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

Oxide films, 7-10 mu m 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 (3) varying within f=0.1-10 kHz and delta=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-, PO32- and PO43- 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 sin(2) psi 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.