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Chobham armour is a composite armour developed in the 1960s at the British tank research centre on Chobham Common. Although the exact composition of Chobham armour remains a secret, it appears to be a composite armour of ceramic tiles within a matrix that is layered between steel armour plating, a combination that is excellent at defeating high explosive anti-tank (HEAT) rounds. Possible ceramics for such armour are: boron carbide, silicon carbide, aluminium oxide (sapphire or "alumina"), aluminium nitride, titanium boride or Syndie, a synthetic diamond composite. Of these boron carbide is the hardest and lightest, but also the most expensive and brittle. Boron carbide composites are today favoured for ceramic plates protecting against smaller projectiles, such as used in body armour and armoured helicopters. Silicon carbide, better suited to protect against larger projectiles, was used in some prototype land vehicles.

The ceramics can be created by cold pressing or hot pressing. A high density is striven for in that air bubbles should be almost absent. Over the years newer and tougher fibre composites have been developed, giving about five times the protection value of the original pure ceramics, the best of which were again about five times as effective as a steel plate of equal weight. These are often a mixture of several ceramic materials. The latest developments involve the use of carbon nanotubes to improve toughness even further. The small hexagonal or square ceramic tiles are encased within a metal (today typically a titanium alloy) matrix, either by isostatically pressing them into the heated matrix, or by glueing them with an epoxy resin. A more general name is therefore: MMC or Metallic Matrix Composite. The word "composite" here refers to "composite armour", not to any composite materials, such as Ceramic Matrix Composites or MMCs in the wider sense, which optionally might be used in its construction. A titanium matrix is extremely expensive to manufacture but the metal is favoured for its lightness, strength and resistance to corrosion, a constant problem with MMCs. The Rank company claims to have invented an alumina matrix for the insertion of boron carbide or silicon carbide tiles. Since the early nineties it is known that bringing the tiles under constant compression by their matrix greatly improves their resistance to kinetic penetrators, which is difficult to achieve when using glues.

The MMC has to be backed by a metal plate, both to reinforce the ceramic tiles from behind and to prevent deformation of the metal matrix by a kinetic impact. Typically the backing plate has one-third of the thickness of the MMC. It can be made from steel, but, as its main function is to improve the stability and stiffness of the assemblage, aluminium is more weight-efficient in light AFVs only to be protected against light anti-tank weapons. The assemblage is again attached to elastic layers. These absorb impacts somewhat, but their main function is to prolong the service life of the MMC by protecting it against vibrations. A deformable composite backing plate can combine the function of a metal backing plate and an elastic layer. Several assemblages can be stacked, depending on the available space; this way the armour can be made of a modular nature, adaptable to the tactical situation. The thickness of a typical assemblage is today about five to six centimetres. Earlier assemblages, so-called DOP (Depth Of Penetration)-matrices, were thicker. The whole is placed within the shell formed by the outer and inner wall of the tank turret or hull, the inner wall being the thickest.

The exact nature of the protection offered is sometimes presented as a bit of a mystery but is in principle a straightforward application of the expected effects caused by the general physical qualities of the ceramics used. Due to their extreme hardness they offer superior resistance against a shaped charge jet and they shatter kinetic energy penetrators. The (pulverised) ceramic also strongly abrades any penetrator. Against lighter projectiles the hardness of the tiles causes a "shatter gap" effect: a higher velocity will within a certain velocity range (the "gap") not lead to a deeper penetration but destroy the projectile itself instead. Because the ceramic is so brittle the entrance channel of a shaped charge jet is not smooth — as it would be when penetrating a metal — but ragged, causing extreme asymmetric pressures which disturb the geometry of the jet, on which its penetrative capabilities are critically dependent as its mass is relatively low. This initiates a vicious circle as the disturbed jet causes still greater irregularities in the ceramic, until in the end it is defeated. The newer composites, though tougher, optimise this effect as tiles made with them have a layered internal structure conducive to it. This mechanism using the jet's own energy against it, has caused some to compare the effects of Chobham to those of reactive armour. This should not be confused with the effect used in many laminate armour of any kind: that of sandwiching an inert but soft elastic material such as rubber, between two of the armour plates. The impact of either a shaped charge jet or long-rod penetrator, after the first layer has been perforated and the rubber layer is being penetrated, will cause the rubber to deform and expand, so deforming both the back and front plates. Both attack methods will suffer from obstruction to their expected paths, so experiencing a greater thickness of armour than is there is nominally, this lowering penetration. Also for rod penetrations, the transverse force experienced due to the deformation may cause the rod to shatter, bend, or just change its path, again lowering penetration.

Ceramic tiles have a "multiple hit capability" problem in that they are unable to sustain successive impacts without quickly losing much of their protective value. To minimise the effects of this the tiles could be made as small as possible, but then the ratio between the area covered by tiles and that covered by the matrix would become more unfavourable, also because the matrix elements cannot be reduced accordingly as they have a minimal practical thickness of about an inch. An equilibrium is usually found at a diameter of about ten centimetres. The backing plate reflects the impact energy back to the ceramic tile in a wider cone. This dissipates the energy, limiting the cracking of the ceramic, but also means a more extended area is damaged. Spalling caused by the reflected energy can be partially prevented by a thin layer of ductile graphite on the face of the ceramic absorbing the energy without making it strongly rebound again as a metal face plate would. Tiles under compression suffer far less from impacts; in their case it can be advantageous to have a metal face plate bringing the tile also under perpendicular compression. The confined ceramic tile then reinforces the metal face plate, a reversal of the normal situation.

Another drawback of ceramic tiles is that sloping them offers little sloped armour advantage as they lack sufficient toughness to significantly deflect heavy penetrators. Indeed, because a single glancing shot could crack many tiles, the placement of the MMCs is chosen so as to optimise the chance of a perpendicular hit, a reversal of the previous desired design feature for conventional armour: to avoid shot traps. As the relative face defeat component of the protective value of a ceramic is much larger than for steel armour, calculating that value by simply measuring the thickness along the LOS (line of sight) would be very inaccurate. Using more, but thinner, MMCs again enlarges that component for the entire armour package, an effect analogous to the use of alternate layers of high hardness and softer steel, which is so typical for the glacis of modern Soviet tanks. Instead of rounded forms the turrets of tanks using Chobham armour have a typical "slab-sided" appearance. However it cannot be safely deduced from the fact that a tank has a squat turret that it would be equipped with ceramic armour; as long rod penetrators are less susceptible to deflection — they are so elongated, dense and fast that they e.g. will ricochet only at very shallow angles above 85° — pure metal armour assemblages no longer seek to completely deflect but to deform and abrade the penetrators by means of the internal assemblage structure, so they too will no longer be rounded.

The armour configuration of the first western tanks using Chobham armour was optimised to defeat shaped charges as guided missiles were seen as the greatest threat. In the eighties however they began to face improved Soviet kinetic energy penetrator rounds of various sorts, which the ceramic layer was not particularly effective against: for the original ceramics the resistance against penetrators was about three times, for the newest composites it is about ten times less than against HEAT rounds. For this reason many modern designs include additional layers of heavy metals to add more density to the overall armour package. The introduction of more effective ceramic composite materials allows for a larger width of these metal layers within the armour shell, given a certain protection level provided by the MMC. The word "add" has caused the misunderstanding that they were literally placed on top of the MMCs or even on the outside of the vehicle, but they typically form an inner layer below them, to prevent extensive damage to the much more expensive MMC should the metal layer strongly deform but not defeat a penetrator. They should however not be confused with the metal backing plates of the MMCs themselves. Using them as a backing plate is possible but compromises the modularity and thus tactical adaptability of the armour system; furthermore due to their extreme hardness they deform insufficiently and would reflect too much of the impact energy to the ceramic tile. The metal used appears to be either a tungsten alloy or, in the case of later M1 Abrams tanks, a depleted uranium alloy. Some companies offer titanium carbide modules. These metal modules (typically employing perpendicular rods) have many perforations or expansion spaces reducing the weight up to about a third while keeping the protective qualities fairly constant. To describe the uranium modules of the M1 the word "mesh" has been used. An early less sophisticated steel version of such armour can be seen attached to the glacis of the German Leopard 1. Such modules are also used by tanks not equipped with Chobham armour. The combination of MMCs and heavy metal modules is sometimes informally referred to as "second generation Chobham"; this usage does not reflect any possible development of the MMCs themselves.

The effectiveness of Chobham armour was demonstrated in the Gulf Wars of 1991 and 2003, where no Coalition tank was destroyed by either the obsolete Iraqi armour or ATGWs. In some cases the tanks in question were subject to multiple hits by both KE-penetrators and HEAT rounds, but the old Russian ammunition used by the Iraqis, in their Polish licence built T-72s, their old T-55s bought from Russia and upgraded with "Enigma" type armour, and T-62 tanks left them completely incapable of penetrating the front armour of Coalition tanks. It is also worth noting that the Iraqis rarely actually hit the Coalition tanks, because of lack of training and inferior optics. To date, only 5-10 Chobham-protected tanks have been defeated by enemy fire in combat, including an M1 that was hit on the side skirts, below the turret ring by a PG-7VR, a tandem charge RPG, in the Iraq War. The jet penetrated the skirting armour and side hull armour, then traversed across the tank's interior and finally penetrated 1.5 to 2 inches into the hull armour on the other side.

The latest version of Chobham armour is used on the Challenger 2 (called Dorchester armour), and (though the composition most probably differs) the M1 Abrams series of tanks, which according to official sources is currently protected by silicon carbide tiles. Given the publicly stated protection level for the earliest M1: 350 mm steel equivalence against KE-penetrators (APFSDS), it seems to have been equipped with alumina tiles. Though it is often claimed to be otherwise, the Leopard 2 does in fact not use Chobham armour, but pure perforated armour, avoiding the very large procurement, maintenance and replacement costs of those ceramic armour systems not based on the cheap but rather ineffective alumina. Ceramic modules will corrode their matrix and gradually fracture during driving and the smallest come at over $100,000. For many modern tanks, such as the Japanese Type 90 and the Italian Ariete, it is yet unknown which type is used. There is a general trend away from ceramic armour towards perforated armour; but even many tanks from the seventies like the Leopard 1A3 and A4, the Italian OF-40 and the French AMX-32 and AMX-40 prototypes used the latter system; the Leclerc has an improved version.

Jeffrey J. Swab (Editor), Dongming Zhu (General Editor), Waltraud M. Kriven (General Editor); Advances in Ceramic Armor: A Collection of Papers Presented at the 29th International Conference on Advanced Ceramics and Composites, January 23-28, 2005, Cocoa Beach, Florida, Ceramic Engineering and Science Proceedings, Volume 26, Number 7; ISBN 1-57498-237-0

• Article on DSTL/QinetiQ Chertsey and Longcross Test Track (Chobham Tank Research Centre)