Formation of grooved and porous coatings on titanium by plasma electrolytic oxidation in H2SO4/H3PO4 electrolytes and effects of coating morphology on adhesive bonding

Abstract

The paper reports a change in the morphology of coatings formed galvanostatically on titanium by plasma electrolytic oxidation in phosphoric/sulfuric acid mixtures, and investigated using scanning electron microscopy, X-ray diffraction, and glow discharge optical emission spectroscopy. An initial grooved morphology, containing anatase, is transformed to a more usual porous morphology, which may also contain rutile. The coatings also contain phosphorus species, but comparatively small amounts of sulfur species. The morphological change occurs over a range of cell charge that is strongly dependent on the molar ratio of the acids but weakly dependent on the applied current. With the change in the coating morphology, the efficiency of coating formation reduces and the sparking becomes more localized and intense. Lap shear tests show that the grooved morphology provides a ~ 60% increase in the strength of adhesively bonded joints compared with a porous morphology.