Corundum armor inserts - Ultra Porcelain

 In 1975, the T-64A turret with corundum filler developed by VNIITM was adopted. The technology of casting turrets with ceramic filler was developed as a result of joint work of VNII-100, Kharkiv plant No. 75, and South Ural radio-ceramics plant.

The armour of the turret consisted of; 115mm cast armour steel > 140 mm ultra-porcelain balls > rear wall 135 mm steel with an inclination angle of 30 degrees.

View of a Corundum ball (Ultra porcelain)

ВНИИТМ (Всероссийский научно-исследовательский институт транспортного машиностроения) VNIITM -- All-Russian Research Institute of Transport Engineering (Mobile Vehicle Engineering Institute)

An array of Corundum ball inserts

Subsequently, the armour protection of Kharkiv tanks was improved, including in the direction of using more advanced barrier materials, at the end of the 1970s and onwards T-64Bs used the BTK-1Sh type steel, made by Electro-Slag remelting. From this armour composition change, the resistance of an equal-thickness sheet obtained by ESR (Electro-Slag remelting) is 10 ... 15% more than armour steels of increased hardness, which was previously used on earlier T-64A variants with 2 internal layers of increased hardness steel. 

T-64A turret with high hardness steel inserts.

Combined armour barrier using ultra porcelain balls in the front of the turret on the Object "432" medium tank.

General view of the entire Corundum ball array.

Taken from: 

-- Main Tank T-64. 50 Years in the ranks Book 

--BTVT