Influence of current density and electrolyte concentration on DC PEO titania coatings

Titania films, 5–9 μm in thickness, were produced on commercial purity titanium by a direct current (DC) plasma electrolytic oxidation (PEO) process using 5 to 20 A dm⁻² current densities in 5 to 15 g L⁻¹ trisodium orthophosphate electrolytes. Phase analyses (composition and crystallite size) were carried out using X-ray diffraction and TEM techniques. Residual stresses associated with one of the crystalline coating phases (anatase) were evaluated using the X-ray diffraction Sin²ψ method. SEM and TEM techniques were utilised to study the cross-section and surface morphologies and nanofeatures of the PEO titania coatings. Variations in the proportions of anatase and rutile phases, surface morphologies, coating thicknesses and associated internal stresses are correlated with the current density and electrolyte concentration used during the DC PEO process on titanium. Direct and shear stresses in the anatase are found in the range −70 to −331 and −17 to −167 MPa respectively.