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.
The Soviet Union has not deployed composite armor on a large scale, deciding instead to focus their efforts on reactive armor. False!
T-64:
First ever production MBT to carry combinational armor. It carried Combination K, which appariently is composed of glass fiber suspended within a plastic resin. Through a mechanism called thixotropy, the resin changes to a fluid under constant pressure, allowing the armor to be molded into curved shapes.
DOI: 1966 RHA verus APFSDS: 410 mm RHA verus HEAT: 500 mm
T-80B:
Along with the T-64B and T-72A, this vehicle substituted a Boron Carbide filled resin aggregate in 2nd generation Combination K, similar to the above mentioned GRP, but was more compact and provided better protection for the same weight.
DOI: 1978 RHA verus APFSDS: 500 mm RHA verus HEAT: 580 mm
T-80BV:
The introduction of ERA on Russian vehicles led to a massive emergancy program on the behalf of NATO to requip their ATGMs with tandem warheads. The first T0W-2A appeared after the T-80U was introduced.
DOI: 1983 RHA verus APFSDS: 500 mm RHA verus HEAT: 1 000 mm
T-80U:
The first vehicle to carry Kontakt-5 EDZ, effective both against HEAT warheads and APFSDS. It also carried an applique armor pack which is composed of a frontal steel plate about 60 mm thick backed by an insert of three layers of inert interlayer reactive armor, composed of steel plates and penapolyurethane filler. Tests by a unified Germany in 1995 found this material to have an Em of about 5.0. Also had significant increases to vehicle survivablity in other areas, mostly the armoring and cellurization of ammunition storage and the incorporation of composite steel/GRP armor on the vehicle's flanks.
DOI: 1985 RHA verus APFSDS: 820 mm RHA verus HEAT: 1 300 mm
T-90
The T-90 was the first vehicle to incorporate inert interlayer reactive armor, along with Kontakt-5, into it's armor system rather than relying on applique packs, lessening the number of null zones on the vehicle.
DOI: 1995 RHA verus APFSDS: 1 005 mm RHA verus HEAT: 1 560 mm