Achieving High Temperature Lubricity Through High Entropy Oxide for Extreme Environment

Traditional Ni- and Co-based aerospace alloys continue to face tribological challenges at elevated temperature, including accelerated oxidation, frictional losses, and wear. To address these challenges, high-entropy oxides (HEOs) are being explored as potential solid-lubricant and protective surface candidates for extreme environments where conventional lubricants rapidly degrade. In this study, a suspension plasma-sprayed (SPS) (Y_0.2Nd_0.2Gd_0.2Sm_0.2Dy_0.2)₂Zr₂O₇ HEO topcoat deposited on an IN625 substrate with an Amdry 386-2.5 bond coat was investigated under high-temperature fretting against a SiC counterface. Fretting tests were conducted at 600 °C and 800 °C, followed by ex situ characterization of worn surfaces using SEM/EDS, and subsurface damage assessment using FIB. The coating exhibited a distinctly lubricious response at both temperatures, reaching steady-state coefficients of friction (CoF) of ~0.1 at 600 °C and ~0.2 at 800 °C. At 600 °C, the CoF stabilized rapidly, and the wear depth remained minimal, with a shallow wear scar covered by a loose oxide/debris layer. At 800 °C, the friction response showed larger fluctuations before stabilization, and the wear depth increased substantially. The wear scar exhibited micro-cracking, chipping, pits/pull-out features, and abundant debris, indicating reduced tribolayer stability and more pronounced damage accumulation. Overall, these results demonstrate the potential of SPS HEO coatings as next-generation protective surfaces for high-temperature tribological interfaces under specific conditions. Furthermore, microstructural stability and tribolayer durability emerge as critical design considerations for maintaining performance at elevated temperatures.