Chobham is type of armor used on advanced main battle tanks, such as the British Challenger 2 and the American M1 Abrams tanks. Although it was developed in the United Kingdom, it was first used by the US Army on the Abrams. The Chobham armor consists of ceramic tiles encased within a metal matrix and attached to a backing plate and several elastic layers. The matrix has to be backed by a special steel alloy plate, both to reinforce the ceramic tiles from behind and to prevent deformation of the metal matrix by a kinetic impact. Due to the extreme hardness of the ceramics used, they offer superior resistance against shaped charges such as high explosive anti-tank rounds and they shatter kinetic energy penetrators. Only the M1 Abrams, Challenger 1, and Challenger 2 tanks have been disclosed as being thus armored.
The Chobham armor features superior resistance against a shaped charge jet and they shatter kinetic energy penetrators. The ceramic also strongly abrades any penetrator. Against lighter projectiles the hardness of the tiles causes a "shatter gap" effect: a higher velocity will not lead to a deeper penetration but destroy the projectile itself instead. Because the ceramic is so brittle the entrance channel of a shaped charge jet is not smooth — as it would be when penetrating a metal — but ragged, causing extreme asymmetric pressures which disturb the geometry of the jet, on which its penetrative capabilities are critically dependent as its mass is relatively low. This initiates a vicious circle as the disturbed jet causes still greater irregularities in the ceramic, until in the end it is defeated.