Study of duty cycle influence on the band gap energy of TiO2/P coatings obtained by PEO process

Abstract

Plasma Electrolytic Oxidation (PEO) processing was used to produce ceramic coatings of TiO2/P, on commercially pure titanium sheets varying the energy per cycle supplied by the power source. Specifically, this work was focused on evaluating the influence of the variations of the duty cycle on the band gap energy of the obtained TiO2/P coatings. The coatings were produced in a base electrolyte composed of Na3PO4 and NaOH. The PEO processing was carried out using a constant voltage of 356 V, a pulse frequency of 2000 Hz and duty cycle variations of 1%, 10%, 30% and 50% with a total processing time of 7 min per sample. X-ray Diffraction (XRD), Raman Spectroscopy (RS), Atomic Force Microscopy (AFM), and Infrared Spectroscopy (IRS) were used to characterize the structural phases of the coatings, the surface morphology, and the concentration of phosphate, respectively. According to the analysis carried out by AFM, as the duty cycle increases, the roughness increases and the pores surrounded by nodular regions begin to appear in the discharge channel during oxidation. XRD showed an amorphization of the anatase phase as the duty cycle increases, which is considered a stable intermediate state between anatase and rutile. From RS, the band corresponding to the rutile structure was evident for the sample with 50% of the duty cycle. Finally, diffuse reflectance spectroscopy revealed that increases in the duty cycle from 10% to 50%, induced a change in the band energy of the coatings from 3.29 to 3.02 eV.