Hard anodizing

Hard anodizing (also known as hard coat anodizing or Type III anodizing) is an advanced electrochemical process that converts the surface of aluminum (and certain other non-ferrous metals) into a thick, dense layer of aluminum oxide. Hard anodizing is a much thicker and harder oxide layer (typically 25–100+ microns) compared to standard (Type II) anodizing.
The resulting coating is ceramic-like, extremely well-integrated with the base material.