Combination K was a type of composite armor.

The mix

Combination K is a three-layer composite armor consisting of outer and inner layer of steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 and middle layer of glass-reinforced plastic

Glass-reinforced plastic

Fiberglass , is a fiber reinforced polymer made of a plastic matrix reinforced by fine fibers of glass. It is also known as GFK ....

 ("steklotekstolit") and a pack of ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 (allegedly corundum

Corundum

Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

) plates.

The T-72

T-72

The T-72 is a Soviet-designed main battle tank that entered production in 1970. It is developed directly from Obyekt-172, and shares parallel features with the T-64A...

 tanks have their gun turret

Gun turret

A gun turret is a weapon mount that protects the crew or mechanism of a projectile-firing weapon and at the same time lets the weapon be aimed and fired in many directions.The turret is also a rotating weapon platform...

 frontal armour composed of three layers: outer and inner layers of steel and middle layer of sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

 or quarts ("kvartz", probably the origin of the "K" in the name; also implies some relationship between the T-64A and the kvartz composite). The gun turrets are cast

Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

 with internal cavities on each side of the main gun

Tank gun

A tank gun is the main armament of a tank. Modern tank guns are large-caliber high-velocity guns, capable of firing kinetic energy penetrators, high explosive anti-tank rounds, and in some cases guided missiles. Anti-aircraft guns can also be mounted to tanks.-Overview:Tank guns are a specific...

, which are later filled with the desired composite material

Composite material

Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

. Some Russian sources describe the material as "peschannye sterzhni" ("sand rods" or more likely "sandbars"), likely made of a form of silica similar to fused silica developed in the USA for the T95

T95

The T95 was an American prototype medium tank developed from 1955 to 1959. These tanks used many advanced or unusual features, such as siliceous-cored armor, a new transmission, and the OPTAC fire-control system...

 prototype medium tank

Medium tank

Medium tank was a classification of tanks; the medium being intermediate in size and weight and armament between heavy tanks and light tanks.The medium tank concept has been eclipsed by the main battle tank.-History:...

s in the 1952 program for equipping tanks with protection against shaped charge

Shaped charge

A shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, to initiate nuclear weapons, to penetrate armor, and in the oil and gas industry...

s and HEAT rounds without sacrificing protection against kinetic energy penetrator

Kinetic energy penetrator

A kinetic energy penetrator is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target....

s. The same design of cast-steel turret with cavities for composite insets is reportedly used also in the more modern T-80

T-80

The T-80 is a main battle tank designed and manufactured in the former Soviet Union. A development of the T-64, it entered service in 1976 and was the first production tank to be equipped with a gas turbine engine for main propulsion.the Swedish Stridsvagn 103 of 1971 used a gas turbine alongside...

 and T-90

T-90

The T-90 is a Russian third-generation main battle tank that is a modernisation of the T-72 . It is currently the most modern tank in service with the Russian Ground Forces and Naval Infantry...

tanks.