A review on lightweight materials for defence applications: Present and future developments

Suchart Siengchin

a National Defence College (Thai NDC 65), National Defence Studies Institute, Vibhavadee-Rangsit Road Dindaeng, Bangkok 10400, Thailand

b The Sirindhorn International Thai-German Graduate School of Engineering (TGGS), King Mongkut's University of Technology North Bangkok (KMUTNB),Bangkok 10800, Thailand

Keywords:Lightweight materials Defence Technologies Developments Applications

ABSTRACT

1. Introduction

Due to the advancement in aerospace technologies,there is a lot of demand for advanced materials. Lightweight composite materials are an emerging technology that had gathered much interest from the space industries. The principle behind lightweight material is the utilization of materials with lower density but ensuring similar or enhanced performance compared to traditional materials. These lightweight composite materials were extensively adopted by the space industries due to their strength-to-weight ratio, reduced carbon emission, good thermal resistance, radiation resistance, fatigue durability, damage resistance, and better mechanical performance [1—3]. The utilization of lightweight composite in aerospace had reduced energy consumption,manufacturing cost, and industrial wastes and improved manufacturability. These advantages had influenced the research community to come up with various perspectives for exploring possibilities to enhance the performance of aircraft and aerospace materials. These lightweight composites are comprised of categories like metal matrix composites (MMC), polymer matrix composites (PMC), ceramic matrix composites (CMC), and fiber composites. Rocket engine parts were some of the common applications of lightweight composites in aerospace.Spacecraft body,fairings,upper/lower wing covers,fuselage skin,vertical/horizontal stabilizers, interior panels, passenger seats, etc. [4—6].

2. Social and economic aspects of the world's defence

Basic economic security should be a human right, and this should be defined in terms of advancing real freedom[7].Economic conditions have also been claimed to be among the most crucial factors behind armed conflicts in developing countries. Sluggish economic growth, increased poverty and skewed income distribution, lack of basic infrastructure and social services, wide regional differences, lack of access to agricultural land, and depletion of natural resources are the notable economic factors responsible for the emergence of civil wars.These economic factors result in increased despair, misery, and frustration for people in marginalized communities and regions, and consequently, they initiate conflict in many parts of the developing world [8—10].

Nowadays, military power is considered an important factor in international relations. It acts as a decisive power in disputes,shapes relationships etc. Moreover, it is a matter of fact that military plays a pivotal role in the safety and security of the human race. Now nuclear weapons are possessed by most of the developing nations.It leaves no time for the country under aggression to assemble their military and industrial forces.The nation once under attack can be practically be destroyed in a matter of minutes[11—13]. Studies found that military power affect patterns of international cooperation, trade policy, economic development,identity construction, other than war formation and termination[14,15]. So, all countries in the world are constantly urged to upgrade and update the military capacity to a larger extend. They remain vigilant at all times to avoid the state of unhappy relationships with their neighboring counterparts.In order to prevent the threats and to ensure peace and stability around the world,it is important to focus more strongly on the continuous transformation and modernization of the defence sector [16]. The defence sector has very strong economic and industrial dimensions.The country's allocations into the defence sector directly depend on the economic, social, and technological welfare of its population and the environment. On the other hand, the sustainable development of all these aspects serves to secure the country's opportunities on the international and domestic planes. Global defence expenditure increased by 4%in 2019,which was the largest increase in the last decade. This was conditioned by the increase in US and Chinese military spending [17,18].

Military power is a function of both quantity and quality. The states of comparable size may still differ in their level of military power because economically developed states field more effective forces[19].Defence spending affects economic growth in multiple ways. Defence spending generates jobs directly and can improve economic output indirectly through the spillover of technology and human capital to the civilian economy [20]. We talk about these absolute effects on growth first. However, defence spending also has opportunity costs because it diverts resources from government programs that might do more to promote growth.Therefore,we also consider the relative effects of defence spending compared with infrastructure investments.The defence and peace economics literature has produced a large number of studies on the relationship between defence spending and economic growth but has not yet reached an agreement on whether the relationship is positive[21—24].

Economists warn that,in the coming years,policymakers might need to look beyond allocating funds within the federal budget to reducing its overall size. But many commentators have correctly noted that defence spending is not the main driver of annual deficits or the growing public debt. For many years, spending on mandatory programs, such as Medicare, has outstripped defence spending.However,our findings suggest that the effects of defence spending on the annual budget and the size of the deficit are meaningful. Prior to the pandemic, defence spending had been about half of annual discretionary spending.A small shift in defence spending can lead to changes in the debt-to-GDP ratio and associated annual interest payments. For example, if the country were to adopt a defence budget of 2.5% of GDP, the annual difference in the defence budget plus associated interest payments in 2030 would be enough to fund a midsize government agency.Therefore,proposed changes in defence spending can have meaningful consequences for the federal budget and the size of the deficit[25].The substantial economic costs of the conflict and elevated uncertainty add to the challenges already facing policymakers from rising inflationary pressures and the imbalanced recovery from the pandemic. Faced with an adverse supply shock of uncertain duration and magnitude from higher commodity prices, monetary policy should remain focused on ensuring well-anchored inflation expectations and intervention if needed to ensure the smooth functioning of financial markets.Additional temporary,timely and well targeted fiscal measures, where feasible, provide the best policy option to cushion the immediate impact of the crisis on consumers and businesses,especially with rising inflation limiting the room for monetary policy maneuver [26].

2.1. Defence companies in the world

Many companies that were funded by the government as well as corporates were contributing to the defence systems all over the world. This report will highlight a few companies that were significantly contributing to the fortification sector.

2.1.1. Lockheed Martin corp

Lockheed Martin is an U.S. based aerospace and global security providing company.They have contributed to many products such as navigation systems,fighter helicopters,autonomous underwater vehicles, fighter aircrafts, military training system software, cyber security systems,career aircrafts etc. [27].

2.1.2. Boeing

Boeing has now acquired a contract from U.S. government for manufacturing replaceable wings for A10 Warthog aircrafts. The A10 Warthog provides excellent protection for the pilot in the form of an armored tub. It weighs about 1200 lbs. And is built fromtitanium sheetsof thickness range of 13—38 mm, capable of withstanding hits from 23 mm cannon rounds[28].

The Unmanned Little Bird H—6U is a scout helicopter.Its unique design combines the autonomous flight capability, payloads and communications of an unmanned aerial vehicle with a combatproven rotorcraft platform. The unmanned system provides overthe-horizon search, communications relay and surveillance capabilities. This little bird is characterized with a low-risk approach,low operational costs and high flight readiness [29].

2.1.3. BAE systems

BvS10 BEOWULF is a amphibious vehicle made by BAE systems,U.K. BvS10 BEOWULF is designed to support the crew in any conditions and in any terrain in missions like transporting equipment and ambulance service on both land and water capable of carrying up to 14 passengers with 8 tons of payload at a maximum speed of 65Kmph [30].

2.1.4. Leonardo

Leonardo is an Italy based defence company which deals with supplying electronics, aviation technologies, cyber-security and so on. Products like ATOS, Falco EVO, Osprey, MIYSIS etc. Have been provided by them offering Advanced multi-domain technological solutions (air, land, sea, space, cyber) for security within and beyond national borders, and for management of complex civil infrastructure. Command, Control, Communication, Computers,Intelligence, Surveillance, Target Acquisition, Reconnaissance(C4ISTAR) sensors and systems, and self-protection equipment giving forces in the field a clear operational picture alongside superior information to support decision-making and intervention throughout the entire chain of command. Leonardo's foundry specializes in the production of Monolithic Microwave Integrated Circuits (MMICs), using Ga-AS (Gallium Arsenide) and Ga—N (Gallium Nitride),a materials essential for the new generation of multifunctional AESA (Active Electronically Scanned Array) radars[31].

2.1.5. Airbus

Airbus is a Netherlands/France based defence company which support shipping and insurance companies, oil, gas & mining organizations,national and private security forces and environmental protection agencies to ensure the safety and efficiency of aviation,space, cyber security and maritime activities around the globe.They've developed a navigation interface called Ocean finder that allows customers to directly order satellite-based maritime detection and identification reports, allows fleet monitoring, relocated hijacked vessels, etc. [32].

2.1.6. Thales

Thales is a global technology leader with more than 81,000 employees on five continents.The Group is investing in digital and"deep tech" innovations — Big Data, artificial intelligence, connectivity, cybersecurity, and quantum technology[33].

2.1.7. Dassault

Dassault is a France based multi-industrial company which mainly focuses on product sustainability by accessing their life cycle(LCA).They provide products for defence,aviation, industrial equipment, maritime transports etc. [34].

2.1.8. Bourque industries

Bourque Industries is an U.S. based advanced materials science company which develops metal alloys through a metal alloying process called Kryron(Carbon Nanotube MMCs).Kryronized alloys have application potential for ballistic armor, mining equipment,aviation, automation, electrical applications medical devices etc.They have been producing armors for vehicles that can provide protection against 50 caliber rounds and IED blasts. They're also providing personnel body armor series like Bourque X-Terminator armor,NIJ-III armor and Kryron based armors which can withstand rifle rounds and armor piercing rounds [35].

2.1.9. Rheinmetall AG

Rheinmetall is a German based defence company which mainly focusses on sustainability and aims to achieve CO2neutrality by 2035. They provide services for military applications, naval and airborne applications. They are working on many battle tanks, All terrain utility vehicles, armored vehicles, unmanned vehicles, unmanned aircrafts, turrets, drones, solar shields, countermines etc,[36].

2.1.10. Ingalls shipbuilding

Ingalls shipbuilding industry is an U.S.based maritime company that has built many naval structures which includes the Aegis DDG 51 class guided missile destroyers, LHA 6 class large deck amphibious ships,National Security Cutters for the U.S.Coast Guard and the Navy's fleet of San Antonio (LPD 17) class amphibious assault ships. They've provided the REMUS 100 unmanned submarines for United Kingdom Navy [37].

3. Lightweight materials and their general studies

Over the past several years, lightweight materials and designs have drawn a great deal of attention due to their major advantages in a number of engineering difficulties that have raised demands in safety,the environment,competition,and cost.Hybrid systems are a successful strategy that aims to utilize the specific functions and unique properties of many materials in lightweight constructions while also improving the efficiency of each material. The fiber reinforced polymer (FRP) composites have garnered a lot of attention lately amongst various lightweight materials, such as aluminum alloy,magnesium alloy,and one's composite forms,due to their incredibly higher modulus/strength-to-weight ratio and great design versatility[38—41].Energy needs in the aerospace and aviation sector are quite high,thus it's important to use lightweight alloys to lower the overall aircraft weight in order to increase cost savings. An aircraft's weight affects how much it costs to move,with fuel accounting for around 30% of the overall expenditures.Consequently, the prospective advantage of adopting lightweight structural materials is obvious given that the amount of global aviation traffic has doubling every 15 years since 1977 and that this tendency looks to keep increasing in the future [42—44]. Lightweight porous materials come in a variety of constituents that are frequently categorized as metallic, nonmetallic, oxides, and polymer materials. Due to its minimal price, template structure, customizability, and biodegradability, natural polymers including starch,cellulose,sodium alginate,and others are preferred to make lighter porous materials [45,46]. Multipurpose, lightweight materials have the potential to significantly increase the entire performance and ecological friendliness of the entire construction system. In order to meet the particular of each purpose or device,lightweight materials are often regarded materials utilizing polymers as hosting matrices or/and composites supplemented with additives.Elastomers,thermoplastics,and thermosets are all types of polymeric material that may be tailored for specific purposes thanks to their diverse physicochemical characteristics. Additionally, in a wide sense, lightweight materials could also comprise inorganic highly dense materials that could be employed sparingly in lightweight systems to perform specialized tasks like selfcleaning or structural health surveillance without affecting the main structure's lightweight properties [47—50]. The increasing demands for fuel efficiency improvement and greenhouse gas emission reduction have propelled manufacturers to build lightweight vehicles.Additionally,a lighter vehicle may function better when recycled and/or driven (e.g., with better fuel efficiency,braking characteristics, and crash safety). Lightweight materials in automobile sector can be broadly categorized into four groups as potential replacements for traditional engineering structures(such as steel and cast iron),including light alloys(such as Al,Mg,and Ti alloys), the HSS family (such as traditional HSSs and AHSSs), composites(CFRP,GFRP,NFRP),and other advanced materials[51—53].The use of several kinds of lightweight materials in the production of various automotive parts is demonstrated in Fig.1 [51].

Fig.1. Incredibly light materials suitable for use in many automotive components [51].

Lighter components and structures for several industrial applications may be produced using cost-effective methods due to the use of light metals (aluminum, magnesium, and titanium) and cutting-edge casting techniques.In order to minimize energy usage and carbon impact,light alloys of Al,Mg,and Ti are being employed more and more in the industrial and transportation sectors.Within past 20 years, cast alloys and techniques for lighter Al, Mg, and Ti applications have advanced significantly. Thin-wall Mg electronic enclosures, Ti engine components, and mega-large Al automobile body castings are just a few examples[54,55].Because they have a low density (about two-thirds that of aluminium) and excellent strength,magnesium alloys have attracted a lot of interest from the railway vehicle sectors lately. Magnesium alloys are currently 1.3—1.5 times more costly than aluminum alloys at the pricing point of raw materials [56]. Liao and team designed a novel lightweight Tix (AlCrNb)100-x(x= 45—80) alloy for transportation vehicles applications. The developed alloys have excellent compression yield strength of around 1500 MPa, higher compression fracture strength of approximately 1800 MPa, and increased compression plasticity of greater than 30%at room temperature,all of which can display desired mechanical characteristics [57]. Recently, an interesting work reported the utilization of magnesium based alloys such as AZ31,AZ61,AZ80,and Mg—Li alloy for the manufacturing of thin-walled aerospace profiles. In order to make various sorts of complicated cross sectional shapes that are of importance to the aviation sector for light extruded items in order to save energy,expenses,etc.By lowering aircraft weight,the capability of showing the optimal materials and processing parameters was demonstrated[58].Shao et al.developed a series of Al—Mg entropic alloy systems, and in-depth examination was conducted to introduce innovative materials. Each of the alloys were discovered to have low densities between 2.64 and 2.75 g/cm3,which were less dense than typical titanium alloys (4.5 g/cm3). The lightweight high entropy alloys that were developed have a high compressive strength of more than 500 MPa at room temperature [59]. Blatnicky and team designed and developed a real model of three wheeler vehicle using EN AW 6063 alloy,and achieved a weight reduction of 40 kg.The simulated model and real model are illustrated in Fig. 2. The acquired knowledge from the considerations of simulations was implemented into the construction design of the frame E3-cycle in accordance with specifications and design guidelines. The sturdiness and structural integrity of the frame are guaranteed in the proposed vehicle since no equipment may create excited vibrations that would induce system resonance and harm the frame [60].

Fig. 2. (a) The calculated stresses in the frame of the planned vehicle's EN AW 6063. T66 material; (b) Real model of the designed vehicle [60].

Composites are being taken into consideration in the transportation industry to create lighter, safer, and more fuel-efficient vehicles. It has been shown that fiber-reinforced polymeric composites, particularly carbon fiber-reinforced plastic (CFRP), can integrate all these properties more effectively than other kind of composites. Its extensive use in aircraft components, which account for almost 50%of the structural mass of the latest airplanes,serves as proof of this. Those materials make use of a flexible polymer matrices as well as the excellent specific stiffness and strength of micro/nano continuous fibers that are both very lightweight[61].Lightweight composites called syntactic foams include hollow particles spread throughout a matrix. Those lighter materials are frequently utilized in applications where weight is an issue, such as maritime boats, aircraft constructions,and pipelines thermal insulation. Metal-reinforced polymer foams has demonstrated high electromagnetic interference shielding at quite low densities [62]. In order to make lightweight components for airplanes like door panels and rudders, natural fiber reinforced composites have become quite popular. Due to their exceptional thermal, mechanical, and biodegradable characteristics, nanoparticles based composites (NBCs) made from polymeric matrix reinforced with natural fibers are widely used in a variety of industries. As illustrated in Fig. 3, NBCs are widely used in the aerospace, automotive, packaging, military, constructing, naval,sporting, medicine, and building block industries to achieve outstanding capabilities [63].

Fig. 3. Applications of biofiber and nanoparticles reinforced composites [63].

Balakrishnan and coworkers examined a variety of natural fiber composites. These natural fiber-reinforced composites' increase in efficiency, cost-effectiveness, and reduced weight are the main driving forces for their use[64].The ECOSHELL idea was created by the European Green Cars Initiative (EGVI) with the intention of creating lightweight vehicles(weight reduction)made of bio-resins and bio-fibers to lessen the environmental dangers associated with decomposition and recyclability. Vegetable fibers may be used to create car body parts, which will have a significant positive influence on the environment and offer several benefits in terms of cost,vehicle weight, CO2emissions, and other factors [65,66]. Two lighter elevators models with walls made of damped aluminum laminate (DAL) and CFRP were the subject of Bae and team's structural project design. The old steel walls' elastic modulus served as the foundation for the building structures of the current elevator walls.The proposed CFRP model is intended to have 20%of the existing wall structure bending stiffness given that the strength in the fibre orientation is roughly nine times greater compared to the current building steel materials, while the aluminum elevator model is intended to have bending stiffness equivalent those of the current steel elevator [67]. Yildiz et al. performed research to develop lighter shipping container alternatives as part of an effort to give a global solution for lowering the energy demand of transporting.The project showcases a brand-new 40-foot container design constructed of laminated carbon fiber. A 40-foot conventional containers has a curb weight that is around 80% less. According to the assessments, the composite container was a very viable option for cutting greenhouse gas (GHG) emissions,conserving fuel, and thus lowering operational transportation expenses [68]. Gypsum-oriented self-compacting lighter composite was developed by Yu and Brouwers.In comparison to conventional gypsum plasterboard, the study shows that this novel composite has much better thermal and mechanical qualities. When compared to conventional gypsum plasterboard having the same density, the novel composite's thermo-physical characteristics show a 30%enhancement[69].The recent trend in nanotechnology is to use the characteristics of nanostructured materials to create enhanced polymer nanocomposites for strong, light, and longlasting radiation-resistant materials. As an alternative, polymeroriented shielding materials are thin, conformable, and may be made using non-toxic, high-Z filler compounds that effectively block X-rays [70]. For a commercial front bumper system comprised of woven carbon fiber composites,Liu et al.developed a structural design and optimization approach.The actual car testing shows that the redesigned bumper system satisfies all toughness and crash safety standards while reducing weight by 31.5%. The suggested design method's efficacy and efficiency are demonstrated through real-world vehicle verification [71].

3.1. Lightweight materials in military sector

In recent times,the quest for searching lightweight materials is high owing to their inherent advantages like the low weight of products, reduced costs, and reduction in overall energy consumption. The twentieth century witnessed a large quantum of innovation in ballistic technology[72,73].Military organizations of every country tried to enhance the vehicle and troops' maneuverability but not at the cost of the protective equipment's performance against high impact.Some of the typical lightweight metals and alloys include aluminium, magnesium, titanium, and nickel During the earlier times, when steel was the only predominant metal, these lightweight materials were deemed inappropriate for usage in military and defence applications owing to their monolithic nature. The emergence of research on alloys and metal coating techniques in the new era has shattered the barrier of using the aforementioned metals as alternatives for steel. All these lightweight metal alloys are characterized by high specific strength and better corrosion resistance [74]. Fig. 4 depicts the comparison of yield strength and density of steel with other engineered lightweight metals and materials [74].

Fig. 4. Comparison of Yield strength and density of steel with other lightweight materials [74].

Metal matrix composites (MMCs) are far better than alloys in terms of performance in the defence sector. Commonly, carbonbased nanomaterials like carbon nanotubes (CNT) and graphene are used as reinforcements in metal matrices which end up in MMCs which could be used at higher operating temperatures and possess high wear resistance for lightweight and vehicle armor applications [75,76]. An aluminium alloy matrix reinforced with CNT has been used to build a ballistic body and vehicle armor named Kryron Terminator. Another Light improved ballistic armor(LIBA) has been developed by Israel which is the best example for metal matrix nanocomposites and these armors withstand up to 20 impacts from 14.5 mm caliber projectiles [75,76].

Cast aluminium alloys find their applications in engine blocks,piston rings, and brake calipers which reduces the overall vehicle weight[77,78].Researchers in the Military and defence sector have started to use magnesium in helicopter gearboxes and generator housings, as a means of providing lightweight resistance to extreme temperatures. Though titanium and nickel are of higher density relatively, their high corrosion and crash resistance paves the way to use them in various military and naval-related components [79,80].

Ceramic materials with high-performance capability have become inevitable in modern defence and weapon systems. Such ceramics are mostly used in aircrafts, military ground vehicles,radar communication systems, soldier armor systems and missile guidance systems. Lightweight ceramic materials are specifically used in military attack helicopters. During the Vietnam conflict,boron carbide ceramics were used as high-strength and lightweight materials in body armor and armor paneling,and from there on the era of advanced ceramics began[81].Ceramics are most commonly used in armor manufacturing owing to their superior projectile resistance, exemplary ballistic performance, lightweight and high compressive strength, and hardness. Ceramics were also used in armor seating which rendered an additional level of protection for the soldiers and pilots during war times, but due to the technological emergence of advanced ceramic materials, these systems were considered to be fundamental armor designs [82,83]. Fig. 5 represents an armored ground vehicle with many ceramic armor shots attached [83].

Fig. 5. Military ground vehicle with ceramic armor shots [83].

Modern applications like ballistic-resistant exterior tilting for drones, planes, and helicopters demand state-of-the-art and advanced ceramic materials such as alumina, silicon carbide, and silicon nitride. Such modern ceramics have replaced metal-based armor plating in tanks and are more prominently used in body armor replacements and in the construction of All protected vehicles (APV). Sizable research efforts have also been made in using advanced ceramic matrix composites in lightweight projectiles and missile radomes which were characterized by higher precision in targeting abilities when compared with the earlier missiles [84].Similarly, telecommunication applications are another area that demands high device precision which contributes greatly towards the emergence of rapid aerospace and military telecommunication systems. Under such situations, researchers focused on using ultralow-temperature ceramics(ULTCC), low-temperature co-fired ceramics (LTCC), ceramic inks/paints and high-temperature cofired ceramics (HTCC) for telecommunications systems in military applications such as active electronically steered antenna (AESA)containing hundreds or thousands of transmitter or receiver modules which operates at various systems and platforms [85].

Polymeric materials are considered to be the most successful materials in defense and military applications to produce durable and lightweight components. Many metallic parts in military applications have been replaced with polymeric materials owing to the numerous advantages including low density, non-conductive nature, durability, and waterproof nature offered by the polymers. Polymers are considered to be safe and strong materials for many military application parts starting from compass cases to precision weapons components. Fig. 6 depicts a typical Russian battle suit made of PNCs which is the latest one. The "Star Wars"high-tech armor has been built on the inherent properties of graphene nanofiller-reinforced polymer composites and possesses the lightweight, high ballistic capacity, better relative strength, high hydrophobic ability, and excellent electrical and thermal conductivity.The armor also contains a helmet with a tinted night vision,gloves,firearm,radio cable,an exoskeleton layer,and padded khaki[86,87].Polymers offer freedom to manufacture in large quantities with less time and cost as their assembling is relatively easier.Hydraulic fittings in military tanks and shields for sea-to-air missiles were manufactured through injection moulding techniques which offer high durability, and resistance to heat, chemicals, and impact. Polymers do not require secondary processing operations like metals and ceramics which cut down production costs also.Some of the commonly used polymers for military and defense components include Polypropylene,polyether ether ketone,acetal,polycarbonate, Thermoplastic elastomer, polyamide 6, polyester,polyphenylene sulphide, and polyvinyl chloride[88,89].

Fig. 6. High-tech armor by the Russian government [95].

Polymer nanocomposites (PNCs) witnessed a greater use in defense and military-related applications in various sectors like smart military structures and textiles,the aerodynamics of military vehicles, weapons, sensors, power generation, medicines for the military,and so on.When nanomaterials are dispersed in polymers,they can be used for making soft body armor for protection.When nanomaterials are hybridized with other reinforcements such as ceramic particles and fibers in polymer matrices, they render interesting material that can be used for military platforms including transparent armor applications [90,91]. In the case of military ships, the use of PNCs is more prevalent. For instance, a ship production industry named Ingall's shipbuilding makes the roofs and deckhouses of its ships by using carbon fiber-reinforced vinyl ester polymer with phenolic fiberglass as hybrid reinforcement. Some other components in the ships like masts, antennas,and transparent radars are also manufactured by using polymer matrix composites lately.

Likewise, the parts of Lockheed Martin F-35 Lightning fighter aircraft like horizontal,and vertical stabilizers,wings,and fuselage were produced out of carbon fiber-reinforced polymer composites and the parts are characterized with additional durability and toughness [92,93]. Upon overcoming existing challenges like durability and high strength equivalent to steel,lightweight metal alloys, ceramic matrix composites, polymer, and fiber-reinforced composites are expected to engulf the entire military industry with their stand-alone properties [94,95].

3.2. Lightweight materials in airforce

The lightweight component has been widely researched and used in numerous sectors,particularly in air force applications,and is related to the green aviation prospect.Lightweight aircraft design might increase flying performance by improving acceleration,strength,and rigidity,and provides better safety compliance (SSDI 1044—5803(95)00066—8).In earlier days,aluminium plays a major role in designing aircraft components for about 70% (from the fuselage to main engine parts) because of its lightweight, high strength, stiffness, inexpensive, widely available, and resistance to corrosion, weather, and stress. Later with the demand for weight reduction and an increase in fuel efficiency, other lightweight metals (titanium, steel, beryllium, lithium, and new aluminium alloys),hybrid materials and composites(carbon fiber,aramid fiber,boron fiber, glass fiber, polymeric and epoxy resins) played a significant role in improving the aircraft design. Due to its superior tensile strength and capacity to survive severe climatic conditions,nickel and high-temperature alloys are widely utilized in the air force specifically in rocket and aircraft structures, external panels,and mounting components[114—116].

Nowadays, over 50% of this fuel-efficient aircraft is built from carbon-reinforced plastic composites following safety standards and regulations [117,118]. Many companies like Airbus, Boeing,Bombardier, BAE Systems, Raytheon, GE Aviation, and Lockheed Martin have also leaned into using thermoplastics and composites in their aircraft and defence related systems. Additive manufacturing(3D printing)brings several benefits to the aviation and propulsion industries by being suitable for different material groups, allowing the production of complicated geometries without tools and dies, and enabling lightweight and functionally better designs.Additive manufacturing is especially helpful for lowvolume production, which is frequent in the aviation sector. It reduces buy-to-fly ratios,eliminates valuable alloy waste,and lowers machining times [119].

Despite carbon fiber and other high-performance composites are getting more attention,aluminium alloys are still being used for most of the structural weight in aircraft. Many lightweight aerospace structural applications, such as the fuselage skin, upper and lower wing skins, and wing stringers, favoring advanced aluminium alloys due to their relatively high specific strength and stiffness, good ductility and corrosion resistance, low price, and excellent manufacturability and reliability. Increases in heattreatment efficiency have allowed the use of high-strength aluminium alloys that can compete with modern composites in a variety of aircraft applications. High density, corrosion and embrittlement vulnerabilities limit the usage of high-strength steels in aircraft systems and components [120]. Gearing, bearings, and undercarriage employ high-strength steel in aircraft applications.Titanium alloys excel in many areas where other metals fail to achieve, including high specific strength, heat resistance,cryogenic embrittlement resistance, and low thermal expansion.Titanium alloys offer a range of benefits over steel and aluminium alloys, making them a good choice for aircraft and engine applications. However, their high cost (often about eight times more than commercial aluminium alloys) and inadequate manufacturability limit their wide applicability. Therefore, titanium alloys are employed in situations that call for both high strength and corrosion resistance. Magnesium alloys also has special characteristics like light weight, high specific strength, shock resistance, strong thermo-conductivity and electromagnetic shielding which finds its usage in aviation and spaceflight applications.These materials can be easily recycled and termed as green material in the 21st century[121]. Rhenium has become a popular metal for aerospace equipment. Rhenium is a rare, temperature-resistant metal used in aerospace and aviation equipment to manufacture precision instrument, engine components and fuel systems. On rockets, missiles, and spacecraft, rhenium coatings reduce arc discharge,mitigate heat impacts and prevent surface wear.In addition,Al—Li alloy and fiber-metal laminates (ARALL; aramid reinforced aluminium laminates) were found to be attractive for aircraft materials because of their fatigue-dominated structural parts i.e.,lower wing skin and pressurized fuselage cabin[122].Beryllium is a military-preferred raw and additive material for targeting systems,electronics, and satellite reflectors. Beryllium-containing components are being replaced by coated metal powder components.The lightweight new material finds its usage in airplane, missile and space applications[123].

Lightweight ceramics, SiC/aramid and Dyneema polyethylene fiber were used as a ballistic protection panel [124]. ALON, transparent ceramic material finds its usage at high temperature and pressure applications. This also provides superior ballistic protection for both air and ground vehicles over traditional glass laminates [125]. Bundles of ceramic nanofibers are lightweight and extremely strong which are used for manufacturing jet engine parts and armour to protect military personnel [126]. 3D-printed preceramic polymers or polymer-derived ceramics(PDCs)can survive extreme temperatures and hypersonic velocities which can be used in thermal protection systems, space vehicles, and lightweight mechanical meta-structures in aviation [127]. Even with the superior high-temperature properties of ceramic composites over conventional metal components, reducing their cost is key to enabling their widespread use in demanding air force applications[128,129].

Because of the development of high-performance and lightweight composite materials, the proportion of composites used in aircraft structures has grown-up rapidly. Polymer and composite materials meet the challenge of reducing aircraft weight by ten times lighter than metal and competing in several fighter jet applications. This sharply lowers lifetime fuel costs, reduces emissions and extends flight range with the additional benefit of radar absorbent[130].Carbon fiber reinforced polymer(CFRP)is the most widely utilized aircraft structural material after aluminium alloys for wing boxes, empennage, fuselage, and control surfaces such as rudders, elevators, and ailerons [131—133]. GFRP is also utilized in radomes and fairings. Impact-resistant aramid fiber polymers are also employed.Fiber metal laminates,notably glass fiber reinforced aluminium (GLARE), have applications in aircraft design owing to their lower density, superior strength, stiffness, and fatigue resistance compared with conventional metals. The fuselage skin and empennage mainly employ GLARE [134]. Shape memory polymer composites(SMPC)are used in morphing-aircraft wing skins,solar arrays, and satellite reflector antennas. SMPCs have lower density,greater shape deformability and recoverability, better processing,and cheaper cost than shape memory alloys. In aviation, carbon/epoxy, carbon/PEEK, carbon/phenolic, kevlar/epoxy, glass fiber,glass/epoxy, glass/phenolic and boron/epoxy were especially used in primary structural, skin material, armour plating, interior fittings, furnishings and composite repair patches [135]. In all those early days, woven cotton or silk fabric skins have always been impregnated in nitrocellulose ‘dope’ to protect against the wind,and laminated wood was reinforced by fabric bonded with adhesives for aircraft wings [136]. Palm kernel shell ash (PKSA) reinforced Al—Mg—Si composites were also used in air force applications particularly in sounding rocket combustion chamber of fighter jets [137]. Studies on fiber/metal composite technology for future aircraft components indicated an increasing demand for reduced operating and maintenance costs [138]. Aerospace-grade polymerssuchaspolyetheretherketone(PEEK),polyphenylsulfone (PPSU), polyamide (PA) and polyetherimide (PEI)provide a reliable and cost-effective way to reduce weight which also have superior corrosion and fatigue resistance, tensile strength, flame retardancy and durability. These polymers find its applications include flight control, fuel systems, aircraft interiors,and engine and aerodynamic-related components. Polyimide (PI)films are widely used due to thermal and static control for their insulating properties,mechanical durability,lightweight,and their resistance to radiation. Its potential applications are light/solar reflective films as well as high-temperature printable films [139](see Table 1).

Table 1Lightweight materials used for military applications.

Design and development of lightweight porous polymer composites (F-NR/CNTs composite foams) exhibiting excellent EMI shielding properties with huge potential for use in the electronics and aerospace industries [140]. Mechanical properties of boron nitride nanotubes(BNNT)and BNNT-polymer interfaces hold great potential for innovative lightweight and high-strength multifunctional aerospace materials (AFRL-AFOSR-VA-TR-2019-0206). Newgeneration microspheres improve flame retardation in composites, minimize aircraft radar detectability, safeguard sensitive electronic components and reduce jet engine vibration. Nanocomposites (e.g., layered silicate, carbon nanotubes, and graphite flakes)allow for redundancy removal and weight reduction,which may improve aircraft component qualities,notably for lightweight applications. Emphasis is placed on microstructural characterization and the relationship between the microstructure and mechanical properties of specific materials systems.However,surface protective coatings (polyurethane, PU and aliphatic diisocyanates)on the aircraft improve performance, increase durability and reduce the frequency of aircraft maintenance [141]. Although lightweight and high-strength materials played a consistent role in constructing fuel-efficient and high-performance aircraft, biocomposites (biomass, plants, crops, micro-organisms, animals,minerals, and bio-wastes) gave new insights to improve the environmental characteristics in the future which is also considered as lightweight, flexible, cost-effective and recyclable. The main application includes cabin, cargo, auxiliary materials, and primary and secondary structures [142]. However, light-weighting reduces energy utilization and improves its performance.This idea is widely used in aircraft components and system design. Advanced lightweight materials and the numerical structural optimization allow for designing the lightweight structure, particularly for aircraft applications.Table 2 shows the various lightweight materials used in the airforce.

Table 2Lightweight materials used in airforce.

3.3. Lightweight materials in Navy

Human civilization needs new materials constantly, thus improving each material to the maximum possible standard is the finest research requirement. To meet the needs of modern technology and develop a product at the most economical price, novel and superior materials are essential.So to keep acceptable levels of safety and functionality,it is necessary to create new materials and enhance the qualities of current ones in tandem with technological advances. Environment-related impacts are experienced by maritime structural components such as boats, submersibles, offshore constructions, and more. Therefore, high-resistance materials that need little to no upkeep for long periods are often considered.Vessels must be lighter and have excellent resistance to corrosion to fulfill technical specifications and operate well. In many cases,the selection of the material for various purposes is comprised of robustness.The hunt for new and better materials is ongoing along with contemporary technical demands for energy-efficient, durable, lightweight, and cost-effective equipment and machines.Finding a single material with the required property profile for engineering applications is almost challenging [148]. Thus, two or more materials, namely composites or alloys, must be blended to achieve the numerous beneficial features of different materials,which may be employed in most engineering applications, in contrast, monolithic materials are used in just a handful. Composites are typically made by dispersing one or more discontinuous phases, called reinforcement materials or reinforcing materials,within a continuous phase, or matrix [149,150]. Based on the matrix,composites are classified as polymer,ceramic and metal matrix[151]. Polymer matrices are sub-divided based on reinforcement:natural fibers and synthetic fibers. The composites have some outstanding properties like reduced fuel consumption, good strength-to-weight and stiffness-to-weight ratios, flatness for stealth requirements, high durability, increased range, increased dimensional stability, manufacturing and maintenance costs,design flexibility,lower electrical and magnetic signatures,reduced wear, increased speed, low moisture absorption, corrosion, and impermeability that encourage their use in the marine industry[152—154].Consequently,marine sectors such as the boat and ship construction sector, renewable energy sector, offshore structural sector, and repairing sector use them extensively for fittings and internal equipment such as pipes,boats,small ships,naval vessels,propellers and propulsion shafts, and warship equipment such as destroyers, frigates, and corvettes [155]. The application of composites in ships and submarines is given in Fig. 7 [156—159].

Fig. 7. Application of composites in ships and submarines [156].

Following Second World War, the United States Navy began using composites for the first time to build small people boats.The fast development of composite usage in other kinds of US Navy vessels between the mid-1940s and the 1960s may be traced backto the success of such boats, which revealed to be rigid, robust,durable, and simple to repair. Over 3000 composite vessels,including personnel boats, river patrol boats, landing craft, and many reconnaissance craft,were in service throughout the Vietnam War. Small ship deckhouses, certain communication ship masts,destroyer pipes, and submarine fair waters and casings were all constructed using composites by the United States Navy.The 1950s saw the introduction of composite ships and submarine construction by other navies.Composites have replaced steel in the construction of bow sonar domes for submarines;for surface ships'communication and surveillance antennas in order to improve acoustic transparency. In the 1970s, composites were used to construct mine hunting vessels for the Norwegian Navy, Royal Navy, and. Royal Swedish Navy [160].

Polymer Composites (Synthetic and Natural fibers based): As said in previous sections, water absorption and shock impact resistance are important for composites to perform better. Generally,there are three main mechanisms by which a material absorbs moisture: bulk diffusion of water through the polymer matrix phase and, in some cases (e.g., organic and plant fibers), through the fibers themselves; the capillary flow of water along the fibermatrix interface; and percolating flow of water through open defects like delamination and cracks.The mechanical performance is widely influenced by water absorption [161—167]. The water absorption of composite materials depends on several parameters,including its kind and volume. One of the relatively few research investigating the effect of the fiber sizing agent on the durability of composites while submerged in water was conducted. Tsenoglou et al. evaluated the wettability of polyester matrix laminates with E-glass fibers that had been de-sized, silane-sized, or elastomer(PDMS)-sized. It was discovered that a higher diffusion coefficient and a higher maximum moisture content at saturation occurred when the connection between the fibers and matrix was less strong[168].Also,certain studies showed that addition of nanoparticles to the fiber reinforced composites or coating the nanoparticles will lead to better strength and fire resistance properties[169].The cost,weight, and structural performance of large patrol boats built of steel, aluminum, and sandwich were compared in several tests[156].It was determined that a patrol boat constructed from a GRP sandwich composite material weighed 10%less than an aluminum boat and 36% less than a steel boat of the same size. Furthermore,hybrid composites,such as hybrid glass-carbon reinforced polymer composite (GCG2C)s, best aid to keep for a long time, the mechanical qualities required by materials for efficient performance in the marine sector. The flexural strength of hybrid (GCG2C) s 462 MPa, and it has the lowest water absorption tendency.Aluminum 6061 may be replaced with a hybridized flax and carbon fiber composite because of its superior vibration-dampening qualities (about 141% better), tensile strength (about 252% better),and low weight(about 49%).Hybrid composite constructions with jute and carbon fiber reinforcements provide better vibrationdampening qualities and lower environmental impact. Natural fibers can be used in some applications but it only as a hybrid with synthetic fiber as said earlier, else water absorption problems will increase. The performance of polymeric composites in fire resistance is also an important aspect that was reviewed by Tran et al.and spotlighted many things to enhance fire resistance in glass fiber-reinforced polymeric composites [170].

Metal matrix composites:There is widespread use of aluminum matrix composites (aluminum alloys and its composites) in the maritime industry. Use of these materials has enabled improvements in the speed, size, and fuel efficiency of ships and boats generally. Using aluminum-based materials usually results in increased maneuverability and access to low drawn ports. Composite materials based on aluminum are up to the task of protecting, building, and preventing fires aboard fast-moving ships.Aluminum is a good choice for carrying heavy cargo when boating at fast speeds [171]. Despite of certain advantages some disadvantages like corrosive attack when used in the outer surface of ships direct contact with water is possible, thus it is necessary to choose the applications.One such application was lightweight ship balcony overhang,that utilized an aluminum honeycomb sandwich structure and Al/Fe structural transition joints obtained by means of the explosion welding technique. Strength, stiffness, and failure modes were determined by experimental research. The results showed that Explosion-welded aluminum-iron (Al/Fe) structural transition joints can be utilized to unite the ship's steel skeleton to an aluminum honeycomb balcony [172,173].

Ceramic matrix composite: Ceramic composites gained more advantage in marine industry due to its strength as well as corrosion resistance. Wang et al. investigated the corrosive and tribological behaviors of Si3N4-hBN composite ceramics in seawater. Corrosion and wear in composite ceramics exposed to seawater have been briefly discussed. Corrosion of Si3N4-20 wt%hBN and Si3N4-30 wt%hBN in saltwater was inhibited after 12 days due to the formation of H3BO3,SiO2,and Si(OH)4compounds on the corrosion surface. The formation of lubricious tribo-chemical coatings comprised of TiO2, SiO2, and Al(OH)3on the worn surfaces during sliding in saltwater resulted in a friction coefficient as low as 0.403 for a Si3N4-30 wt% hBN/Ti6Al4V sliding pair. The friction coefficient of the sliding pair also increased due to the corrosion of Si3N4-30 wt% hBN proving that it could be used in marine engineering applications[174].Table 3 gives the information about various lightweight materials used in different parts of marine applications. From this information, it is clear that lightweight materials especially composite materials with fiber reinforced have a good scope in the marine applications subjected to their performance in water and impact resistance with economical aspect.The hull applications can be done with hybrid fiber polymer matrix composites for better performance. The focus on fire resistance should be made while choosing the composite,which is one of the important problems in marine industry.The reliable tools to access the failures caused by explosions, impacts, collisions, and fire to made.

Table 3Various lightweight materials used in different parts of marine applications.

3.4. Lightweight materials in space

Several studies reported that aluminum and titanium alloys are widely used lightweight metal matrix composites for aerospace applications. Recently, magnetic aluminum metal matrix composites became more popular due to their magnetic and lightweight properties.These magnetic properties were induced by reinforcing the aluminum matrix with Fe3O4particles [179]. Some of the common applications of aluminum MMC in aerospace are fuselage skin, upper and lower wing skins, wing stringers, etc. Some of the commonly used aluminum alloys in aerospace industries are Al—Cu, Al—Zn, and Al—Li [180]. Additionally, titanium alloys (TA)were extensively used in aerospace because of their high specific strength.The TA has excellent properties like corrosion resistance,heat resistance, cryogenic embrittlement resistance, fracture toughness, and low thermal expansion. It is an ideal replacement for steel and aluminum alloys for building airframes and engines in aerospace industries [181].

Recently fiber reinforced composites (FRC) and fiber metal laminates have gained more interest in aerospace applications.Among various FRC carbon fiber reinforced composites are widely used to develop structural components like wing boxes, empennage, and control surfaces [182].Glass fiber reinforced composites are used in semi-structural and aramid fiber reinforced composites are used where there is a requirement of high impact resistance[183].On the other hand,the fiber metal laminates,and hybrid fiber composites are revolutionizing the aerospace industries due to their reduced density,high stiffness,and fatigue resistance,hence it is used as fuselage skin and empennage in Airbus A380 [184].Recently there are other advancements in polymer matrix composites like shape memory polymer composites (SMPC) and selfhealing polymer composites (SHPC). The SMPCs are used in the skin of morphing wing aircraft,solar arrays,and reflector antennae of satellites. These SMPCs can regain their original form when it gets distorted due to temperature, electric or magnetic field [185].The SHPCs are very fascinating due to their self-healing properties when subjected to structural damage. The self-healing process is achieved by encapsulating self-healing agents,which are activated utilizing heat or light.More prominent research is undergoing due to its application advantage because the spacecraft, satellites, and space rockets are subjected to continuous damage caused by spacedebris [186].

Ceramic matrix composites are the next-generation composites developed for the harsh aerospace environment. The rocket engines and parts that are designed for outer space must require materials that withstand higher thermal and thermomechanical conditions as temperatures exceed 2500?C. When the space shuttles have a reentry into the earth's atmosphere, they will be exposed to higher thermal shocks and the body temperature increases up to 1500?C. The ceramic matrix composites are the only materials that can satisfy such conditions of higher mechanical shocks,mechanical stress,and temperatures beyond 1700?C[187].At these higher thermal loads, the silicon carbide would lose its properties due to the recrystallization and hence silicon carbide matrix was reinforced with carbon fibers for these applications[188]. Some of the common ceramic materials used in CMCs are silicon carbide, carbon, silicon nitride, alumina, and mullite. These CMCs differ depending upon the reinforcement fiber and the matrix, some of which is a commercially available example: carbon/carbon, carbon/silicon carbide, silicon carbide/silicon carbide,alumina/alumina, and different combinations [189]. The CMCs are manufactured through different techniques like gas mixture deposition, pyrolysis, sintering, and electrophoretic process. The electrophoretic process is not utilized on an industrial scale and further research is required. These CMCs are highly adapted to space applications because it also exhibits high-temperature creep resistance properties. The lightweight materials and their applications in space are tabulated in Table 4.

Table 4Lightweight materials and their uses space applications.

4. Future trends and directions

The future of defence technology is now on the path of attaining heights, and mainly focus on these areas for the growth in this sector. Intelligence, surveillance, and reconnaissance (ISR) operations benefit significantly from using Artificial intelligence in defence since it improves computational military reasoning. A decrease in injuries among troops is a direct result of the use of computer vision in managing the security of military equipment and enhancing the capabilities of autonomous weapon systems.Next is that the defence is upgrading its defensive technology to be more innovative and effective against modern threats. Various technological advances, including hypersonic travel, directed energy weapons, and space militarization, are now under development. Armies must accomplish several goals, including force protection, situational awareness improvement, lowering the cognitive and physical burden on troops, and easing mobility in rugged terrain.The other task is defence forces that may fulfill their goals of controlling territory, protecting civilians, and solidifying their achievements using Robotics & Autonomous Systems technology. Connecting ships, aircraft, tanks, drones,troops, and bases is the beginning of how the Internet of Things may be used for defence.This improves cognition,field knowledge,awareness,and reaction time.Hackers may quickly gain access to military networks and cause disruption or even destruction of critical infrastructure or sensitive data. Cybersecurity, artificial intelligence, and automated threat detection and mitigation constitute prescriptive security technology, which is used to prevent cyberattacks before they compromise defensive cyberwarfare. Next, for purposes like flying or warfare training, immersive technologies make it simple to create reproducible and adaptable experiences.New companies are using virtual reality (VR) to create simulated learning environments(STE).The Defence equipment's speed,capacity,and fuel consumption may all be significantly enhanced by reducing its weight. Unlike conventional manufacturing methods, 3D printing allows for manufacturing components and parts using substantiallyfewer raw materials.Information and the capacity to conclude will be more important in future conflicts.Then Armed forces with the ability to evaluate the most critical data and distribute the results will have a significant tactical advantage swiftly and effectively.Big data analytics helps by revealing patterns in previously unconnected datasets. To have accurate and timely intelligence for any military action, the rapid data transfer of 5G facilitates immediate military decision-making. This system claims to provide both hyper-converged connections and secure data networks. Finally,Blockchain allows for safe data transfer to all parties involved. For this reason, new defence industry companies are developing blockchain-based solutions to safeguard sensitive military data and combat cyber-attacks.

New developments in directed energy weapons, such as lightweight pods, make it possible to disable drones and destroy vital electronic components. Military infrastructure adopts flexible lightweight materials to accommodate global deployments. Some typical applications that might benefit from the use of lightweight materials include water tanks, cisterns, fuel tanks, and mast systems. The greatest strengths include expert knowledge of lightweight components and the capability to perform repair services in very sensitive regions.Carbon fiber reinforced plastic(CFRP)offers better durability than aluminium and steel, making it an ideal material for novel and resilient solutions in defence technology.Addition to CFRP, glass fiber reinforced plastic (GRP) is used in many applications. In defence sectors, GRP is used for many new aspects,such as bridges,containers,pontoon bridges,storage tanks,water tanks, observation platforms, and even transmitter masts.Because of their lightweight, these systems may be quickly assembled and disassembled. Compared to metal systems, GRP's corrosion resistance,acoustic insulation,and thermal and chemical resistance provide significant benefits in the applications. Selfhealing carbon fiber reinforcing polymers have recently been developed, which are very useful in military aircraft, unmanned aerial vehicles(UAVs),naval ships and weapons since they mitigate the effects of damage to composite materials.Lightweight materials assist in enhancing aircraft and UAV's range and payloads with decreasing fuel consumption. Defence sector is looking for novel high-strength, extremely lightweight materials that can be incorporated with major structures and protect combat vehicles against future weaponry. One of the driving factors by defence agencies is to upgrade the cutting-edge personal protection equipment towards the creation of lightweight ballistic material.The advantages of comfort and greater protection for ground troops have prompted several nations to work on developing better alternatives, such as ballistic inserts, lightweight under suits, combat helmets, antimine boots, and flame-resistant outfits. Apart from all the above points, lightweight materials are too costly for broad application,therefore cost is a fundamental obstacle the defence sector must overcome.So,efforts should be made to lower the production costs of lightweight material equipments via research and development.Lightweight materials need further study to enhance their recycling qualities and for better integration into the end-products.

The defence sector depends only on the below-given scenarios for becoming an outstanding performer. Firstly the cyber battle fields, where the software and algorithms will determine military missions’ success more than platforms, like the introduction of Artificial Intelligence and network-based systems. The second is space as an operational military domain where all the global powers will work on them. This scenario will call for the establishment of new concepts of operations, regulatory frameworks,and international agreements. The third one is Extended and augmented reality everywhere, where the merger of braincomputer interfaces and augmented/virtual reality in the battlefield will lead to a pervasive robotization of the battlefield and the rise of (mis-)information-driven capabilities. Fourth, the Dominance of unmanned and autonomous platforms on the battlefield.Fifth, Hybrid human-machine teaming, where a seamless integration and cooperation between humans and machines.The sixth one will be the development of more sophisticated biological weapons.This will also cause new biohazards emerging from synthetic biology and gene edition technologies, it may be used as a bioweapon or biothreat. The seventh one is the Enhanced cognitive abilities of soldiers (human enhancement), where the critical impact advances in biotechnology, synthetic biology, gene edition technology, and brain-computer interfaces will be coupled with soldiers and computers. Then the Real-time mapping of dynamic environments where the quantum sensing, and quantum navigation will have made leaps ahead in the development resulting in real-time mapping capabilities of ever-changing environments.The critical scenario is the use of misinformation which will become a weapon targeting all information systems and enabling a scale of influencing operations challenging the capability of nation-states to counter or control. Then, environmental problems, energy supply,and climate change will become geostrategic drivers for conflicts.Energy supply will remain a significant factor of strategic dominance. From a defence point of view, despite the enormous investments made in new energy generation and storage systems,energy will remain a critical challenge for most defence systems and operations.

In some military aircraft applications, carbon fiber epoxy composites are used. Currently, typhoon aircraft use carbon fiber composites for up to 30 of their structural weight and 70%of their surface area while F-18 aircraft use 10%of the structural weight and 50%of the surface area.It is more likely to increase in the future and the same trend will be followed for all types of military aircraft.The strength of lightweight materials plays a vital role in defence applications,but destructive testing consumes both time and money.The use of simulation software for testing lightweight materials reduces the time and cost of designing and fabricating a new material considerably. In recent days, expensive prototyping using lightweight materials is reduced and the use of simulation and optimization software like SolidWorks, ANSYS, Hypermesh has increased. Integration of nanotechnology with lightweight materials for developing military-grade materials has a wide future scope for developing sensor materials, EMI shielding materials,corrosion detection, radiation, temperature, gases, and strain resistance, and so on. Nanotechnology has a significant contribution towards intelligent systems, signal processing, laser weapon systems, and autonomous defence systems for mortars, artilleries,and missiles.

The development of ballistic shields and armor is another area of scope covered under the purview of lightweight materials.Lightweight ceramics can be a wise choice for building a ballistic shield for armor applications. They cope with the need for the fabrication of smaller and more complex shapes which can be readily used for military and defence applications. Ballistic shields for machine gunners are also one of the potential futuristic applications of lightweight ceramics since a machine gun is an effective weapon to lay down a large fire field when compared with other gunners.In the case of military body armor,complex shapes can be fabricated with the aid of reinforced ceramics or hybrid ceramic composites and this material can be used for additional protection.Utilization of natural fiber-reinforced polymer composites in ballistic applications has been carried out earlier and the future of such studies looks bright.Natural fiber-reinforced composites find their suitability in the ballistic field and their application range widens to the automobile, military, aircraft, and other defence applications.

5. Conclusions

This review will aid in the future development of lightweight materials based on composites made of polymers, metals, and ceramics for defence systems.Additionally,this review may pave the way for the research and development of additional lightweight materials for defence applications. So leading countries are interested in cutting-edge technologies that use lightweight materials in defence applications. The defence R&D is increasing its ability to respond to new threats by developing more complex and advanced lightweight defence equipment made of lightweight materials.This review concluded that lightweight materials can be a futuristic material that can develop the capability of defence equipments to enhance their strength, resistance towards high temperature,abrasion, wear and tear, and adverse environmental exposure.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.