Design and Characterization of Self Lubricating Refractory High Entropy Alloy Based Multilayered Films

TL;DR

This paper presents the design and characterization of NbMoWTa/Ag multilayered refractory high entropy alloy films that achieve low friction and high hardness through controlled interface coherence and phase structure, promising for protective coatings.

Contribution

It introduces a novel multilayered RHEA film with optimized interface coherence and phase structure, enhancing tribological performance compared to existing self-lubricating films.

Findings

Multilayered films with h=2.5 nm show superior tribological performance.

Coherent interfaces significantly improve hardness and wear resistance.

Phase transition from incoherent to coherent interfaces enhances properties.

Abstract

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.