Refractory high entropy alloys (RHEAs) have been proven to have excellent mechanical properties with a potential use as protective thin films. However, the combination of high hardness with low friction and wear is a major challenge in the design of RHEA films. In this study, we show that NbMoWTa/Ag self-lubricating multilayered films give a remarkable reduction in friction and at same time maintain high hardness. Interestingly, it is found that the bcc superlattice dominates in both NbMoWTa and Ag layers and the interfaces become coherent when the individual layer thickness h is reduced below 10 nm. The film properties are then strongly dependent on h ranging from 100 to 2.5 nm, and the most promising properties are obtained when the interface structure transforms from incoherent to coherent one. Especially, the multilayer with h = 2.5 nm exhibits superior tribological performance over monolithic NbMoWTa due to the significant coherent strengthening along with the self-lubricating ability in the multilayer. This tailored phase transition and coherent structure between the matrix and lubrication phases can also provide an optimal wear rate-coefficient of friction combination, which is higher than most of the Ag-containing self-lubricating films. The current work might open a new route toward the development of innovative self-lubricating RHEA films with excellent tribological properties.
Keywords: magnetron sputtering; mechanical property; multilayer; refractory high entropy alloy; tribology.