Investigation of microstructural features and the wide-temperature tribological behavior of pre-oxidation treated FeCoCrNiMo0.8 high-entropy alloy coatings

Surface oxide film stability is critical to the wide-temperature tribological performance of FeCoCrNiMo0.8 high-entropy alloy (HEA) coatings. Pre-oxidation is a promising strategy for the in-situ formation of such protective oxide films. However, the evolution of the pre-formed oxide film and its effect on wear across a wide temperature range remain insufficiently understood. FeCoCrNiMo0.8 HEA coatings were fabricated via laser cladding and pre-oxidized at 800 °C for 10-40 h. Their oxide-film evolution, mechanical properties, and tribological behavior were systematically investigated. The oxide film transformed from dispersed island-like particles to a continuous dense structure with increasing pre-oxidation time, and its thickening followed a classical parabolic growth law (k p = 1.66 μm 2 /h, R 2 = 0.97). The S30 sample (30 h) formed a bilayer oxide film, comprising an outer spinel layer (FeCr 2 O 4 , NiFe 2 O 4 ) and an inner Cr-rich Cr 2 O 3 layer. S30 exhibited the highest H/E and H 3 /E 2 ratios among the four samples, indicating enhanced load-bearing capacity and resistance to plastic deformation. Across 25-800 °C, S30 consistently exhibited the lowest friction coefficients (0.57-0.29) and wear rates ((32.65-0.87) ×10 −6 mm 3 /N·m). At 25 °C, the pre-oxidized film experienced localized shear-induced failure, with wear dominated by adhesive and minor abrasive/fatigue mechanisms. At 400 °C, friction-induced chemical reactions promoted partial oxide formation, shifting wear toward mild fatigue and abrasive mechanisms. At 800 °C, a continuous, stable composite oxide film formed on the wear track, reducing wear primarily through mild abrasive mechanisms. Compared with the as-cladded FeCoCrNiMo0.8 HEA coating, pre-oxidation substantially improved wear resistance across 25-800 °C while affecting friction only moderately, indicating that the bilayer oxide film primarily contributes to wear resistance. These findings clarify the formation pathway and tribological role of the pre-oxidized bilayer film, providing a basis for designing pre-oxidation treatments of HEA coatings in wide-temperature applications.