Achieving low friction at high temperature through high-entropy oxide coatings for extreme environments

Conventional Ni- and Co-based aerospace alloys continue to face tribological challenges at elevated tempera-
tures, including frictional losses, oxidation-assisted damage, and wear degradation. High-entropy oxides (HEOs),
as a class of high-entropy ceramics, offer a promising pathway for developing thermally stable protective
coatings for extreme environments where conventional surface materials and lubricants can degrade. In this
study, a suspension plasma-sprayed (SPS) (Y0.2Nd0.2Gd0.2Sm0.2Dy0.2)2Zr2O7 HEO topcoat deposited on an IN625
substrate with an Amdry 386–2.5 bond coat was evaluated under high-temperature fretting against a SiC
counterface. The novelty of this work lies in investigating the fretting response of an SPS-deposited rare-earth
zirconate HEO coating at 600 ◦C and 800 ◦C, where limited studies have addressed the combined effects of
temperature, fretting motion, third-body formation, and coating damage evolution in high-entropy ceramic
coating systems. The coating showed competitive low-friction behaviour compared with conventional high-
temperature metallic alloys and several reported ceramic-based tribological coatings, reaching steady-state co-
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efficients of friction of approximately 0.1 at 600 ◦C and 0.25 at 800 ◦C. At 600 ◦C, the low and stable friction
response was associated with the formation and retention of fine oxide-rich third-body debris, resulting in a
shallow wear depth of approximately 7 μm. At 800 ◦C, the coefficient of friction remained relatively low but
became more unstable, while the wear depth increased to approximately 43 μm. SEM/EDS and FIB analyses
revealed microcracking, chipping, pull-out features, and debris accumulation, indicating reduced tribolayer
stability and increased damage accumulation at the higher temperature. Overall, this study highlights the po-
tential of HEOs within the broader family of high-entropy ceramics for high-temperature fretting applications,
while also showing that wear resistance is strongly governed by temperature-dependent adhesion, debris sta-
bility, and fracture processes.