Chobham

The varying substances of Chobham are each designed to defend against different threats. The basic armor plate is shaped and surrounded with RHA, which in addition to giving the armor its basic shape matrix is the best compromise between plasma and kinetic penetration resistance. Additional layers likely include depleted uranium for mass, harder metals such as titanium or perhaps iridium, and ceramics for resistance to plasma from HEAT sandwiched between metal layers to minimize the brittle nature of ceramic materials. DU layers are typically kept near the outside of the armor, sometimes as an applique, due to its toxicity and the pyrophoric nature of uranium - if it is penetrated, it tends to catch fire and flow in the direction of the shot. Since burning DU is highly lethal in its own right, better to have that on the outer layers, although if the armor is penetrated the chances of crew survival aren't great, given the energy levels involved.

As best as I have been able to determine, Chobham armor is a multi-layer sandwich of rolled homogenous steel- essentially flat, smooth plates of uniform steel- ceramic plating- which is extremely hard, and effective at stopping most AP rounds- and titanium alloy, which is incredibly strong, but mind-bogglingly difficult with which to work. Also included is a layer of some sort substance on the interior of the tank to stop spall- fragments formed from the tank's own armor- which is likely kevlar. The M1, M1A1, and M1A2 all include a layer of depleted uranium- U²³⁵ or U²³⁸ that is no longer radioactive- which is so incredibly dense as to easily defeat most anti-armor munitions. Both the M1 Abrams and Challenger Main Battle Tanks use Chobham.

It's opined in Tom Clancy's book 'Armored Cav', that Chobham is mostly layers of Rolled Homogenous Armor with 2-3 small layers of ceramics. Tanks are fought in three manners, misdirection, brute force on the head of a pin, and with Chemistry professors. The brute force method uses an 'Aluminum Sabotted DU Long-Rod Penetrator', and the Chem tack uses a HEAT round. Basically a javelin of ultra-hot gases/plasma that can penetrate 80 cm of RHA.

The NATO answer to HEAT rounds, and Long-Rod Penetrators, is Chobham. To reduce the penetration of HEAT rounds, thin layers of heat resistant, and brittle ceramics are sandwiched inside thick layers of Depleted Uranium, which address the threat of the Kinetic energy weapons. A 45 CM piece of Chobham plate protects as well as something like 80cm of RHA against penetrators, and 130cm of RHA against HEAT rounds.

see also High-Velocity apfsds for info on the Armor Piercing Aluminum Sabotted Depleted Uranium Long Rod Penetrators (thanks 'The Custodian')