Institute of Chemistry of Far Eastern Branch of Russian Academy of Sciences, Vladivostok, Russia:

V. S. Egorkin, Senior Researcher of Laboratory of Nonstationary Surface Processes, е-mail: egorkin@ich.dvo.ru
I. E. Vyaliy, Junior Researcher of Laboratory of Nonstationary Surface Processes
S. L. Sinebryukhov, Associate Professor; Head of Laboratory of Nonstationary Surface Processes
S. V. Gnedenkov, Dep. Director for Science; Head of the Department of Electrochemical Systems and Surface Modification Processes

The paper presents the results of a study of the chemical composition, morphology, and tribological properties of coatings produced on an aluminum alloy D16 in a tartrate-containing electrolyte by plasma electrolytic oxidation (PEO). In order to form the coatings, a transistor power supply was used, providing a 5 s square-wave voltage pulses to the treated sample. The power supply is capable to control the pauses between pulses to achieve the required duty cycle values D. The influence of the parameters of the polarizing signal and the time of oxidation on the properties of the obtained PEO coatings is studied. It is established that the change in the duty cycle of the polarizing signal affects the chemical composition, thickness and properties of the formed oxide coatings. The results of X-ray diffraction data indicate that the main component of the PEO layers under investigation is a cubic modification of aluminum oxide (γ-Al₂O₃). In addition to cubic, the -alumina (β-Al₂O₃), aluminum phosphate (AlPO₄) and molybdenum and aluminum carbide (Al₂Mo₃C), which are formed as a result of plasma-chemical reactions involving the processed alloy and electrolyte components, are also present in the PEO layers. The analysis of the dependence of the friction coefficient  on the number of cycles shows that the PEO coatings formed at larger values of the duty cycle and oxidation time can withstand substantially more abrasion cycles. With the increase in the amount of electricity consumed to create the coating, thicker PEO layers are formed, which contributes to the improvement of tribological properties. It is established that an increase in the duty cycle of the polarizing signal leads to the reduction of the apparent porosity of the formed PEO layers, formation of more aluminum and molybdenum carbides in their composition, which leads to an increase in the wear resistance and hardness of the coatings.

This work was supported by the Russian Science Foundation (No. 14-33-00009).

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