INFLUENCE OF OXIDATION TIME PARAMETERS ON THE COMPOSITION AND MORPHOLOGY OF Al2O3 Al2O3•CoXOY CATALYTIC COATINGS

Abstract

The process of formation of mixed oxide coatings on a high-silicon aluminum alloy in the cobalt-containing diphosphate electrolyte by the method of plasma-electrolytic oxidation is investigated. The chronographs of the molding voltage of the investigated system have a classical form with the division into characteristic regions. It was shown that the heterogeneity of the chemical composition of AL25 causes the consumption of part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It was established that the growth of the relative mass of the formed layer of mixed oxides Al2O3·CoxOy is a function of time. Dependence is extreme with a maximum of 55 minutes. The chemical composition and surface morphology of the oxide layer formed depend on the time of oxidation. The content of the catalytic component in the surface layers varies from 0.2 to 23.3 at. % with an increase in processing time of 10 to 60 minutes. Maximum incorporation of cobalt into the oxide layer occurs at PEO for 35–50 minutes, while the silicon content in the surface layers does not exceed 2 at. %, which is favorable for the catalytic properties of the material. The inclusion of cobalt is visualized by the intersperses of blue-violet color in the places of combustion of micro-arc discharges. The formed mixed layer of alumina and cobalt oxides are characterized by a developed microglobular structure formed by conglomerate spheroids with an average size of 1–2 microns. The formed oxide layer consists of α-Al2O3 with incorporated Co3O4 phases. The presence of an amorphous halo is due to the formation of a structure in non-equilibrium conditions. The set of detected factors is a prerequisite for the high catalytic properties of the resulting coatings. A promising field of application of Al2O3·CoxOy systems is an internal cylinder catalysis of internal combustion engines.