The First world tanks and early tank production used Armoured plate steel which is carbonised by heating the steel in the presence of carbon (usually charcoal) for long durations (often several weeks), this allowed the carbon to be absorbed into the surface of the steel plate to depths of up to 20 thousandths of an inch, the subsequent heat treatment of the plate produced a material with a malleable and shock resistant core and an extremely hard outer face. Unfortunately this type of steel was very prone to splinters breaking off the surface when hit, causing injury to the crew inside.
Homogeneous Armour Plate (developed between the wars for tank production)
Armour steel must be hard yet also be able to withstand shock in order to resist high velocity metal projectiles and one way of producing this is called Rolled Homogeneous Armour. Steel with these characteristics is produced by rolling appropriately sized cast steel billets into sheet form. The grain structure which exists in any cast metal creates lines of weakness throughout the casting making it very brittle, by rolling the cast steel billets the grain structure is stretched to form long lines, eliminating these areas of weakness and enabling the stresses within the steel to flow throughout and the effects of impact to be dissipated.
Rolled homogenous armour is so called because, due to the rolling process its structure and composition is uniform throughout its section.
Spaced amour is exactly what it says it is. A second layer of armour added to the outside of the vehicle with a gap in between (rather like a cavity wall), the principle is that the hollow space disrupts the shockwave caused by the impact, deflecting the projectile and forcing it to lose momentum, also a shaped charge round is detonated before it reaches the main armour and its forces dissipated in a similar manner. The second layer of armour need not necessarily be steel but in general practice, it is. Spaced armour first came into use towards the end of the Second World War; in archive film clips you can see German vehicles riding around with additional armour plates added on.
Today we use bar armour or slat armour and its purpose is to detonate war heads (anti tank rocket propelled grenades) before they reach the main body armour. Bar armour was developed to keep the weight down while still maintaining the capability to detonate the war head.
Because it is still covered by the Official Secrets Act little is known about Cobham Armour, although it is believed to be made from many layers of different energy absorbing materials, compressed together under extreme pressure and heat to make a very dense material which is then added in blocks to the exterior of the vehicle.