FWIW, Fatshark did mention details on the brittleness coating in their news post about the DLC class:
When using the Brittleness coating, the weapon applies on-hit a small amount of stacks of an unique Hive Scum Brittleness debuff, granting additional damage from any source against the more hardened armour types.
This effect has a limit of 15 stacks (for a total of 37.5% Brittleness), but can be applied in addition to other forms of Brittleness without sharing the same maximum cap.