Tribological Property of V₂AlC MAX Phase Coating Over Wide Temperature Range

MAX phase coating with laminar structure possesses the superior combination of ceramics and metals. Particularly, the rich M-position elements in MAX phase can be oxidized easily to form the lubricating phase at high temperature friction, which makes it a promising self-adaptive lubrication, excellent oxidation and corrosion resistance over a wide temperature range. In this study, the dense V₂AlC MAX phase coating was successfully fabricated by arc/sputtering hybrid deposition technology and post-annealing.

The V₂AlC MAX phase coatings were fabricated on nickel-base superalloy substrates (Φ30 mm×3 mm) using a home-made combined arc/sputter equipment, followed by vacuum annealing. The rectangular aluminum target (mass fraction: 99.9%; 400 mm×100 mm×7 mm) and the circular vanadium target (mass fraction: 99.9%; Φ128 mm×15 mm) were applied as sputtering source and cathode arc source, respectively. All eight substrates were suspended on a pre-fixed rack by iron wire and apart 3 cm. Prior to deposition, the chamber was evacuated to a base pressure of 1.5×10⁻³ Pa to eliminate air, and then the chamber was heated to 100 ℃. To improve the adhesion strength, all substrates were pre-etched by Ar⁺ bombardment for 60 min, and then a V interlayer (thickness of ~1 000 nm) was deposited by pure arc technology. During V-Al-C coatings deposition, the reactive gas of CH₄ and Ar (flow rate ratio: Ar/CH₄ = 5/40) to keep a total pressure of 1.3 Pa. To ensure all substrate surface were coated uniformly, the substrates were rotated on their axis in the front of Al targets with a target-substrate distance of 5 cm. During deposition, the power of V and Al targets was kept at 17 W and 3.2 kW, respectively. Substrate negative bias voltage of -200 V was applied to the substrate during both etching and deposition processes. After deposition, a post-annealing treatment was applied to transform the V-Al-C coating to V₂AlC MAX phase coating at the condition of 750 ℃ and 2.0×10⁻² Pa for 1 h.

Subsequently, the friction behavior and the wear mechanism were focused over a wide temperature range from 25 ℃ to 700 ℃. The results showed that V₂AlC coating has serious shear deformation at 25 ℃ to 300 ℃, and wear failure was mainly caused by abrasive wear. The main friction failure at 500 ℃ was dominated by adhesion wear and abrasive wear, and the top layer of the coating showed signs of oxidation. However, when the temperature was beyond 600 ℃, the diffusion and oxidation of V element occurred and the layered V₂O₅ lubricating phase emerged, which favored the significant reduction of friction coefficient, and the wear type was mainly oxidation wear. As a result, the V₂AlC coating presented both the low wear rate and excellent tribological properties at high temperature of 600 ℃. At 700 ℃, the oxidation degree of the coating increased, and the oxidation wear was the main reason for the lubrication failure of the coating. It was worth noting that V₂AlC coating had poor lubrication performance in the low and medium temperature range. The low shear oxide V₂O₅ lubricating phase formed at 600 ℃ was the key to maintaining low friction and wear and long-term stability of the coating. However, V element was easily oxidized at high temperature, which limited the protection application of V₂AlC coating at higher temperatures.