Tribological Characteristics Improvement of Wear Resistant MAO-Coatings

Currently, the most promising technology of coating formation is microarc oxidation (MAO) with unique properties of the surface layer, which combine high wear resistance, corrosion resistance, and heat and erosion resistance. Microarc oxidation can be used for parts and components manufacturing in various segments of industries. However, the technology improvement by improving the tribological characteristics of MAO-coatings can not only enhance economic effect, but also expand its application. 1. Introduction Microarc oxidation (MAO) method was achieved in the early 1970s of the 20th century as the result of discovering microarc discharges phenomenon in electrolyte by Markov in the Institute of Inorganic Chemistry [1, 2]. This is quite a new method of improving the surface strength. Coatings with high wear resistance and high adhesive strength to base material can be achieved by this method. The essence of the method is the collaboration of electrochemical oxidation and electrodischarges phenomena at the anode-electrolyte border. The term “microarc oxidation” means electrochemical oxidation of the anode (or detail part) under the voltage higher than the voltage of sparking. The most important advantages of the method include(i)coating deposition without any pretreatment (grinding, degreasing, pickling, etc.), which significantly reduces the cost and complexity of work;(ii)the possibility of coating figurine details (including internal and threaded surfaces, hidden cavities);(iii)coating formation with thickness from 0.05 to 0.2？mm and 0.3 to 1.0？mm and adhesion, comparable to the strength of the base material;(iv)significant oxidation process time reduction (2–2.5 times) in comparison with thick anodizing;(v)the possibility of full process automation; (vi)cheapness and availability of reagents and materials; (vii)wide range of process speed regulations; (viii)environmental friendliness of the process (does not require the use of special sewage treatment plants). Microarc oxidation is mainly used for valve metals. The composition of MAO-coatings is a complex of aluminum oxides, including high-temperature phase (α-Al2O3) as well as element compounds which are included in the base material and the electrolyte. Today, ceramic coatings are widely used on aluminum alloys. The driving force behind this has been the high chemical affinity for oxygen and the unique properties of Al2O3 oxide. Among the positive qualities of aluminum alloys, their lightness, high relative strength, high corrosion resistance, cold resistance, good weld ability, and high