Self-generating lubricious oxides for friction reduction in FeCoNiMo and CrCoNiMo from room temperature to 1000 °C

For decades, reducing wear failure in metals and alloys in high-temperature, oxidizing environments has been a critical challenge due to their thermal softening, loose oxidation layers, and inability to use solid lubricants. Thus, the formation of self-generating lubricious tribo-oxide layers is highly desirable for reducing friction and wear in materials without additives. Here, we introduce Mo to two popular medium-entropy alloys (MEAs, i.e., FeCoNi and CrCoNi) and develop two Mo-containing MEAs (i.e., FeCoNiMo and CrCoNiMo). We measure the coefficients of friction (CoFs) and wear rates of the alloys from room temperature to 1000 °C and characterize the wear tracks. For both MEAs, the incorporation of 3d transition metal cations into MoO 3 considerably enhances the lubricity, maintaining CoFs below 0.4 from room temperature to 1000 °C, with the lowest CoFs of ∼0.10 at 800 °C and 1000 °C for CrCoNiMo. From room temperature to 600 °C, FeCoNiMo shows higher wear resistance than CrCoNiMo, attributed to the rapid formation of glaze layers consisting of Fe-based spinel oxides and molybdates. At 800 °C and 1000 °C, the wear resistance of FeCoNiMo significantly decreases due to the instability of the glaze layer and the volatility of MoO 3 . In contrast, CrCoNiMo shows higher wear resistance than FeCoNiMo at 800 °C and 1000 °C, due to the formation of a stable glaze layer composed of NiO-(Ni,Co)MoO 4 and an underlying chromite layer. We demonstrate a strategy to enhance the high-temperature wear resistance of multicomponent alloys by forming lubricious and stable tribo-oxidation layers. • Record high-temperature hardness and wear behaviours of MEAs up to 1000 °C. • Propose a new strategy to add 3d transition metals into MoO 3 to reduce volatility. • Demonstrate NiO-molybdates oxide systems with ultra-low friction (0.1) at 1000 °C.