Purified titanium oxide with novel morphologies upon spark anodization of Ti alloys in mixed H2So4/H3PO4 electrolytes

Abstract

The spark anodization behavior of alpha/beta Ti6Al4V and Ti6Al7Nb alloys and of alpha c.p. Ti in H2SO4, H3PO4, and mixtures of these acids was studied. Chemical depth profiling revealed oxides purified with respect to the substrate alloying elements. This was particularly pronounced on Ti alloys spark anodized in H2SO4/H3PO4 mixtures, the Al content decreasing continuously towards the surface, and V and Nb hardly detectable in the outermost 200 nm. The incorporation of S was significantly reduced in mixed electrolytes, while about 8 at-% P was present. A novel oxide morphology with "worm-like" features in the micrometer range, very different from well-known nano/microporous oxides, was found in mixed electrolytes under suitable conditions. Similar but more porous-like morphologies were formed on Ti. Simple alpha/beta substrate microstructural considerations cannot explain the morphological and chemical observations. Raman spectroscopy indicated the presence of mixed anatase, rutile, and brookite phase on anodized Ti alloys. Bond strengths of 34 MPa for worm-like and 40-50 MPa for nano/microporous morphologies as well as excellent abrasion behavior were found. The compatibility of grit-blasting with the spark anodization process for creating multitopography surfaces was demonstrated. Neither the observed chemical effects, nor the observed particular morphology or the presence of brookite have been reported before.