“Although the construction details of the Chobham armour remain a secret, it has been described as being composed of ceramic tiles encased within a metal framework and bonded to a backing plate and several elastic layers. Owing to the extreme hardness of the ceramics used, they offer superior resistance against shaped charges such as high-explosive anti-tank (HEAT) rounds and they shatter kinetic energy penetrators.”
“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. The newer composites, though tougher, optimise this effect as tiles made with them have a layered internal structure conducive to it...”
https://en.wikipedia.org/wiki/Chobham_armour