Compositional Complexity-Induced Ultralow Friction in Medium-Entropy MXenes

TL;DR

This study demonstrates that medium-entropy MXenes exhibit ultralow friction and superlubricity due to their compositional complexity and surface chemistry, outperforming other 2D materials.

Contribution

It reveals how compositional complexity and surface chemistry in medium-entropy MXenes lead to ultralow friction and superlubricity, advancing solid lubricant design.

Findings

Annealing reduces adhesion and friction across all MXenes studied.

Medium-entropy MXenes achieve superlubricity with friction as low as 0.0022.

Higher initial OH content and out-of-plane stiffness contribute to ultralow friction.

Abstract

Two-dimensional MXenes are promising solid lubricants, but the roles of compositional complexity and surface chemistry in governing interfacial friction remain unclear. Here, we systematically investigate the adhesion and friction behaviors of medium-entropy (ME) MXenes, TiVNbMoC3 and TiVCrMoC3, and compare them with conventional titanium carbide MXenes, Ti2C and Ti3C2, using a SiO2 colloidal atomic force microscopy probe. Thermal annealing at 200 C converts OH surface terminations to O terminations, leading to pronounced reductions in adhesion energy and friction force across all MXenes studied. ME MXenes exhibit larger adhesion reductions because of their higher initial OH contents and more extensive OH-to-O conversion. In addition, their intrinsically higher out-of-plane bending stiffness suppresses energy dissipation during sliding, enabling ultralow friction. Notably, superlubricity is achieved in ME MXenes, with annealed TiVCrMoC3 exhibiting a coefficient of friction as low as 0.0022, outperforming graphene, MoSe2, and other MXenes evaluated using the same experimental approach. These findings identify compositional complexity as a powerful strategy for engineering MXenes with exceptional tribological performance and establish ME MXenes as a new class of solid lubricants.