Chobham armour
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[[Image:SolOline-2005-04-27-095150.jpg|thumb|The [[M1 Abrams]], a [[main battle tank]] protected by Chobham armour]]
'''Chobham armour''' is the name informally given to a [[composite armour]] developed in the 1960s at the [[United Kingdom|British]] [[tank]] research centre on [[Chobham Common]]. The name has since become the common generic term for ceramic [[vehicle armour]].
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 charge]]s such as [[high explosive anti-tank]] (HEAT) rounds and they shatter [[kinetic energy penetrator]]s.
== Structure ==
Ceramic tiles have a "multiple hit capability" problem in that they are unable to sustain successive impacts without quickly losing much of their protective value.<ref>W.S. de Rosset and J.K. Wald, "Analysis of Multiple-Hit Criterion for Ceramic Armor", ''US Army Research Laboratory TR-2861, September 2002''</ref> 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 an inch / 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,<ref>Bruchey, W., Horwath, E., Templeton, D. and Bishnoi, K.,"System Design Methodology for the Development of High Efficiency Ceramic Armors", ''Proceedings of the 17th International Symposium on Ballistics, Volume 3, Midrand, South Africa, March 23-27, 1998'', p.167-174</ref> or by glueing them with an [[epoxy]] resin. Since the early nineties it is known that bringing the tiles under constant compression by their matrix greatly improves their resistance to kinetic penetrators, which is difficult to achieve when using glues.<ref>Hauver, G.E., Netherwood, P.H., Benck, R.F. and Kecskes, L.J., 1994, "Enhanced Ballistic Performance of Ceramics", ''19th Army Science Conference, Orlando, FL, June 20-24, 1994'', p. 1633-1640</ref>
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.<ref> V. Hohler, K. Weber, R. Tham, B. James, A. Barker and I. Pickup, "Comparative Analysis of Oblique Impact on Ceramic Composite Systems", ''International Journal of Impact Engineering'' '''26''' (2001) p. 342</ref> 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 [[vibration]]s. 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.<ref>D. Yaziv1, S. Chocron, C.E. Anderson, Jr. and D. J. Grosch, "Oblique Penetration in Ceramic Targets", ''19th International Symposium of Ballistics, 7–11 May 2001, Interlaken, Switzerland'' '''TB27''' p. 1264</ref> 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 thin layer of ductile [[graphite]] 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 took place: ceramic tiles, as such 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 is added to optimise impact resistance.<ref>Yiwang Bao, Shengbiao Su, Jianjun Yang, Qisheng Fan, "Prestressed ceramics and improvement of impact resistance", ''Materials Letters'' '''57''' (2002) p. 523</ref> To confine the ceramic core several advanced techniques are used, supplementing the traditional machining and welding, including [[sintering]] the supension material around the core; squeeze casting of molten metal around the core and spraying the molten metal onto the ceramic tile<ref>Chu, Henry S; McHugh, Kevin M and Lillo, Thomas M, "Manufacturing Encapsulated Ceramic Armor System Using
Spray Forming Technology" ''Publications Idaho National Engineering and Environmental Laboratory'', Idaho Falls, 2001</ref>.
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 composite]]s which combine ceramic compounds with metal. The latest developments involve the use of [[carbon nanotube]]s 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 Syndie, 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 plate]]s protecting against smaller projectiles, such as used in [[body armour]] and armoured [[helicopter]]s; this was in fact in the early sixties the first general application of ceramic armour<ref>S. Yadav and G. Ravichandran, "Penetration resistance of laminated ceramic/polymer structures", ''International Journal of Impact Engineering'', '''28''' (2003) p. 557''</ref>. 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 [[armoured fighting vehicle|AFVs]] only to be protected against light [[anti-tank weapon]]s. A deformable composite backing plate can combine the function of a metal backing plate and an elastic layer.
=== Heavy metal modules ===<!-- This section is linked from [[Depleted uranium]] -->
The armour configuration of the first western tanks using Chobham armour was optimised to defeat shaped charges as [[guided missile]]s 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. They typically form an inner layer placed below the much more expensive matrix<ref>[[Tom Clancy|Clancy, Tom]], ''Armored Cav — a guided Tour of an Armored Cavalry Regiment'', New York 1994, p. 65</ref>, 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; furthermore, due to their extreme hardness, they deform insufficiently and would reflect too much of the impact energy to the ceramic tile. Metals used include a [[tungsten]] alloy for the [[Challenger 2]]<ref>Claessen, Luitenant-kolonel A.H.J., ''Tanks & Pantserwagens — De Technische Ontwikkeling'', Blaricum, 2003, p. 96</ref> or, in the case of the [[M1 Abrams|M1A1HA]] (Heavy Armor) and later American tank variants, a [[depleted uranium]] alloy<ref>''M1 Abrams Main Battle Tank'', p. 13</ref>.
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"<ref>Gelbart, Marsh, ''Tanks — Main Battle Tanks and Light Tanks'', London 1996, p. 126</ref> 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"<ref>''Armored Cav — a guided Tour of an Armored Cavalry Regiment'', p. 61</ref>.
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"<ref>Gelbart, Marsh, ''Tanks — Main Battle Tanks and Light Tanks'', London 1996, p. 114</ref>.
== 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 penetrator]]s. The (pulverised) ceramic also strongly [[abrasion|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<ref>Chang, Albert L. and Bodt Barry E., "JTCG/AS Interlaboratory Ballistic Test Program — Final Report", ''Army Research Laboratory - TR-1577 - December 1977'' p. 12</ref> . 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"<ref>Chan, H.M., "Layered ceramics: processing and mechanical behavior", ''Ann Rev Mater Sci 1997''; '''27''': p. 249–82</ref>. This mechanism using the jet's own energy against it, has caused some to compare the effects of Chobham to those of [[reactive armour]]. This should not be confused with the effect used in many laminate armour 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 [[APFSDS|long-rod penetrator]], after the first layer has been perforated and 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 is 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.
The effectiveness of Chobham armour was demonstrated in the [[Iraq war (disambiguation)|Gulf Wars]] of 1991 and 2003, where no Coalition tank was destroyed by either the obsolete Iraqi armour or [[Anti-tank guided missile|ATGW]]s. In some cases the tanks in question were subject to multiple hits by both [[kinetic energy penetrator|KE-penetrators]] and [[HEAT]] rounds, but the old Russian ammunition used by the Iraqis, in their Polish licence built [[T-72]]s, their old [[T-55]]s bought from [[Russia]] and upgraded with "Enigma" type armour, and [[T-62]] tanks left them completely incapable of penetrating the front armour of Coalition tanks. It is also worth noting that the Iraqis rarely actually hit the Coalition tanks, because of lack of training and inferior optics. To date, only 5-10 Chobham-protected tanks have been defeated by enemy fire in combat, including an M1 that was hit on the side skirts, below the turret ring by a [[PG-7VR]], a [[tandem charge]] [[rocket propelled grenade|RPG]], in the [[Iraq War]]. The jet penetrated the skirting armour and side hull armour, then traversed across the tank's interior and finally penetrated 1.5 to 2 inches into the hull armour on the other side.
== Development and application ==
Since the early sixties there were in the USA extensive research programmes ongoing aimed at investigating the prospects of employing composite ceramic materials as vehicle armour<ref>Hanby, K.R., ''Fiber-Reinforced Metal-Matrix Composites-1967'', Defense Metals Information Center DMIC-S-21, MCIC-005839 PL-011311 MMC-700204</ref>. This research mainly focused on the use of an aluminum metal matrix composite reinforced by silicon carbide whiskers, to be produced in the form of large sheets.<ref>Kolkowitz, W. and Stanislaw, T.S., "Extrusion and Hot Rolling - Two Advanced Fabrication Techniques for the Preparation of Whisker-Metal Composites", ''Proceedings of the 14th National Symposium and Exhibit, Vol. 14 - 'Advanced Techniques for Material Investigation and Fabrication', 5-7 Nov 68, Cocoa Beach, Florida, Paper No. 11-4A-3''</ref> The reinforced light metal sheets were to be sandwiched between steel layers.<ref>''M1 Abrams Main Battle Tank'', p. 5</ref> 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<ref>Trinks, Walter, "Hohlladungen und Panzerschutz — Ihre wechselweise weiterentwicklung", ''Jahrbuch der Wehrtechnik 8'', 1974, p. 156</ref>.
An alternative technology developed in the USA was based on the use of glass modules to be inserted into the main armour;<ref>''M1 Abrams Main Battle Tank'', p. 5</ref> 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]];<ref>''Soviet/Russian Armor and Artillery Design Practices'', p. 88</ref> 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.<ref>''Soviet/Russian Armor and Artillery Design Practices'', p. 92</ref> It was, later in improved forms, part of 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 and the introduction of a market system forced the Russian industries to find new customers by highlighting its good qualities;<ref>''Soviet/Russian Armor and Artillery Design Practices'', p. 164-169</ref> it is today rarely referred to as Chobham armour.
Meanwhile in the United Kingdom another line of ceramic armour development had been started, meant to improve the existing cast turret configuration of the [[Chieftain tank|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, was thus strongly oriented at optimising the ceramic composite system for defeating shaped charge attack<ref>Kelly, Orr ''King of the Killing Zone: The Story of the M-1, America's Super Tank'', New York 1989, p. 111</ref>. The British system consisted of a honeycomb matrix with ceramic tiles backed by ballistic nylon<ref>Long, D., ''Modern Ballistic Armor — Clothing, Bomb Blankets, Shields, Vehicle Protection'', Boulder 1986, pp. 82-84</ref>, placed on top of the cast main armour.<ref>''M1 Abrams Main Battle Tank'', p. 5</ref> 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. 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 with 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 exemplified by the events of 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.<ref>''M1 Abrams Main Battle Tank'', p. 6</ref>
However, on 11 December 1974 a Memorandum of Understanding was signed between the Federal Republic of Germany and the USA 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 to adopt ''Burlington'' for this purpose; the Germans however in response that year initiated a new armour development programme of their own.<ref>Spielberger Walter J., ''Von der Zugmachine zum Leopard 2'', München 1980, p.230</ref> 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<ref>Van Zelm, G. and Fonck B.A., "Leopard-1 Gevechtstank", ''De Tank, Juni 1991'' p. 53</ref> as fitted to the [[Leopard 1]]A3, 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<ref>Claessen, Luitenant-kolonel A.H.J., ''Tanks & Pantserwagens — De Technische Ontwikkeling'', Blaricum, 2003, p. 95</ref>. 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:<ref>''Armored Cav — a guided Tour of an Armored Cavalry Regiment'', p. 5</ref> the total armour system would have a [[RHA]] equivalence against them of about 350 mm (compared to about 700 mm against shaped charges).<ref>''M1 Abrams Main Battle Tank'', p. 9-10</ref> No consensus developing, General [[Creighton Abrams]] himself decided the issue in favour of Burlington.<ref>Kelly, Orr, ''King of the Killing Zone: The Story of the M-1, America's Super Tank'', New York 1989, p. 121</ref> Eventually each army would procure its separate 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.
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. The [[Iran]]ian government had ordered 1,225 vehicles of an upgraded Chieftain type, the ''Shir-2'' (FV 4030/3), with Chobham armour added 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]].
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 ([[Kinetic energy penetrator|APFSDS]]), it seems to have been equipped with [[alumina]] tiles.{{Or|date=September 2007}}
Though it is often claimed to be otherwise, the [[Leopard 2]] in fact does not use Chobham armour, but pure [[perforated armour]], avoiding the very large procurement, maintenance and replacement costs of those ceramic armour systems not based on the cheap but rather ineffective alumina. For many modern tanks, such as the Japanese [[Type 90]] and the Italian [[Ariete]], it is yet unknown which type is used. There is a general trend away from ceramic armour towards perforated armour; but even many tanks from the seventies like the Leopard 1A3 and A4, the Italian [[OF-40]] and the French [[AMX-32]] and [[AMX-40]] prototypes used the latter system; the [[Leclerc]] has an improved version.
== Notes ==
{{reflist|2}}
== References ==
{{refbegin}}
* {{ cite book | last = Hull | first = Andrew W | coauthors = Markov, David R. and Zaloga, Steven J. | title = Soviet/Russian Armor and Artillery Design Practices: 1945 to Present | publisher = Darlington Productions, Darlington | year = 2000 }}
* {{ cite book | last = Zaloga | first = Steve | title = M1 Abrams Main Battle Tank 1982-1992 | publisher = Osprey Publishing Ltd., London | year = 1993 }}
* {{ cite book | last = Clancy | first = Tom | title = Armored Cav — a guided Tour of an Armored Cavalry Regiment | publisher = Berkley Books, New York | year = 1994 }}
{{refend}}
== Further reading ==
Jeffrey J. Swab (Editor), Dongming Zhu (General Editor), Waltraud M. Kriven (General Editor); ''Advances in Ceramic Armor: A Collection of Papers Presented at the 29th International Conference on Advanced Ceramics and Composites, January 23-28, 2005, Cocoa Beach, Florida, Ceramic Engineering and Science Proceedings, Volume 26, Number 7''; ISBN 1-57498-237-0
== External links ==
* [http://homepage.ntlworld.com/alan-turnbull/eye-spy-uk-secrets3.htm Article on DSTL/QinetiQ Chertsey and Longcross Test Track (Chobham Tank Research Centre)]
[[Category:Vehicle armour]]
[[Category:Composite materials]]
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