Defence materials evolve in 21st Century

Materials World magazine
,
30 May 2019

Explore the new challenges facing the interaction of defence and materials.

Raw materials will continue to play an essential role in defence and aerospace for the foreseeable future. While advances in technology have greatly improved the opportunities with which materials can be applied to improve the speed, weight and strength of end-products, the value each element brings is by no means devalued.

The investment involved in defence and the longevity of machines can slow down the rate of change in developments. But opportunities abound with composites and computational methods have helped drive technology forward. Fortunately, the fields of aerospace and defence remain supportive and robust to materials innovation and consideration.

Iron to bronze

Personal protection and weapons, such as swords, primitive firearms and cannon were the primary need for defence materials before motorised military vehicles arrived in the 19th Century. Early forms of body armour were complex systems of different materials. For example, Chinese lamellar armour consisted of small plates of bronze or iron held together using leather laces. This developed through scale armour and the metal cuirass into the plate armour, recognisable as the suit worn by knights in the Middle Ages. Chainmail, made of stainless steel, is still manufactured and sold today for protection against stabbing, and in gloves used in the butchering industry.

Wootz, more commonly called Damascus steel, was used in swords of the 13th Century. Its microstructure continues to fascinate metallurgists today, especially since discovering that it contains carbon nanotubes, first recognised in 1991. Both bronze and iron were used for cannons, although iron cannons were usually made of relatively poor quality wrought iron with an unfortunate tendency to burst when fired. Iron easily rusts, which was a problem as many cannons were used on ships. Bronze, although 20% heavier than iron, resulted in a lighter gun because its superior properties allowed for a thinner barrel. Bronze was for many years the alloy of choice, but, as always, there is a balance between performance and price, and in the 16th Century the cost of bronze was three times that of iron, which led to 17th Century investments in manufacturing technology to improve the quality of iron. Both iron and bronze cannons were used on the UK warship Mary Rose but the Vasa, a Swedish warship famed for sinking on its maiden voyage in 1628, used bronze cannons. A reproduction of a Vasa cannon was cast and test fired successfully in 2014. Splinter patterns and behind armour effects were recorded.

Adapting alloys

Today we expect our military equipment to be built of the most advanced materials available. However, we are often unaware that it can take 25 years for a new one to progress from the laboratory to the battlefield. Those used in today’s ships, airplanes and vehicles may well have originated in a laboratory at the end of the last century.

Carbon-fibre composites are essential. Our aircraft would not get off the ground without them. For example, around 35% of the weight of the F-35 multirole aircraft (see pages 41-43) consists of carbon-fibre composite, with epoxy matrix, although bismaleimide is used for areas with higher operating temperatures.

Widely used aerospace alloys include titanium-aluminium-vanadium (Ti-Al-V) for structural components, titanium-aluminium-tin (Ti-Al-Sn) for higher temperatures, and ultrahigh strength 300M steel for landing gear components. A wide range of alloys are needed for jet engines, from the single-crystal nickel alloys used in the hot section of the TP400 engine in the Airbus A400M transport aircraft
to the titanium fan blades in the Trent 1000 and high temperature steel for the shafts. While high-strength and high-temperature materials are needed to power an aircraft, advanced materials such
as carbon-carbon composites used in aircraft brakes are needed to bring them to a halt.

Camouflage and signature management is essential for survival. The objective is not to be invisible, but to be difficult to detect against any background, and if detected, to be difficult to identify. Also, modern weapons are expensive – Meteor missiles cost around US$2mln each, an Excalibur 155mm round US$70,000 and even an anti-tank warhead for the widely fielded Carl-Gustav weapon US$3,000, so a wasted shot can be very costly. Recent conflicts in Afghanistan, Syria and countries in Africa have not required advanced camouflage, but increasing tension in Europe has resulted in growing interest in multispectral camouflage – visual and infrared – and mobile camouflage kits for ground vehicles.

Affordability is a key concept. As military equipment seems to get more and more expensive, there is always a difficult balance between performance and cost. Cost is linked to design and manufacturing, and an important development is additive manufacturing. While the technology is not new, there have been important advances during the past decade, and a number of components for aerospace, space and engine applications are now in production. The technology is still being explored, and new alloys and methods of testing and quality assurance being developed. The additive manufacturing of obsolete parts for which there is no available supplier is already in use, and several nations, including the UK, are studying mobile additive manufacturing in the field or onboard a ship. Design tools for topological optimisation are also priority.

Multimaterial production

Materials are agnostic. The materials used in defence systems are by and large those used in civil aeroplanes, spacecraft, energy systems, functional textiles and electronics. Defence needs no longer drive materials development because the market is small compared with civil ones. This alleviates some of the economic pressure on basic materials research for defence, but places greater demands on our ability to monitor and evaluate the potential offered by new technologies such as metamaterials and graphene.

By 2050 many of today’s aircraft, vehicles, weapons systems and ships will have reached the end of their useful and/or economic life. Some emerging materials will have matured and been certified, and others will have been discontinued, but many materials in use today will still be in use in 2050 although their methods of manufacture and assembly will be improved to suit more complex, multimaterial structures.

We will see titanium alloys used in greater volumes than today and in more varied applications, in components made by additive manufacturing optimised using topological design tools. New generations of ferrous and non-ferrous alloys will emerge using integrated computational materials engineering to optimise multiple properties simultaneously. This approach is already leading to steels with improved corrosion and fatigue resistance, and is being extended to wear-resistant materials.

Fuel is critical on the battlefield, and today we use electrical energy across many applications. New materials include those for generation and storage, such as structural batteries and ultracapacitors using graphene, and reduced rare-earth metal electric motors and generators. These will enable the trend for more electric ‘everything’ including vehicles (which initially will be hybrid concepts), aircraft and ships. For the latter, there are already prototype superconducting electric motors delivering 35MW.

Will we see electric weapons such as railguns? Existing prototypes use conventional materials including copper and aluminium, but could use superconducting materials. Energy might be stored in flywheels for rapid discharge in a weapon, or in flow batteries for storing electricity from solar cells to supply a base.

Relatively new on the list of materials for defence are semiconductor materials, of which the most prominent is gallium nitride (GaN). With its high power capability GaN is being introduced into advanced radar systems. Other electronic materials include electrochromic materials for adaptive camouflage, and the family of two-dimensional materials including graphene for broadband photodetectors.

Metamaterials have potential in many defence applications, from low-profile and compact antennae to radar absorbers. Cost-effective manufacturing is a critical parameter which needs to be studied.

Nanotechnology, widely hyped, should not be neglected.  Nanomaterials are maturing, slowly but surely. Defence applications include transparent armour such as aluminium oxide and spinels where a nanometer grain size is essential for strength, hardness, toughness and transparency. Other possible applications for nanomaterials include self-decontaminating coatings, including metal-oxide frameworks on textiles for battledress and camouflage, and corrosion-resistant coatings in a wide range of marine applications.

It is useful to reflect on those materials which will not be used in 2050, such as those affected by the Registration, Evaluation, Authorisation and restriction of Chemicals (REACH). Cadmium-plated fasteners, hard-chrome plated gun barrels and various organic chemicals such as pyrethroids used as insecticides will have been phased out and replaced with materials having reduced, ideally zero, environmental impact. An important challenge in this respect is to replace the hardeners used in epoxy resins, which are hazardous in
the workplace.

Suitable warfare

Defence applications are not immune from the broader challenges facing society today. These include the need to pack more functionality into a smaller volume and use less power and the growing awareness of husbanding resources. Raw materials are limited, and many are sourced from non-European suppliers. There is a clear risk that in a crisis supplies will be unavailable. To minimise this risk, the concept of critical materials has been formalised, and there are efforts to replace these with more readily available materials. Not immediately obvious is the essential capability to manufacture and maintain. Defence systems are replaced infrequently and lifetimes are often extended, leading to reduced production of new systems. Staff retirement results in the loss of personnel competence and low volume and sporadic production leads to manufacturers looking for business elsewhere. The profit margins in consumer goods are far higher than in defence products.

On the positive side, many new materials are being developed through civil investments. Lightweight, high-strength materials for automotives are equally useful in military vehicles. New manufacturing technology such as additive manufacturing, multimaterial joining and composite production are useful for military aircraft. Two-dimensional materials such as graphene, but including others such as disilicides are being studied for many applications including photodetectors.

Reuse and recycling are more of a challenge for defence than for civil products. They may be particularly hazardous materials, especially older systems, and issues of secrecy to manage. Disposal after decommissioning is not an area which has received much attention from defence authorities, so the question of ‘what does one do with an old aircraft carrier?’ remains unanswered.

A wider use of computational methods offers important opportunities to improve existing materials and to identify completely new ones. Ab initio calculations show promise to identify new energy storage materials, and thermodynamic and kinetic calculations, proven for many years useful to improve ferrous alloys should be extended to non-ferrous systems. Magnesium alloys are likely to be used in a wider range of applications.

Quantum computing is starting to show promise and will revolutionise our understanding and manipulation of the materials world. Offering the numerical power to address calculations that are beyond even the fastest current supercomputers, the ability to understand, predict and design microstructural material solutions will open up a completely new world of possibilities.

There are many novel but technically feasible defence concepts on the drawing boards, and all depend on cost-effective materials and manufacturing technologies. Electric armour will demand new materials to be practically useful, as will systems such as hypersonic aeroplanes. For the latter, sub-systems such as the heat exchanger of the engine have been successfully demonstrated, but to build aircraft – or more likely unmanned platforms capable of sustained hypersonic flight – it will demand novel materials including ceramic-ceramic components.


Dr Savage is Swedish Defence Research Agency Research Director – Materials Technology and IOM3 Defence, Safety and Security Committee Corresponding Member.