FRICTION PROPERTIES OF MАO COATINGS ON ALUMINUM ALLOYS
Authors: Subbotin Oleksandr , Bilozerov Valery , Volkov Oleg , Subbotinа Valeria , Shevtsov Vadym 
Published: December 2022

Aluminum alloys are widely used in mechanical engineering due to their physical and mechanical properties. However, their low wear resistance and burr resistance limit their use in friction nodes. In this regard, parts made of aluminum alloys operating under conditions of sliding friction require surface strengthening, which determines the friction wear parameters. Research works in this direction allow increasing the reliability and service life of friction nodes. Ensuring the compatibility of friction pairs allows reducing the coefficient of friction and, as a result, to increase wear resistance. One of the effective methods of forming wear-resistant coatings on aluminum alloys is the method of micro-arc oxidation (MAO). The use of micro-arc discharges as highly concentrated energy sources to create conditions for high-temperature electrotechnical processes and phase transformations in the surface layer makes it possible to form structures based on high-temperature oxides on the surface of metals, which provides qualitatively new surface properties: high hardness, wear resistance, corrosion resistance in various environments. However, the use of oxide coatings in bearings is mainly due to their high wear resistance and corrosion resistance. Information on the antifriction properties of aluminum oxide-based coatings is limited. The purpose of this work is to determine the coefficient of friction of MAO-coatings on D16 aluminum alloy in a pair with cast iron, steel and MAO-coating under conditions of oil and water lubrication, and to find out the feasibility of using MAO technology to create sliding bearings. The formation of MAO coatings was carried out in an alkaline-silicate electrolyte in the cathodic-anodic mode. The structure and morphology of the working layer of the MAO-coating surface, which works in friction pairs with cast iron, steel and MAO-coating under conditions of oil and water lubrication, was investigated. The benefit of the available porosity of the working layer of the MAO coating, which is 5-10% and contributes to the retention of lubricant in the friction zone, is shown. The phase composition, hardness of the coatings were studied and the coefficient of sliding friction was determined on the SMC-2 friction machine according to the "disk-pad" scheme when using water and oil as lubricants; the load varied from 0.2 to 2 kN, the nominal pressure from 1 to 11 MPa. It was established that the MAO coating has a crystalline structure, the phase composition is high-temperature modifications of aluminum oxide (α-Al2O3 and γ- Al2O3), and the hardness is 18000-20000 MPa. It was established that the coefficient of sliding friction of gray cast iron - MAO-coating, steel - MAO-coating and MAO on MAO decreases to the value of 0.01 - 0.013 under lubrication conditions. It was found that in a pair of MAO-coating - MAO-coating, anti-friction properties are manifested both under conditions of lubrication with oil and water. The low coefficient of friction for the coating-coating pair in the case of water lubrication is explained by the high hydrophilic properties of the oxide contacting surfaces, which makes it possible to recommend them for use in sliding bearings (liners) of hydroturbines. A conclusion was drawn regarding the reasons for the low coefficient of friction of the studied samples.
Keywords: D16 aluminum alloy, cast iron, steel, micro-arc treatment, coating, phase composition, crystal structure, aluminum oxides, coating thickness, coating hardness, friction coefficient, antifriction properties