Tribological Behavior of VAlN-Ag Coating Prepared by Direct Current Combined with High Power Pulsed Magnetron Sputtering Over a Wide Temperature Range

With the development of aviation engine technology, the high flow ratio and thrust weight ratio is gradually improved, especially components such as brush seal and foil air bearing will serve in harsh environment of high-speed and wide temperature from 25 ℃ to 650 ℃. The insufficient lubrication will cause friction and wear of surface, decreasing sealing performance, shortening service life and affecting safe and reliable operation of engine. Therefore, developing low friction coating with good lubrication, long service life and wear resistance over wide temperature range to prevent wear and failure of component, and improve the service life. The VAlN hard coating with high hardness, wear resistance, and good thermal stability was selected to prepare chameleon composite coating combining with soft metal Ag. In view of the high power pulsed magnetron sputtering (HiPIMS) had the excellent discharge characteristics of low discharge frequency, high plasma density and high ionization rate, which was conducive to the preparation of low defect, dense and smooth surface of coatings. Meanwhile, the deposition rate of sputtering soft metal was lower than that of direct current magnetron sputtering (DCMS) under the same power. Therefore, during the preparation process, the DCMS technology combined with HiPIMS technology was utilized to prepare the chameleon composite coating by the home-made multi-target reactive magnetron sputtering equipment. Among them, DCMS technology was utilized to sputter VAl alloy targets, and regulated target power through the HiPIMS to control Ag contents, then fabricated the VAlN/VAlN-Ag composite coatings with different Ag contents (11.4%, 19.8%, 24.5%). The tribological behavior at 25 ℃, 300 ℃ and 650 °C were investigated in this article. The friction coefficient of three coatings were close to each other and relatively high at 25 ℃, but with the increase of temperature to 300 °C and 650 °C, the friction coefficient reduced with increasing of Ag content, as well as the coating with 24.5% Ag obtained the lowest friction coefficient of 0.45 and 0.23. The wear rate increased with the rise of temperature from 25 ℃ to 650 ℃, the S1 coating with the best mechanical properties possessed the lowest wear rate, the composite coatings exhibited great tribological behavior over wide temperature range. It was found that the phase structure, element valence and chemical bond of coating unchanged after friction at 25 ℃ and 300 ℃, and the behavior of elements diffusion was not obviously. The tribochemical reaction was occurred during friction test at 650 ℃, thus the valence states of V and Ag elements increased and the AgVO₃ and Ag₃VO₄ with layered structure were mainly formed. The formed high-temperature lubricating phase played the key factor to effectively reduce friction coefficient, improve the friction and wear resistance of coatings, as well as tolerable higher temperature. The diffusion of V and Ag elements to the coating surface occurred during high temperature friction process, which adjusted the elemental composition and structure of contact interface. As a results, the internal microstructure changed obviously, which were the surface layer mainly composed of bimetallic lubricating phase and dense Al₂O₃ protective layer, the intermediate layer mainly composed of V₂O₅ and NiO phases, and the bottom layer dominated by Ti transition layer. The wear mechanism was mainly adhesive wear and oxidative wear during the friction at 650 ℃. This article provided a theoretical basis and experimental basis for the design and preparation of low friction coatings over wide temperature range.