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.