Chobham Armour - Structure

Structure

Ceramic tiles have a "multiple hit capability" problem in that they cannot sustain successive impacts without quickly losing much of their protective value. To minimise the effects of this the tiles are made as small as possible, but the matrix elements have a minimal practical thickness of about one inch (25 mm), and the ratio of coverage provided by tiles would become unfavourable, placing a practical limit at a diameter of about four inches (ten centimetres). The small hexagonal or square ceramic tiles are encased within the matrix either by isostatically pressing them into the heated matrix, or by gluing them with an epoxy resin. Since the early nineties it has been known that holding the tiles under constant compression by their matrix greatly improves their resistance to kinetic penetrators, which is difficult to achieve when using glues.

The matrix has to be backed by a plate, both to reinforce the ceramic tiles from behind and to prevent deformation of the metal matrix by a kinetic impact. Typically the backing plate has half of the mass of the composite matrix. The assemblage is again attached to elastic layers. These absorb impacts somewhat, but their main function is to prolong the service life of the composite matrix by protecting it against vibrations. Several assemblages can be stacked, depending on the available space; this way the armour can be made of a modular nature, adaptable to the tactical situation. The thickness of a typical assemblage is today about five to six centimetres. Earlier assemblages, so-called DOP (Depth Of Penetration) -matrices, were thicker. The relative interface defeat component of the protective value of a ceramic is much larger than for steel armour. Using a number of thinner matrices again enlarges that component for the entire armour package, an effect analogous to the use of alternate layers of high hardness and softer steel, which is typical for the glacis of modern Soviet tanks.

Ceramic tiles draw little or no advantage from sloped armour as they lack sufficient toughness to significantly deflect heavy penetrators. Indeed, because a single glancing shot could crack many tiles, the placement of the matrix is chosen so as to optimise the chance of a perpendicular hit, a reversal of the previous desired design feature for conventional armour. Ceramic armour normally even offers better protection for a given areal density when placed perpendicularly than when placed obliquely, because the cracking propagates along the surface normal of the plate. Instead of rounded forms, the turrets of tanks using Chobham armour typically have a slab-sided appearance.

The backing plate reflects the impact energy back to the ceramic tile in a wider cone. This dissipates the energy, limiting the cracking of the ceramic, but also means a more extended area is damaged. Spalling caused by the reflected energy can be partially prevented by a malleable thin graphite layer on the face of the ceramic absorbing the energy without making it strongly rebound again as a metal face plate would.

Tiles under compression suffer far less from impacts; in their case it can be advantageous to have a metal face plate bringing the tile also under perpendicular compression. The confined ceramic tile then reinforces the metal face plate, a reversal of the normal situation.

A gradual technological development has taken place in ceramic armour: ceramic tiles, in themselves vulnerable to low energy impacts, were first reinforced by glueing them to a backplate; in the nineties their resistance was increased by bringing them under compression on two axes; in the final phase a third compression axis was added to optimise impact resistance. To confine the ceramic core several advanced techniques are used, supplementing the traditional machining and welding, including sintering the suspension material around the core; squeeze casting of molten metal around the core and spraying the molten metal onto the ceramic tile.

The whole is placed within the shell formed by the outer and inner wall of the tank turret or hull, the inner wall being the thicker.

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