Crystallization of anodic titania on titanium and its alloys

Abstract

Crystallization of amorphous anodic films grown at constant current density on sputtering-deposited titanium, and Ti–Si and Ti–Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35–38% of the film thickness. Oxide zones, of size ∼ 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at ∼40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy.