Mechanical Behavior of Graphene Nanofiller Grafted Carbon Fiber Reinforced Polymer Composites

Abstract

Globally fiber reinforced polymer (FRP) composites are drawing tremendous attention recently for their use in critical applications like structurals and bridges as a substitute for traditional metallic materials. They have an added advantage over traditional metals in terms of high corrosion resistance, greater strength and modulus. The use of carbon fiber reinforced polymer (CFRP) composites is progressively increasing in the fields of aerospace, cryogenic storage tanks, marine and so on because of their high specific strength, chemical resistance and thermal and electrical conductivity. The poor interfacial adhesion between reinforcing carbon fiber (CF) surface and polymer materials affects the interface dependent critical mechanical properties like interlaminar shear strength and flexural strength. The improvement in interfacial properties can be carried out by introducing a third phase into the composites, i.e., nanofiller. Currently, decoration of carbon fiber (CF) surface with graphene based nanofillers (GBNs) by electrophoretic deposition (EPD) route is a trend to enhance the interfacial performance of CFRP composites. The present investigation begins with assessing the improvement in the flexural and interlaminar properties of CFRP composites by surface modification of carbon fiber via EPD technique using different graphene based nanofillers like graphene (G), graphene oxide (G-O), graphene hydroxyl (G-OH) and graphene carboxyl (G-COOH) and selecting a better nanofiller out of these for further studies. The highest flexural strength and interlaminar shear strength (ILSS) were obtained for the G-COOH grafted CFRP (G-COOH/CFRP) composites, which were 9.6 % and 22.9 % higher than that of control CFRP composites, respectively. Thermo-mechanical analysis was conducted in the temperature range of 30 ºC to 180 ºC to understand the temperature dependent mechanical behavior of all the composites. FTIR analysis of nanofillers was carried out to confirm the functionalization of nano fillers. From the previous study, it was learnt that grafting G-COOH on carbon fiber surface by electrophoretic deposition (EPD) is a promising route to improve the mechanical properties of CFRP composites. Methyl violet (MV) was used in the cathodic EPD technique to maintain stability of the suspension and to impart a positive charge to the nanofillers there by facilitating the cathodic EPD. Before going for thermal annealing of decorated carbon fibers, they were washed with acetone to remove methyl violet. This step of removing MV was one of the longest time consuming step as it involves 24 hours of air drying, followed by washing with acetone. These steps might hinder the commercialization of cathodic EPD technique due to the long processing times involved. Therefore, in the current research study, an attempt was made to analyse the necessity of removing MV by washing the decorated fibers with acetone, post EPD and its effect on the mechanical behavior of the modified CFRP composite by carrying out flexural and short beam shear tests. The G-COOH/CFRP composite in which the MV was not washed has shown a decrement of 20.82 % and 4.90 % in ILSS and flexural strength respectively in comparison to neat CFRP composites. Carbon fiber surface topography was analysed using a scanning electron microscope (SEM) before and after acetone washing. Fractography analysis was carried out to understand the failure mechanism and dominant mode of failure using SEM. FTIR analysis of neat and G-COOH loaded epoxy was carried out to understand the effect of MV on the epoxy/G-COOH interface. The next objective was aimed at analysing the mechanical behavior of G-COOH/CFRP composites when they are tested in-situ at room temperature, cryogenic temperatures (CT) and elevated temperature (ET). Along with this, the effect of a prominent processing parameter, which was the nanofiller concentration in the EPD bath on the deposition morphology, and mechanical behavior of G-COOH/CFRP composites, was also studied. Experimental results showed that out of three different concentrations (0.5 g/L, 1.0 g/L, 1.5 g/L) used, composites made with 1.5 g/L bath concentration have shown the best mechanical behavior at both room temperature (RT) and cryogenic temperature (CT). At RT, improvements of 25.38 % and 17.02 % were observed in ILSS and flexural strength, respectively in comparison to neat CFRP composites. At CT, the highest improvements observed in ILSS and flexural strength were 20.78 % and 5.34 %, respectively in comparison to neat CFRP composites. Similarly, composites made with 1.5 g/L EPD bath concentration had shown a maximum improvement in energy absorbed before failure of 33.25 % at RT and 22.54 % at ET for flexural testing (flexural toughness) and in case of short beam shear tests (interlaminar toughness), improvements of 35 % at RT and 78 % at CT were observed in comparison to that of neat CFRP composites at respective test temperatures. However, at ET, modified composites exhibited lower flexural strength and interlaminar shear strength (ILSS) values in comparison to that of neat CFRP composites. Viscoelastic behavior of all composites was studied to understand bath concentration's effect on thermal behavior via dynamic mechanical thermal analysis (DMTA). Fractography of tested samples (both ET and RT) was performed utilizing a scanning electron microscope (SEM) to determine the prominent failure mode. As the next step towards a comprehensive understanding of EPD process, the effect of electrophoretic deposition time on the resultant carbon fiber surface morphology and mechanical behavior of G-COOH/CFRP composites was analysed at room temperature and elevated temperatures in this study. The laminates fabricated (both neat and modified) were subjected to short beam shear tests (SBS) at room temperature (RT) and different elevated temperatures (ET), i.e. 70 ºC, 100 ºC, 120 ºC and the role of deposition time at each testing temperature was also analysed. CFRP composites fabricated with 60 minutes of deposition time have shown an improvement of 35.0 % in ILSS, when compared to that of control specimen at RT. Modified composites showed a maximum improvement of 16 % and 13 % in ILSS values over neat CFRP composites at RT, 70 ºC and 120 ºC, respectively. However, interestingly at 100 ºC, modified composites have shown inferior shear behavior in comparison to neat CFRP composite. Scanning electron microscope (SEM) was used to observe the tested samples to find out the dominant mode of failure.