Chobham armour

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 combination of ceramic layered between armour steel 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 remained a mystery for some time, but it was eventually revealed that Chobham armour works in a manner somewhat similar to reactive armour. When the armour is hit by a HEAT round the ceramic layer shatters under the impact point, forming a dust under high pressure. When the HEAT round "burns through" the outer layers of armour and reaches the ceramic, the dust comes flying back out the hole, slowing the jet of metal.

Modern tanks also have to face KE-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 armor package. The metal used appears to be either tungsten or, in the case of later M1 Abrams tanks, depleted uranium. 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 armor. In some cases the tanks in question were subject to multiple point-blank hits by both KE-penetrators and HEAT rounds, but the old Russian ammunition used by the Iraqis, in their Polish licence built T-72's, their old T-55's 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 in a weak spot by an RPG-7 in the Second Gulf War.

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 presently 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 horrendous 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.