Harder, better, stronger: keeping armour bullet proof
One of the many challenges facing the world of materials innovation is in the area of protection from high speed impact: armour. From the metallic full-body armours of knights in the Middle Ages to the light bulletproof jacket of modern day armies, the development of ballistic armour is undoubtedly a story of remarkable success in the field of defence technology.
With the growing capability of ballistic rifles, the effectiveness of armours is ever increasing. Depending on the nature of the rifle, the velocity of a bullet can range anywhere between 300 and 900 m/s, with the best capable of even piercing through the best armours of the age (aptly called the 'armour piercing' rifles). Hence, it requires little imagination to visualise that a slab made of any commonly known material lying in the path of such a high-energy projectile would easily disintegrate into fragments. However, substances used for armour are an altogether different category of material that are able to absorb all the energy of the bullet so are able to withstand the impact of such strikes with little harm. Analysis of videos of a bullet hitting an armour plate taken by high-speed cameras (capable of capturing a million frames per second) shows various stages of interaction of the bullet with the armour. What happens first is that the pointed tip of the bullet gets blunted, then the energy from the bullet is dissipated in shock waves, which radiate out from the point of contact and then finally the steady erosion of the bullet as it progresses into the armour plate.
The materials of choice for such commercial ballistic armours are either made of steel or ceramics. However, whatever the material, the key objective is that the armour has to be able to absorb the energy of the strike. When a material is subjected to external force or loading, there are two stages of deformation. The first stage is a stage of reversible deformation, during which, if the load, or force, is removed the material goes back to its exact initial dimension, much like a rubber band. The material behaviour under such criterion is termed 'elastic'. However, beyond a certain limit of 'stretching' the material reaches the second stage, which is when the deformation in the material is no longer reversible, i.e. a permanent offset is induced in the material which cannot be undone. When examined at an atomic level, such deformation causes a permanent offset in the relative distances between the atoms. This is when the material is said to be 'plastic'.
A hard material is one which can resist this very plastic deformation up to a large amount of pressure. Ceramics such as silicon carbide, boron carbide and certain grades of steel belong to this class of extremely hard materials. For these materials a force even as high as impact of a bullet is surprisingly not strong enough to bring about a significant plastic deformation - this is why these materials are the perfect candidates for ballistic armour plates. However, in comparison with steel armours, ceramic armours are much lighter, which obviously offers an added benefit if they are to be used either for individual or vehicular use. The density of boron carbide is significantly lower which offers a three times reduction in the weight of the same size of armour made of steel and is even lighter than silicon carbide. Boron carbide is also the third hardest material known to mankind, following naturally occurring diamond and boron nitride. Besides being hard and light, the other major properties desirable in advanced armour materials are superior resistance to wear, low friction and capability to withstand high temperatures due to the range of threats that might be encountered. The fact that modern day ceramics, resulting from years of continual improvements through extensive research, are able to tick almost all these requirements certainly underlines the importance of these classes of materials in ballistic armour applications. So the question arises - what makes ceramics such an exceptionally hard material?
The heart of this question actually lies in the basic difference between the nature of chemical bonds that exist in metals and ceramics. The type of bonds between atoms in a metal is called a 'metallic bond' while those in ceramics are called 'covalent bonds'. Covalent bonds involve interactions between two participating atoms at their electronic level. Such bonds are much shorter and stronger than metallic bonds. This high strength of the bonds in ceramic materials provides the requisite stiffness to these materials against high impact bullet strikes. However, it would be wrong to claim that armours made out of the best available ceramic material would be able to pass the 'bullet test' without a scar. In spite of the advances in the area of ceramic science, there are only a certain number of strikes that a piece of armour can resist. Currently, a continuous effort is being directed towards understanding the mechanism behind the damage of armours in order to improve overall design and the material's performance. Studies are now being undertaken to better understand the modes of interaction between bullets and armour, and to discover the mechanisms behind armour failings in order to negate these. In future, both computer modelling of materials as well as improved state-of-the-art characterisation techniques would enable the development of more protective, lightweight ballistic armours for the future.