for widespread industrial applications.