Composite armour is a type of vehicle armour consisting of layers of different materials 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 composite armour 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. Its primary purpose is to help defeat high-explosive anti-tank (HEAT) projectiles.
HEAT had posed a serious threat to armoured vehicles since its introduction in World War II. Lightweight and small, HEAT projectiles could nevertheless penetrate hundreds of millimetres of the most resistant steel armours. The capability of most materials for defeating HEAT follows the "density law", which states that the penetration of shaped charge jets is proportional to the square root of the shaped charge liner density (typically copper) divided by the square root of the target density. On a weight basis, lighter targets are more advantageous than heavier targets, but using large quantities of lightweight materials has obvious disadvantages in terms of mechanical layout. Certain materials have an optimal compromise in terms of density that makes them particularly useful in this role.
History
The earliest known composite armour for armoured vehicles was developed as part of the US Army's T95 experimental series from the mid-1950s. The T95 featured siliceous-cored armour which contained a plate of fused silica glass between rolled steel plates. The stopping power of glass exceeds that of steel armour on a thickness basis and in many cases glass is more than twice as effective as steel on a thickness basis. Although the T95 never entered production, a number of its concepts were used on the M60 Patton, and during the development stage (as the XM60) the siliceous-cored armour was at least considered for use, although it was not a feature of the production vehicles.
The first widespread use of a composite armour appears to have been on the Soviet T-64. It used an armour known as combination K, which apparently is glass-reinforced plastic sandwiched between inner and outer steel layers. Through a mechanism called thixotropy, the resin changes to a fluid under constant pressure, allowing the armour to be moulded into curved shapes. Later models of the T-64, along with newer designs, use a boron carbide-filled resin aggregate for greatly improved protection . The Soviets also invested heavily in reactive armour, which allowed them some ability to control quality, even after production.
Among NATO nations and allies, the most common type of composite armour today is Chobham armour, first developed and used by the British in the experimental FV 4211 tank, which was based on Chieftain tank components. Chobham uses multiple non-explosive reactive armour plates (NERA), which sandwich a layer of elastomer (like rubber) between two plates of steel armour. This was shown to dramatically increase the resistance to HEAT projectiles, even in comparison to other composite armour designs. Chobham was such an improvement that it was soon used on the new U.S. M1 Abrams main battle tank (MBT) as well. The need to mount multiple angles plates, along with an outer steel layer to protect the armour array, gives the Challenger and Abrams their "slab sided" look.
The Soviets/Russians had a similar composite armour to the West's own "NERA", with rubber sandwiches between plates of steel. This armour was confirmed to be inside the T-72B's "Super Dolly Parton" armour, but is suspected to be inside the T-80A as well, since it is unlikely the Soviets would put worse armour in their "premier" tank.