Application of optically black lightabsorbing coating on aluminum and titanium alloys

In the aerospace industry aluminum and titanium alloys are the bulk of engineering materials. This is due to their high specific characteristics, as well as manufacturability. An aircraft in outer space is exposed to a number of factors: flows of high-energy ions, plasma, solar energy, heat flows, meteor particles, etc. As a result of their impact, processes can occur in the materials of a spacecraft (SC) that cause a change in the initial properties of the material or SC equipment. At the launch stage the spacecraft experiences significant vibration and thermal loads. Based on this, it is of great interest to improve the characteristics of the structural engineering materials used.

One such method is micro-arc oxidation (MAO). MAO is an electrochemical process of modifying the surface of valve metals, which is accompanied by the action of plasma discharges on the workpiece. A distinctive feature of micro-arc oxidation is the participation of surface microdischarges in the process of coating formation, which have an enormous and specific impact on the coating being formed, as a result of which the composition and structure of the obtained oxide layers significantly differ, and the properties are greatly improved compared to the conventional anode films [1–3].

The properties of MAO coatings are determined by their structure and composition, which, in turn, depend on the base material, electrolyte composition, and processing mode. When describing the characteristics of the coating, the following parameters are taken into account: thickness, microhardness, breakdown voltage, heat resistance, corrosion resistance, wear resistance, porosity, etc.

Coatings obtained by micro-arc oxidation own unique physical and mechanical properties, however, in practice researchers are faced with the problem of obtaining the coating for the rocket and space industry within a certain color range.

Therefore, for example, products with black MAO coating can be used for cases involving onboard spacecraft systems or in the thermal control system to maintain the required operating temperature of the spacecraft elements due to the property of black color to absorb the entire visible light spectrum.

However, obtaining coating of a certain color is associated with a number of factors that affect the final result. First, the composition of the electrolyte has a significant impact on the color of the formed coating. The authors of various works have repeatedly noted that the key role in the formation of a certain color of the MAO coating is played by the content of transition metal salts in the electrolyte [1–6]. In this paper, electrolytes for the formation of a black coating are considered.

Secondly, compound and main components of metal alloys react with electrolyte components, forming a coating on the sample surface; therefore, the MAO coating color changes on different alloys of the same valve metal. So, for example, if in a slightly alkaline electrolyte process two different aluminum alloys, one of which contains 5% manganese, it will have a white tint, the other aluminum alloy with a copper content of 5% will posess a greenish tint after processing. A similar result is observed in the processing of various valve metals and their alloys.

Thirdly, processing parameters, such as duration, current density, the ratio of the anode and cathode components of the current and others are important in obtaining a coating with a certain color shade and physical and mechanical characteristics.

The aim of the work is to determine the composition, concentration of electrolyte components and processing modes of MAO aluminum and titanium alloys to obtain optically black light-absorbing coating with high physical and mechanical characteristics.