National University of Science and Technology “MISiS”, Moscow, Russia:

A. G. Rakoch, Professor of a Chair of Metal Protection and Surface Technology
D. M. Strekalina, Assistant, e-mail: dasha_367@mail.ru
A. A. Gladkova, Assistant

Technological mode of plasma-electrolytic oxidation of VT6 (ВТ6) alloy was defined for obtaining of composite coating, consisting of double oxide (TiAl₂O₅) with high temperature modification of aluminium oxide (α-Al₂O₃). This coating increases the wear resistance of titanium alloy by more than 6 times. There were offered the mechanisms of high temperature modification of aluminium oxide (α-Al₂O₃) in coating, including its outer layer, formed after plasma-electrolytic oxidation of VT6 (ВТ6) alloy in cooled alkaline water solution (pH = 12.8), containing up to 40 g/l of sodium aluminate. These mechanisms are based on exothermal processes of formation of titanium aluminide and/or solid solution of oxides of aluminium (basis), titanium, alloying element of the alloy and low through porosity of coating. In such electrolyte, the average growing rate of the coating on VT6 (ВТ6) alloy with given density (15 A/dm²) is high (1.3 μm/min) because of not only the alloy oxidation but also the plasma thermochemical treatment of electrolyte layer, adjacent to metallic base and containing sodium aluminate. The latter leads to inclusion of aluminium oxide into the coating. Wear resistance of VT6 (ВТ6) alloy with and without coating was studied with automatic friction machine High-temperature Tribometer (CSM Instruments, Switzerland). The “ball – disc” layout was used for specimen testing. The X-ray spectrum recording of specimens with coatings was carried out on the X-ray diffraction meter Rigaku Ultima V (TOKYO BOEKI) using monochromated CoK_α radiation and both symmetrical and asymmetrical (α = 5^o) exposures. Special sofware was used for the quantity analysis of phase composition with accuracy of minimum 5% (wt.) and 5% (vol.) of phases with known structure included in the coating. Microhardness of coatings (60 'm thickness) was measured on sections with microhardness tester Buehler Micro Met^® 5101. The Omni Met^® software was used for mathematical processing of experimental results for the purpose of the average microhardness acquiring (min. 15 measures). Coating thickness was measured with thickness tester Fischer Duel Scope FNP 10. The distribution of ultimate composition on the coating thickness was studied with the scanning electrone microscope Sphinx 133 Camscan.

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