Nanoscale Advances REVIEW View Article Online View Journal | View Issue Mechanical properties of aerospace epoxy composites reinforced with 2D nano-fillers: current Cite this: Nanoscale Adv.,2021,3,2741 status and road to industrialization Radhika Wazalwar, Megha Sahu and Ashok M. Raichur * High-performance epoxy composites find application in the aerospace industry. Although epoxy is a high- performance polymer, its fracture toughness is compromised due to its highly cross-linked nature. Nanomaterials such as carbon nanotubes (CNTs), graphene derivatives, and inorganic 2-dimensional (2D) nanomaterials are being explored to improve epoxy composites' mechanical properties. Graphene is one of the most popular 2D nano-reinforcing agents for epoxy composites. Following graphene discovery, the research community's attention was brought to various other few-atom thick 2D nanomaterials. Hence, apart from graphene, inorganic nanosheets such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), etc., are also being studied as modifiers for enhancing the mechanical performance of epoxy Creative Commons Attribution-NonCommercial 3.0 Unported Licence. composites. Graphene, TMDs and hBN are known to possess a high aspect ratio, high specificsurfacearea and inherently high mechanical strength and stiffness, contributing to a stronger and tougher composite. Despite that, the challenges associated with these nanomaterials, such as dispersion issues, lack of standardization, underlying health hazards, etc., have hampered their commercialization. It has been long past Received 20th January 2021 adecadesincethediscoveryofgraphene,yetthereareconcernsregardingthelabtoindustryscale-up,and Accepted 24th March 2021 health and environmental hazards associated with nanomaterials for the fabrication of aerospace composites. DOI: 10.1039/d1na00050k This review offers a comprehensive literature survey and a perspective into the possible ways of bridging the rsc.li/nanoscale-advances gaps between the laboratory research and industrialization of 2D nanosheet-filled epoxy composites. This article is licensed under a 1. Introduction Metal alloys, especially aluminum alloys, have been an indis- Open Access Article. Published on 29 March 2021. Downloaded 9/24/2021 9:46:36 AM. pensable part of the aerospace industry for a long time. A large Department of Materials Engineering, Indian Institute of Science, Bengaluru, India. portion of the product range offered by aircra manufacturers E-mail: [email protected]; [email protected]; Tel: +91-80-22933238 Radhika Wazalwar is currently Dr Megha Sahu is currently a Ph.D. student at the Depart- working as a Materials and ment of Materials Engineering, Process Senior Engineer at Boe- Indian Institute of Science, Ben- ing India Private Limited, Ben- galuru, India. Radhika gradu- galuru, India. She obtained her ated with a Bachelors in Ph.D. from the Department of Technology from the Depart- Materials Engineering, Indian ment of Metallurgical and Institute of Science, Bengaluru, Materials Engineering, Visves- India in 2017 and holds varaya National Institute of a Masters degree in Chemistry Technology, Nagpur, India in from Lucknow University, India. 2016. Presently she is working Dr Sahu has led more than 15 on chemical modication of 2D patents in the eld of polymers, nano-llers for enhancing the mechanical behaviour of epoxy additive manufacturing and composites. Her research work composites under the supervision of Prof. Ashok M. Raichur. includes graphene based advanced epoxy composites, nanomaterial synthesis and chemical alteration of nanomaterials for application related to improved mechanical behaviour of polymer composites. © 2021 The Author(s). Published by the Royal Society of Chemistry Nanoscale Adv.,2021,3, 2741–2776 | 2741 View Article Online Nanoscale Advances Review still comprises of metal aircra. However, since the last decade, was also mentioned. A review article by Marouf et al.11 discussed there has been a shi towards polymer composites. Modern and compared the effect of one, two and three-dimensional planes such as Boeing 787 and Airbus A380, which came into nano-llers on the fracture properties of epoxy composites. service in the last decade, utilize more than 50% by weight The underlying toughening mechanisms in binary and ternary carbon ber reinforced epoxy composites in the aircra fuse- epoxy composites reinforced with nanollers such as nano- lage, wings and empennage assemblies. In comparison to silica, nano-rubber, CNTs, nanoclay and graphene were dis- metals, polymer composites, especially epoxy composites, offer cussed in detail in this article. Szeluga et al.12 reviewed graphene several advantages. They possess higher corrosion resistance, llers' effect on the mechanical, thermal, electrical and ame better strength to weight ratio, require fewer sub-assemblies, retardant properties of epoxy composites. The effect of ller consume less fuel due to reduced weight, offer easier repair size, degree of exfoliation and functionality on the properties of and maintenance, etc. However, there are some limitations epoxy composites was summarized. The authors pointed out associated with epoxy composites. Like most other polymers, that there is insufficient literature to assess the ller-matrix epoxy is also thermally and electrically non-conducting, which interaction accurately. Atif et al.13 reviewed epoxy composites leads to its inability to deal with electromagnetic interference reinforced with graphene. They established a correlation of the and lightning strikes. Another limitation of epoxy is that it is ller size, morphology, and functionalization extent with the highly brittle due to its heavily cross-linked network. Aircra nal epoxy composites' mechanical, thermal, and electrical structures undergo extreme thermal cycles, mechanical load properties. The authors pointed out the lack of consensus and fatigue and need to have good fracture toughness so that regarding graphene's effect on the properties of epoxy catastrophic failure does not occur. Researchers are continu- nanocomposites. ously trying to improve the mechanical properties of epoxy In a review by Kulkarni et al.,14 the authors described the composites, focusing on fracture toughness. Many toughening covalent and non-covalent functionalization approaches for agents such as rubber,1,2 thermoplastic polymer particles, silica surface modication of graphene. The authors discussed the 3 4 ff Creative Commons Attribution-NonCommercial 3.0 Unported Licence. nanoparticles, and nanoclay have been explored to enhance e ects of processing protocols, ller dispersion, and ller epoxy's mechanical properties. Several researchers have been surface modication on graphene–epoxy composites' electrical, exploring CNTs,5,6 graphene, and more recently, inorganic 2D mechanical, and thermal properties. Singh et al.15 reviewed nanomaterials7,8 for the improved mechanical performance of graphene, CNTs, and graphene/CNT hybrid reinforced epoxy epoxy composites. Amongst these, 2D nanomaterials are composites. The inuence of the ller's type and functionality popular due to their high aspect ratio and inherently high on the mechanical, thermal, electrical and ame-retardant mechanical strength.9 properties of epoxy composites was discussed. The various Many review articles have been published on the subject of applications of such composites, especially in aircra body, nano-llers for epoxy. Domun et al.10 comprehensively reviewed electro-magnetic shielding, corrosion-resistant coatings, etc., This article is licensed under a the status of epoxy composites reinforced with single and were discussed. multiwalled CNTs, graphene, nano-silica and nano-clay. The Research groups worldwide have extensively worked on authors summarized the effect of these nano-llers on the epoxy reinforced with graphene and inorganic 2D nanollers Open Access Article. Published on 29 March 2021. Downloaded 9/24/2021 9:46:36 AM. tensile and fracture properties of epoxy composites. They and have produced promising results in terms of the mechan- pointed out the lack of uniformity in the composite fabrication ical and thermomechanical performance of these composites. methods, ller functionalization, and dispersion techniques Despite the abundant literature on 2D nanomaterial-lled used, restricting the understanding of the composites' epoxy resins, it has still not graduated from the laboratory to mechanical properties. The lack of suitable quantitative the aircra industry. Hence, a perspective is needed in terms of methods to assess ller dispersion and ller-matrix interaction the research conducted in this eld, its challenges, and the possible solutions to these challenges. The review articles on 2D nanoller reinforced epoxy Ashok M. Raichur is currently composites available so far have majorly taken into consider- a professor at the Department of ation graphene and its derivatives. To the best of our knowl- Materials Engineering, Indian edge, the status of inorganic 2D nano-llers, especially TMDs Institute of Science (IISc), Benga- and hBN and their various organic and inorganic multiscale luru, India. He received his Ph.D. hybrids, which are being extensively explored to reinforce epoxy from the Department of Metal- composites, has not been reviewed yet. Many authors have lurgical Engineering, University discussed the problems associated with the processing and of Nevada, Reno, USA in 1996 dispersion of nano-llers. However, successful commercializa- and joined IISc as a faculty in tion