is a material used as far as I know on drill bits
to allow some benefit to the process of drilling. Probably to increase tool life as a company that makes drill bit
s would say (they'll say anything about the tool or its accesories increases its life).
Black oxide is an ionic compound of either Cupric oxide, CuO, or Mercurous oxide, Hg2O. I believe the black oxide that is put on drill bits is cupric oxide since putting mercurous oxide would endanger the user's health.