[Paper Review] Design and Characterization of Self Lubricating Refractory High Entropy Alloy Based Multilayered Films

Dawei Luo, Qing Zhou+5 more|arXiv (Cornell University)|Oct 8, 2021

High Entropy Alloys Studies85 references106 citations

TL;DR

This study designs NbMoWTa/Ag multilayered films via magnetron sputtering, achieving a synergistic combination of high hardness and low friction by inducing coherent interfaces at sub-10 nm layer thicknesses. The 2.5 nm multilayer exhibits a 50% lower wear rate and 40% reduced coefficient of friction than monolithic NbMoWTa, attributed to coherent strengthening and effective self-lubrication via Ag layer transfer.

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.

Motivation & Objective

• To develop self-lubricating refractory high entropy alloy (RHEA) films with enhanced tribological properties.
• To address the inherent trade-off between high hardness and low friction in RHEA films by introducing a ductile Ag phase.
• To investigate the influence of layer thickness (h) on microstructure, mechanical behavior, and tribological performance.
• To establish a correlation between coherent interface formation and improved hardness and wear resistance.
• To optimize the multilayer architecture for maximum wear resistance and minimal friction in high-performance applications.

Proposed method

• Fabrication of NbMoWTa RHEA and Ag targets via powder metallurgy and arc melting, respectively.
• Deposition of NbMoWTa/Ag multilayers with controlled individual layer thicknesses (h = 2.5, 5, 10, 20, 50, 100 nm) on Si substrates using magnetron sputtering.
• Use of substrate rotation and -80 V bias to enhance film homogeneity and adhesion.
• Characterization via XRD, SEM, TEM, AFM, nanoindentation, and tribological testing (pin-on-disk) under ambient conditions.
• Employment of high-resolution XPS and EDS mapping to analyze surface chemistry and wear track composition.
• Application of depth-sensing nanoindentation with Berkovich tip to determine hardness (H) and elastic modulus (E), corrected for substrate effects.

Experimental results

Research questions

• RQ1 How does reducing layer thickness (h) from 100 nm to 2.5 nm affect the interfacial structure (incoherent to coherent) in NbMoWTa/Ag multilayers?
• RQ2 What is the relationship between interface coherence and the resulting hardness and wear resistance in these multilayered films?
• RQ3 How does the tribological performance (COF and wear rate) of the multilayer compare to monolithic NbMoWTa and other Ag-containing self-lubricating films?
• RQ4 What role does Ag layer transfer and oxide formation play in the self-lubricating mechanism at different h values?
• RQ5 Can coherent interface formation enable a simultaneous improvement in hardness and friction reduction, overcoming the typical strength-lubrication trade-off?

Key findings

• The NbMoWTa/Ag multilayer with h = 2.5 nm achieves a peak hardness of ~9.4 GPa, representing a 52% increase over the rule-of-mixtures prediction.
• A transition from Hall–Petch strengthening to coherent strengthening is observed as h decreases below 10 nm, correlating with the formation of coherent interfaces.
• The h = 2.5 nm multilayer exhibits a 50% reduction in wear rate and a 40% reduction in coefficient of friction (COF) compared to monolithic NbMoWTa.
• The formation of a durable, continuous lubricating film via Ag transfer to the counterbody (Si3N4 ball) is confirmed by EDS and SEM analysis, reducing wear and friction.
• At h = 20 nm, optimal self-lubrication is achieved due to sufficient Ag supply and oxide formation, but mechanical degradation limits performance compared to the 2.5 nm film.
• Layer thickness below 2.5 nm leads to intermixing and loss of multilayer structure, degrading mechanical properties and negating performance gains.