Structural characteristics and residual stresses in oxide films produced on Ti by pulsed unipolar plasma electrolytic oxidation

Oxide films, 7-10 μm thick, were produced on commercially pure titanium by plasma electrolytic oxidation in a sodium orthophosphate electrolyte using a pulsed unipolar current with frequency (f) and duty cycle (δ) varying within f = 0.1-10 kHz and δ = 0.8-0.2, respectively. The coatings comprised a mixture of an amorphous phase with nanocrystalline anatase and rutile phases, where the relative rutile content range was 17-25 wt%. Incorporation of phosphorus from the electrolyte into the coating in the form of PO₂^-, PO₃^2- and PO₄ ^3-, as demonstrated by EDX and FT-IR analyses, contributed to the formation of the amorphous phase. Residual stresses associated with the crystalline coating phase constituents were evaluated using the X-ray diffraction sin² ψ method. It was found that, depending on the treatment parameters, internal direct and shear stresses in anatase ranged from-205 (±17) to-431 (±27) MPa and from-98 (±6) to-145 (±10) MPa, respectively, whereas the rutile structure is comparatively stress-free.