Pulsed Plasma Electrolytic Oxidation as Strategy to Produce Optical Selective Surfaces

Porous alumina (PA) films have been investigated for selective optical surface application. In this work, we use pulsed plasma electrolytic oxidation (PPEO) to produce porous surfaces in aluminum. During the PPEO processing, image of plasma on the samples and active species present in micro-discharges were documented using CCD camera and standard optical emission spectroscopy (OES) technique. Porosity was controlled by varying the pulsewidth ( $T_{\mathrm {on}}$ ) and treatment time, keeping the pulse repetition time ( $T_{\mathrm {off}}$ ) constant. The optical reflectance and thermal efficiency of the samples were investigated for samples with different microstructure, crystalline phase and chemical composition. It is shown that coatings produced by shorter pulses ( $T_{\mathrm {on}}$ ) with smaller time of treatment resulted in smallest pore size and more uniform distribution. These results positively influenced the reflectance and thermal efficiency of the surfaces.