composite spaced armour with hardened steel outer layer (often filled with NERA or ceramic inserts) are becoming more common on most advanced light battle tanks (ZTQ-15) and IFVs (Namer, Puma). Materials As designs became more specialized, more and more materials were used. The most important are: Elastomers Some modern main battle tanks (MBTs) and IFVs carry rubber or steel (hardened in some cases) skirts to protect their relatively fragile suspension and lower side armour and lower glacis, often combining the two. Some elastomer fillings (e.g. M551's floating cells and screens, T-72B's radiation protection layer) behave like spaced armour, where the elastic layer effectively lowers the concentration of the jet of HEAT warheads. Leopard 1A3 and 1A4 and add-on armour of T-55 and T-62 used dense polystyrene filling to increasing the effectiveness of spaced armour. The early second generation Russian MBTs used dense glass-fiber reinforced pressured plastic as filling in the frontal upper glacis spaced armour which is even more effective than the pure elastomer. NERA armour also use elastomers pressed between two or three sheets steel or aluminium layers, it acts as ERA armour with lesser effectiveness, but is not destroyed during operation, so it can hold multiple hits in the same place. Most of the modern MBTs use some NERA layer within their spaced armour, or as an outer layer. Hardened steel Whereas normal armour must compromise between hardness and ductility, spaced armour can be constructed from plates with differing material properties to increase effectiveness against kinetic energy penetrators. Most of the Cold War and modern spaced armours use rolled homogeneous armour as the inner layer and a thin (10–30 mm) face-hardened, semi-hardened steel plate as the outer layer. The thin but very hard outer layer acts as a burster and shatter plate, which allows the main armour to be designed much thinner with the same protection level. The most advanced designs use triple- or high-hardened steel. In some cases aluminium is added to hardened steel armour as a softer inter-layer to destabilise the projectiles and HEAT jets by density changes. The Leopard 2 uses a slanted first armour stage (disturber), a specially hardened second stage (disrupter) and a softer, high ductility third stage (absorber). The disturber is designed to either entirely deflect or manipulate the direction of incoming kinetic energy penetrators. If penetration does occur, the projectile is then shattered and fragmented when striking the disrupter. Assuming the