Chobham armour



Chobham armour is the name informally given to a composite armour developed in the 1960s at the British tank research centre on Chobham Common, Surrey, England. The name has since become the common generic term for ceramic vehicle armour. Other names informally given to Chobham Armour include "Burlington" and "Dorchester."

Although the construction details of the Chobham Common armour remain a secret, it has been described as being composed of ceramic tiles encased within a metal matrix and bonded to a backing plate and several elastic layers. Due to the extreme hardness of the ceramics used, they offer superior resistance against shaped charges such as high explosive anti-tank (HEAT) rounds and they shatter kinetic energy penetrators. Only the M1 Abrams, Challenger 1, and Challenger 2 tanks have been disclosed as being thus armoured. The armour was first tested in the context of the development of a British prototype vehicle, the FV4211. Despite being a British invention, the armour type was first implemented on the American Abrams tank.

Protective qualities
Due to the extreme hardness of the ceramics used, they offer superior resistance against a shaped charge jet and they shatter kinetic energy penetrators (KE-penetrators). The (pulverised) 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 (the "gap"), not lead to a deeper penetration but destroy the projectile itself 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, on which its penetrative capabilities are critically dependent as its mass is relatively low. 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 the 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 many laminate armours of any kind: that of sandwiching an inert but soft elastic material such as rubber, between two of the armour plates. The impact of either a shaped charge jet or long-rod penetrator 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 experiencing 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 just change its path, again lowering penetration.

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.

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.

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 with metal. 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 (sapphire or "alumina"), aluminium nitride, titanium boride and Syndite, a synthetic diamond composite. Of these boron carbide is the hardest and lightest, but also the most expensive 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 fact in the early sixties the first general application of ceramic armour. Silicon carbide, better suited to protect against larger projectiles, was at that time only used in some prototype land vehicles, such as the MBT-70. The ceramics can be created by cold pressing or hot pressing. A high density is striven for in that air bubbles should be almost absent.

A matrix using a titanium alloy is extremely expensive to manufacture but the metal is favoured for its lightness, strength and resistance to corrosion, which is a constant problem. The Rank company claims to have invented an alumina matrix for the insertion of boron carbide or silicon carbide tiles.

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 AFVs 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.

Heavy metal modules
The armour configuration of the first western tanks using Chobham armour was optimised to defeat shaped charges as guided missiles were seen as the greatest threat. In the eighties however they began to face improved Soviet kinetic energy penetrator rounds of various sorts, which the ceramic layer was not particularly effective against: the original ceramics had a resistance against penetrators of about a third compared to that against HEAT rounds, for the newest composites it is about one-tenth. For this reason many modern designs include additional layers of heavy metals to add more density to the overall armour package.

The introduction of more effective ceramic composite materials allows for a larger width of these metal layers within the armour shell: given a certain protection level provided by the composite matrix, it can be thinner. They typically form an inner layer placed below the much more expensive matrix, to prevent extensive damage to it should the metal layer strongly deform but not defeat a penetrator. They can also be used as the backing plate for the matrix itself, but this compromises the modularity and thus tactical adaptability of the armour system: ceramic and metal modules can then no longer be replaced independently. Furthermore, due to their extreme hardness, they deform insufficiently and would reflect too much of the impact energy, and in a too wide cone, to the ceramic tile, damaging it even further. Metals used include a tungsten alloy for the Challenger 2 or, in the case of the M1A1HA (Heavy Armor) and later American tank variants, a depleted uranium alloy. Some companies offer titanium carbide modules.

These metal modules (typically employing perpendicular rods) have many perforations or expansion spaces reducing the weight up to about a third while keeping the protective qualities fairly constant. The depleted uranium alloy of the M1 has been described as "arranged in a type of armour matrix" and a single module as a "stainless-steel shell surrounding a layer (probably an inch or two thick) of depleted uranium, woven into a wire-mesh blanket".

Such modules are also used by tanks not equipped with Chobham armour. The combination of a composite matrix and heavy metal modules is sometimes informally referred to as "second generation Chobham".

Development and application


The concept of ceramic armour goes back to 1918, when Major Neville Monroe Hopkins discovered that a plate of ballistic steel was much more resistant to penetration if covered with a thin (1–2 millimetres) layer of enamel.

Since the early sixties there were, in the US, extensive research programmes ongoing aimed at investigating the prospects of employing composite ceramic materials as vehicle armour. This research mainly focused on the use of an aluminium metal matrix composite reinforced by silicon carbide whiskers, to be produced in the form of large sheets. The reinforced light metal sheets were to be sandwiched between steel layers. This arrangement had the advantage of having a good multiple-hit capability and of being able to be curved, allowing the main armour to benefit from a sloped armour effect. However, this composite with a high metal content was primarily intended to increase the protection against KE-penetrators for a given armour weight; its performance against shaped charge attack was mediocre and would have to be improved by means of a laminate spaced armour effect, as researched by the Germans within the joint MBT-70 project.

An alternative technology developed in the US was based on the use of glass modules to be inserted into the main armour; although this arrangement offered a better shaped charge protection, its multiple hit capability was poor. A similar system using glass inserts in the main steel armour was from the late fifties researched for the Soviet Obiekt 430 prototype of the T-64; this was later developed into the "Combination-K" type, having a ceramic compound mixed with the silicon oxide inserts, which offered about 50% better protection against both shaped charge and KE-penetrator threats, compared with a steel armour of the same weight. It was, later in several improved forms, incorporated into all subsequent Soviet main battle tank designs. After an initial period of speculation in the West as to its true nature, the characteristics of this type were disclosed when the dissolution of the Soviet Union in 1991 and the introduction of a market system forced the Russian industries to find new customers by highlighting its good qualities; it is today rarely referred to as Chobham armour.

In the United Kingdom another line of ceramic armour development had been started in the early 1960s, meant to improve the existing cast turret configuration of the Chieftain that already offered excellent heavy penetrator protection; the research by a team headed by Gilbert Harvey of the Fighting Vehicles Research and Development Establishment (FVRDE), therefore was strongly oriented at optimising the ceramic composite system for defeating shaped charge attack. The British system consisted of a honeycomb matrix with ceramic tiles backed by ballistic nylon, placed on top of the cast main armour. In July 1973 an American delegation, in search of a new armour type for the XM815 tank prototype, now that the MBT-70 project had failed, visited Chobham Common to be informed about the British system, the development of which had then cost about ₤6,000,000; earlier information had already been divulged to the US in 1965 and 1968. It was very impressed by the excellent shaped-charge protection combined with the penetrator impact damage limitation, inherent to the principle of using tiles. The Ballistic Research Laboratory at the Aberdeen Proving Ground that year initiated the development of a version, named Burlington, adapted to the specific American situation, characterised by a much higher projected tank production run and the use of a thinner rolled steel main armour. The increased threat posed by a new generation of Soviet guided missiles armed with a shaped charge warhead—as demonstrated in the Yom Kippur War of October 1973, when even older-generation missiles caused considerable tank losses on the Israeli side—made Burlington the preferred choice for the armour configuration of the XM1 (the renamed XM815) prototype.

However, on 11 December 1974 a Memorandum of Understanding was signed between the Federal Republic of Germany and the US about the common future production of a main battle tank; this made any application of Chobham armour dependent on the eventual choice for a tank type. Earlier in 1974 the Americans had asked the Germans to redesign the existing Leopard 2 prototypes, considered by them too lightly armoured, and had suggested adoption of Burlington for this purpose, of which type the Germans had already been informed in March 1970; the Germans however in response in 1974 initiated a new armour development programme of their own. Having already designed a system that in their opinion offered satisfactory protection against shaped charges, consisting of multiple-laminate spaced armour with the spaces filled with ceramic polystyrene foam as fitted to the Leopard 1A3, they put a clear emphasis on improving KE-penetrator protection, reworking the system into a perforated metal module armour. A version with added Burlington was considered, including ceramic inserts in the various spaces, but rejected as it would push vehicle weight well over sixty metric tonnes, a weight then seen as prohibitive by both armies. The US Army in the summer of 1974 faced the choice between the German system and their own Burlington, a decision made more difficult by the fact that Burlington offered, compared with steel armour, no weight advantage against KE-penetrators: the total armour system would have a RHA equivalence against them of about 350 mm (compared to about 700 mm against shaped charges). No consensus developing, General Creighton Abrams himself decided the issue in favour of Burlington. Eventually each army procured its own national tank design, the project of a common tank failing in 1976. In February 1978 the first tanks protected by Burlington left the factory when the first of eleven pilot M1 tanks were delivered to the US Army.

Beside these state projects, private enterprise in the US during the seventies also developed ceramic armour types, like the Noroc armour made by the Protective Products Division of the Norton Company, consisting of boron carbide sheets backed by resin-bonded glass cloth.



In the United Kingdom application of Chobham armour was delayed by the failure of several advanced tank projects: first that of a joint German-British main battle tank; then the purely British MBT-80 programme. A first directive to prepare Chobham armour technology for application in 1975 was already given in 1969. It was determined by a study of a possible Chobham-armour protected MICV that a completely new design using only Chobham armour for the most vulnerable front and side sectors (thus without an underlying steel main armour) could be 10% lighter for the same level of protection against KE-ammunition, but to limit costs it was decided to base the first design on the conventional Chieftain. The prototype, FV 4211 or the "Aluminium Chieftain", was fitted with a welded aluminium add-on armour, in essence a box on the front hull and front and side turret to contain the ceramic modules, of which box the fifty millimetre thick inner wall due to its relative softness could serve as their backing plate. The extra weight of the aluminium was limited to less than two tonnes and it was shown that it was not overly susceptible to cracking, as first feared. Ten test vehicles were ordered but only the original one had been built when the project was cancelled in favour of the more advanced programmes. However, the Iranian government ordered 1,225 vehicles of an upgraded Chieftain type, the Shir-2 (FV 4030/3), using the same technology of adding Chobham armour to the main cast armour, bringing total weight to 62 metric tonnes. When this order was cancelled in February 1979 because of the Iranian Revolution, the British government, under pressure to modernise its tank fleet to maintain a qualitative superiority relative to the Soviet tank forces, decided to use the sudden surplus production capacity to procure a number of vehicles very close in design to the Shir-2, called the Challenger 1. On 12 April 1983 the first British tank protected by Chobham armour was delivered to the Royal Hussars.

In France from 1966 GIAT Industries performed experiments aimed at developing a light vehicle ceramic armour, in 1970 resulting in the CERALU-system consisting of aluminium-backed alumina weldable to the vehicle, offering a 50% increase in weight-efficiency against ballistic threats compared to steel plate. An improved version was later applied in helicopter seats.

The latest version of Chobham armour is used on the Challenger 2 (called Dorchester armour), and (though the composition most probably differs) the M1 Abrams series of tanks, which according to official sources is currently protected by silicon carbide tiles. Given the publicly stated protection level for the earliest M1: 350 mm steel equivalence against KE-penetrators (APFSDS), it seems to have been equipped with alumina tiles.

Though it is often claimed to be otherwise, the original production model of the Leopard 2 did not use Chobham armour, but a combined spaced armour and perforated armour configuration, cheaper in terms of procurement, maintenance and replacement than a ceramic armour system. For many modern tanks, such the Italian Ariete, it is yet unknown which type is used. There was a general trend in the eighties away from ceramic armour towards perforated armour, but even many tanks from the seventies like the Leopard 1A3 and A4, the French AMX 32 and AMX 40 prototypes used the latter system; the Leclerc has an improved version.

Aerospace applications
The first ceramic plates found application in the aerospace sector: in 1965, the helicopter UH-1 Huey was modified with HFC (Hard-Faced-Composite) around the seats of pilot and copilot, protecting them against small arms fire. The plates were in boron carbide, which, though exceedingly costly due to its superior lightness, has remained the material of choice for aerospace applications. An example among many, the modern V-22 Osprey is protected similarly.