Design of a Two-Layer Al–Al2O3 Coating with an Oxide Layer Formed by the Plasma Electrolytic Oxidation of Al for the Corrosion and Wear Protections of Steel

The article analyses technologies for the formation of coatings to protect machine and equipment parts from corrosion and wear in aggressive environments. The study considers plasma electrolytic oxidation (PEO) regimes of valve metal materials on the core, microstructure, and physical and mechanical properties of oxide coatings. To solve this problem, it is promising to develop a combined technology: application of a layer of aluminium on the surface of steel parts followed by its PEO. The purpose of this work is a theoretical study of the temperature distribution in a steel cylinder covered with a layer of aluminium during PEO to substantiate the thickness of the layers of the two-layer aluminium–aluminium oxide coating. An installation for PEO and a technological process of forming a two-layer Al–Al2O3 coating on long-dimensional parts are developed. A mathematical model is created for an infinite three-layer cylinder with an internal coaxial surface cylindrical source of heat that moves at a constant speed in the radial direction deep into the aluminium layer of the cylinder. It is based on solving the boundary value problem of thermal conductivity with the condition of ideal thermal contact between the layers. During PEO, a solid oxide layer is formed, but the thickness of the unoxidized aluminium layer decreases. The thermophysical characteristics of such a cylinder are functions of both the radial co-ordinate of the cylinder and the time. During the construction of a two-layer coating, it is advisable to use the results of thermal calculations to justify the thickness of the unoxidized aluminium layer adjacent to the surface of the steel under the condition of ensuring the flow of mutual diffusion processes at the interface between aluminium and steel to increase the adhesion strength of the coating to the base due to heating by instantaneous heat sources caused by the action of electric-spark discharges in the PEO process. The thickness of the outer solid layer of aluminium oxide is chosen based on the condition of ensuring the necessary service life of machine parts for wear with a certain reservation. The results of mechanical, tribological, and corrosive cracking tests for steel samples with the developed two-layer Al2O3 coating show its high operational properties.