Phase and tribological behavior of Al_0.5Ti₂NbVZr_x lightweight refractory HEA coatings

To extend the service life of high-temperature components, lightweight refractory high-entropy alloy (LRHEA) coatings of Al_0.5Ti₂NbVZr_x (x = 0, 0.5, 1, 1.5) were prepared on Ti6Al4V substrates by laser cladding. Their microstructure, phase composition, mechanical properties, and tribological behavior over a wide temperature range were systematically investigated. The results showed that the Zr addition is correlated with the suppression of needle-like BCC precipitates. A single β phase with BCC structure was obtained at x = 0.5, whereas excessive Zr content (x ≥ 1) induced the Al₂Zr precipitation. The microhardness was improved from 505.53 HV_0.3 (Zr₀) to 715.97 HV_0.3 (Zr_1.5) with Zr content elevation, while nanoindentation revealed enhanced resistance to plastic deformation, primarily attributed to solid-solution strengthening and grain refinement. Furthermore, secondary phase strengthening was evident in the Zr₁ and Zr_1.5 coatings. The Zr_0.5 coating demonstrated the best wear resistance at 25 °C, 400 °C, and 600 °C. Especially at 600 °C, the wear rate reached its lowest value of 3.04 × 10⁻⁵ mm³/(N·m). XPS analysis revealed that a uniform and compact oxide layer consisting of ZrO₂, Al₂O₃, and TiO₂ formed on the surface of the Zr_0.5 coating during the wear process, indicating that appropriate Zr addition enhanced its high-temperature wear resistance. This study offers valuable guidance on the design of Zr-doped LRHEA coatings for high-temperature friction applications.