MIL-A-8625 This specification covers the requirements for SIX types and two classes of electrolyticallyformed anodic coatings on aluminum and aluminum alloys for non-architecturalapplications.

It is typically applied to heavy wear industrial parts intended for use in aggressive or highly corrosive applications.  This type of coating is typically for thicker and harder than normal anodizing on CNC machined parts. 

Again, this type of anodize will lend the parts a durability approaching that of hard faced or case hardened steel.  Hard coated metal usually becomes a dark gray to a black finish.  This can vary depending on the aluminum alloy.  One of the main purposes of hard anodizing is to make the aluminum more resistant to corrosion.  The thicker oxidized layer protects the finished part from being exposed to moisture, oxygen, and various other factors.  Sealed items are even more corrosion resistant. 

The outer coating is also extremely hard, typically much harder than the original metal.  In many cases, a thick hard anodized coating can be as hard as tool steel.  The oxide layer is part of the metal, it won't peel off and the surface finish will increase.  Hard anodized metals are usually very well insulated and they don't conduct heat or electricity well.  Thus this process being especially useful for applications that require the part be used at high temperatures. Therefore, if the threaded hole needs to be conductive it should be masked in anodizing process or should be tapped afterwards.

MIL-A-63576, MILITARY SPECIFICATION: ALUMINUM OXIDE COATINGS, LUBRICATIVE, FOR ALUMINUM AND ALUMINUM ALLOY. This specification covers the requirements for an electrochemical process for building a lubricative anodic coating on aluminum and aluminum alloys. The unsealed anodic coating is impregnated with polytetrafluoroethylene (PTFE) or coated with a resin-bonded materials containing PTFE.

This process also have brandname AnoLube or Altef.