Chobham armour


Chobham armour is the informal name of a composite armour developed in the 1960s at the Military Vehicles and Engineering Establishment, a British tank research centre on Chobham Lane in Chertsey. The name has since become the common generic term for composite ceramic vehicle armour. Other names informally given to Chobham armour include Burlington and Dorchester. Special armour is a broader informal term referring to any armour arrangement comprising sandwich reactive plates, including Chobham armour.
Within the Ministry of Defence, Chobham usually refers specifically to the non-explosive reactive armor and ceramic composites, while Dorchester is usually in reference to additional armour packages, primarily composed of explosive reactive armour and spaced armour, although these are often conflated when in colloquial usage.
Although the construction details of the Chobham armour remain a secret, it has been described as being composed of ceramic tiles encased within a metal framework and bonded to a backing plate and several elastic layers. Owing to the extreme hardness of the ceramics used, they offer superior resistance against shaped charges such as high-explosive anti-tank rounds and they shatter kinetic energy penetrators.
The armour was first tested in the context of the development of a British prototype vehicle, the FV4211, and first applied on the preseries of the American M1. Only the M1 Abrams, Challenger 1, Challenger 2, and K1 88-Tank have been disclosed as being thus armoured. The framework holding the ceramics is usually produced in large blocks, giving these tanks, and especially their turrets, a distinctive angled appearance.

Protective qualities

Due to the extreme hardness of the ceramics used, the tiles offer superior resistance against a shaped charge jet and they shatter kinetic energy penetrators. The ceramic also strongly abrades any penetrator. Against lighter projectiles, the hardness of the tiles causes a shatter gap effect: a higher velocity will, within a certain velocity range, not lead to a deeper penetration but destroy the projectile instead.
Because the ceramic is so brittle, the entrance channel of a shaped charge jet is not smooth—as it would be when penetrating a metal—but ragged, causing extreme asymmetric pressures which disturb the geometry of the jet. This initiates a vicious circle as the disturbed jet causes still greater irregularities in the ceramic, until in the end it is defeated. The newer composites, though tougher, optimise this effect as tiles made with them have a layered internal structure conducive to it, causing "crack deflection". This mechanism, using a jet's own energy against it, has caused the effects of Chobham to be compared to those of reactive armour.
This should not be confused with the effect used in non-explosive reactive armour: that of sandwiching an inert but soft elastic material such as rubber, between two armour plates. The impact of either a shaped charge jet or armour-piercing fin-stabilized discarding sabot kinetic energy long rod penetrators, after the first layer has been perforated and while the rubber layer is being penetrated will cause the rubber to deform and expand, so deforming both the back and front plates. Both attack methods will suffer from obstruction to their expected paths, so experience a greater thickness of armour than there is nominally, thus lowering penetration. Also for rod penetrations, the transverse force experienced due to the deformation may cause the rod to shatter, bend, or only change its path, again lowering penetration.
All versions of Chobham armour have incorporated a large volume of non-energetic reactive armour plates, with added hard armour ahead of the NERA and/or behind the NERA (intended to catch the fragments of long rods or HEAT jets after they have been fractured or disrupted by the front plate and NERA. This is another factor favouring a slab-sided or wedge-like turret: the amount of material the expanding plates push into the path of an attack increases as they are placed closer to parallel to the direction of that attack.
To date, few Chobham armour-protected tanks have been defeated by enemy fire in combat; the relevance of individual cases of lost tanks for determining the protective qualities of Chobham armour is difficult to ascertain as the extent to which such tanks are protected by ceramic modules is undisclosed.
During the second Iraq war in 2003, a Challenger 2 tank became stuck in a ditch while fighting in Basra against Iraqi forces. The crew remained safe inside for many hours, the Burlington LV2 composite armour protecting them from enemy fire, including multiple rocket-propelled grenades.

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 25 mm, and the ratio of coverage provided by tiles would become unfavourable, placing a practical limit at a diameter of about 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 1990s 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 modular, to be replaceable, and more adaptable to varied tactical situations. The thickness of a typical assemblage is today about five to six centimetres. Earlier assemblages, so-called 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 reduced 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 gluing 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.

Material

Over the years newer and tougher composites have been developed, giving about five times the protection value of the original pure ceramics, the best of which were again about five times as effective as a steel plate of equal weight. These are often a mixture of several ceramic materials, or metal matrix composites which combine ceramic compounds within a metal matrix. The latest developments involve the use of carbon nanotubes to improve toughness even further. Commercially produced or researched ceramics for such type of armour include boron carbide, silicon carbide, aluminium oxide, aluminium nitride, titanium boride and Syndite, a synthetic diamond composite. Of these boron carbide is the hardest and lightest, but also the most costly and brittle. Boron carbide composites are today favoured for ceramic plates protecting against smaller projectiles, such as used in body armour and armoured helicopters; this was, in the early sixties, the first general application of ceramic armour. Silicon carbide is better suited to protect against larger projectiles than boron carbide as the latter material suffers a phase collapse when impacted by a projectile travelling at a speed over. The ceramics can be created by pressureless sintering or hot pressing. A high density is required, so residual porosity must be minimised in the final part.
A matrix using a titanium alloy is very costly to produce but the metal is favoured for its lightness, strength, and resistance to corrosion, which is a constant problem.
The backing plate can be made from steel, but, as its main function is to improve the stability and stiffness of the assemblage, aluminium is more weight-efficient in light armoured fighting vehicles only to be protected against light anti-tank weapons. A deformable composite backing plate can combine the function of a metal backing plate and an elastic layer.