  Refractory high entropy alloys (RHEA) 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 self lubricating RHEA films. In this study, we show
that designing of NbMoWTa/Ag multilayered films give a remarkable reduction in
friction and at same time maintain high hardness. Interestingly, it's found
that the bcc superlattice dominates in both layers and the interfaces are
highly 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 ones. Specially, 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 matrix and lubrication phases can also provide an optimal
wear rate-coefficient of friction (COF) combination, which is higher than most
of the Ag containing self lubricating films. The current work might open a new
route towards the development of innovative self lubricating RHEA films with
excellent tribological properties in general.
