Low-temperature deposition process for black phosphate-selenide coatings

Abstract

Oxide coatings, which can be obtained by thermal, thermochemical, chemical and electrochemical methods, are widely used as protective and decorative coatings on ferrous metals under non-harsh operating conditions. Each of them finds its own, the most appropriate area of application. For protective and decorative oxidation, the most widely used chemical oxidation method with the use of alkaline solutions containing 600–800 g/l NaOH, which makes it possible to obtain black or dark blue coatings up to 3 μm thick at temperatures of 130–150 °С. The obvious disadvantages of the alkaline oxidation process are high energy consumption, difficult working conditions and aggressiveness of the solutions used. In addition, coatings formed in alkaline solutions are highly porous and therefore can be used as protective coatings only in light climatic conditions [1, 2]. A more environmentally friendly and energy efficient alternative to protective and decorative black oxide coatings could be black phosphate coatings formed in low-temperature solutions. The relevance of the development of a lowtemperature process for the deposition of black coatings is also due to the following fact. It is known that to harden the surfaces of machine parts, the durability of which is determined by their wear resistance, they are thermally laser treated. In this case, surface hardening is the more effective, the higher the absorption coefficient of the treated surface. Processes for applying black phosphate coatings with a high absorption coefficient (0.8–0.9) are being developed especially for laser heat treatment. Known black phosphating processes are also high-temperature (95–98 °С). They have not found wide practical application due to such disadvantages as the instability of solutions and the irreproducibility of the results [3, 4]. Low-temperature deposition process for black phosphate-selenide coatings