Busoshoku: Koka is a specific form of Busoshoku Haki.
This technique somehow uses Busoshoku Haki to harden the body (or parts of it) and weapon which the user is holding. It was first seen used by Monkey D. Luffy, in combination with Gear Third, in order to use his Gomu Gomu no Elephant Gun against the Kraken. The arm became black and shiny and notably did not cause the usual side effect for Gear Third on release. Vergo, Smoker and Zephyr are able to either use this technique or do something similar to it as they have demonstrated an exact likeness of this Haki technique; Zephyr had earned the nickname "Black Arm" because of his mastery of it. It can also be imbued into weapons, as Vergo utilized it on a bamboo stick, and Smoker on his Nanashaku Jitte, the latter indicating Busoshoku Haki can be used in conjuncture with Kairoseki.
Allows the user to add 1/3rd Willpower to their attacks and Defense. This effect replaces normal Armament Haki.Weapons infused with Armament: Hardening do not extend the effect bonus to ranged projectiles such as Vacuum Waves or bullets, as those use standard Armament Haki.