In Situ Crystalline-Amorphous Composite Oxide Structure Suppressing Thermal Wear Instability for Refractory Multi-Principal Element Alloy

Refractory multi-principal element alloys (RMPEAs) are attractive for elevated-temperature tribology, yet many exhibit pronounced wear-resistance degradation from ambient to medium temperatures (approximately 25°C–500°C). Conventional approaches typically rely on temperature-specific, in situ tribo-oxide formation, which limits the improvement to narrow operating windows. Here we demonstrate an oxygen-induced amorphization strategy in NbMoWTaTi that produces a mechanically robust composite oxide tribo-layer and suppresses medium-temperature wear degradation. Oxygen incorporation into the chemically short-range ordered NbMoWTaTi matrix promotes uniformly dispersed Ti─O clusters, leading to local lattice instability and spatially uniform amorphization. During sliding at 500°C–650°C, this process generates a composite oxide tribo-layer comprising an amorphous matrix containing ultrafine WO₃ nanocrystals (∼5 nm), which exhibits high hardness and stiffness and supports ultra-low wear (on the order of 10⁻⁷ mm³·N⁻¹·m⁻¹). In addition, pre-fabrication of a comparable composite oxide layer reduces the wear rate by one to two orders of magnitude from 25°C to 400°C relative to the untreated alloy. These results establish oxygen-induced amorphization as a design strategy for forming stable, high-strength oxide tribo-layers and achieving enhanced wear resistance across a broad service-temperature range.