Robust Low-Friction and Low-Wear TiNbMoTaCr High-Entropy Film Enabled by Periodically Inserting Curved MoS Sheets.

The well-known integration of physical, chemical, and mechanical properties enables high-entropy alloys (HEAs) to be applied in various fields; however, refractory HEAs are brittle and susceptible to abrasive wear at high coefficients of friction (COF), resulting in insufficient mechanical durability against abrasion. Herein, curved MoS nanosheets are periodically introduced into the TiNbMoTaCr film for triggering the self-assembly mixed metal oxides @MoS nanoscrolls, which contain hard mixed metal oxides cores and the low-shearing lubricant MoS shells, during the friction in the air environment; such mixed metal oxides@MoS nanoscrolls in the friction interfaces can contribute to the robust low friction and low wear. Compared to the pure TiNbMoTaCr film (with high COF of ∼0.78, low abrasive durability identified by worn-out event), the periodic incorporation of 10 nm thickness curved MoS sheets can successfully achieve a low COF of ∼0.08 and low wear rate of ∼9.561 × 10 mm/ Nm, much lower than the pure MoS film (COF = ∼ 0.21, wear rate = ∼ 1.03 × 10 mm/ Nm). Such superior tribological properties originate from the cooperative interaction of TiNbMoTaCr nanolayers and curved MoS nanosheets, accompanied by the self-assembly of mixed metal oxides@MoS nanoscrolls. In these nanoscrolls, TiNbMoTaCr can act as an 'air-absorbing agent' to form high-loading mixed metal oxide cores and serve as an 'oxygen sacrificer,' preventing the low-shearing lubricant curved MoS nanosheets from oxidation. In addition, even with the soft MoS, the hardness of the TiNbMoTaCr/MoS nanomultilayers can still be well maintained and increased above the calculated values by mixing law, further favoring superior mechanical durability. The synergetic effect of TiNbMoTaCr and curved MoS nanosheets during the friction in air can provide a route to design HEA films with enhanced tribological properties for better mechanical durability and broader application prospects.