Chobham armour
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Chobham armour is a composite armour developed 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 of ceramic layered between steel armour plating, a combination that is excellent at defeating high explosive anti-tank (HEAT) rounds. Possible ceramics for such armours are: boron carbide, silicon carbide, aluminium oxide (sapphire or "alumina"), titanium boride or Syndie, a synthetic diamond composite. Of these boron carbide is the hardest and lightest, but also the most expensive and brittle. Over the years newer 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. The ceramic tiles are encased within a metal (today typically titanium) matrix, either by isostatically pressing them into the heated matrix, or by glueing them with an epoxy resin. A more general name is therefore: CMC or Ceramic Matrix Composite. 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 CMC's. The Rank company claims to have invented an alumina matrix for the insertion of boron carbide or silicon carbide tiles.
The exact nature of the protection offered by this layering remains classified, but it has been suggested that some part of Chobham armour works in a manner somewhat similar to reactive armour. This may be referring to the effect 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 in reality, 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.
Modern tanks also have to face kinetic energy penetrator rounds of various sorts, which the ceramic layer is 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 metal used appears to be either tungsten or, in the case of later M1 Abrams tanks, depleted uranium. Some companies offer titanium carbide modules. These metal modules or rods have many perforations or expansion spaces reducing the weight up to about a third while keeping the protective qualities fairly constant.
The effectiveness of Chobham armour was demonstrated in the first Gulf War, where no Coalition tank was destroyed by the obsolete Iraqi armour. 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 Coalition armour. It's 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 Second Gulf War. The jet penetrated the skirting armour, side hull armour, traversed across the tanks interior and penetrating a further 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 French Leclerc 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.