From Gineipaedia, the Legend of Galactic Heroes wiki
However in-depth observation of the ship construction facilities in the Free Planets Alliance by civilian experts allowed for some deduction to be made. It is thought that ship armour was composed of ceramic composites and monocrystalline metals.
Analysis was also possible on an Alliance public release of a 3 dimensional image of a destroyed Imperial battleship from the Battle of Amritsar. The destroyed ship had numerous hull breaches allowing for a cross-sectional examination of Imperial armour. It showed a 7 to 8 layer construction, composed of monocrystalline metal layers sandwiching foam ceramic layers.
Civilian estimates place standard battleship armour thickness at approximately 5000mm around the bridge, engines, and other critical areas, and thinning to 500mm to 1000mm on the ship's flanks. (Data Book: Mechanic & Seiyū Encyclopaedia, p. 189-190)