Composite armourComposite armour is a type of armour consisting of layers of different material such as metals, plastics, ceramics or air. Most composite armours are lighter than their all-metal equivalent, but instead occupy a larger volume for the same resistance to penetration. It is possible to design a composite armour both stronger, lighter and less voluminous than traditional armour, but the cost is often prohibitively high, restricting its use to especially vulnerable parts of a vehicle.
The most common type of composite armour today is Chobham armour, first developed by the British in the 1970s for their new Challenger tank. Chobham sandwiches a layer of ceramic between two plates of steel armor, which was shown to dramatically increase the resistance to high explosive anti-tank (HEAT) rounds. HEAT had seriously challenged the ability of armor to survive since its introduction in WWII, and Chobham was such an improvement that it was soon copied on the new US M1 Abrams main battle tank as well (although there it is referred to as Special Armor). It is the fabrication of the ceramic in large tiles that gives the Challenger and Abrams their "slab sided" look.
Chobham's precise mechanism for defeating HEAT was something of a mystery until the 1980s. High speed photography showed that the ceramic material shatters as the HEAT round penetrates, blowing up to a huge volume which then expands back out the hole and pushes the metal jet of the HEAT with it. The effectiveness of the system was amply demonstrated in Desert Storm, where a handful of Challengers destroyed 300 Iraqi tanks without loss, one at over five miles range.
Newer versions of Chobham include open spaces, depleted uranium and other layers in addition to the original steel/ceramic layering. The uranium layers are included primarily to increase the total mass of metal while not being larger physically.