Role of surface terminations on MXenes' stability and performance in tribological applications.

Abstract

MXenes, a growing class of two-dimensional transition-metal carbides, nitrides, and carbonitrides, have become increasingly relevant in many fields, ranging from energy storage to biology and tribology, due to their excellent mechanical and surface properties. MXenes are synthesized by selective etching processes, resulting in surface terminations, usually -O, -OH, and -F, whose relative abundance depends on the etching procedure adopted. In this article, we examine the effect of MXenes' surface terminations on their stability and tribological performance, with a particular emphasis on recent advances in the synthesis routes, allowing for better control over the terminations' composition and uniformity. We then discuss the stability of MXenes under different conditions, such as in solution or within coatings, summarizing experimental evidence for oxidation and hydrolysis in air and aqueous media, focusing on how surface and edge passivation influences degradation pathways and long-term stability. Subsequently, we focus on tribological applications, exploring MXenes behavior under tribological conditions, for which contact pressure, shear, and local temperature can activate tribochemical reactions, promote termination redistribution, and drive the formation of protective tribofilms that can reduce wear of the substrate. Finally, we analyze how surface terminations affect lubricity by modifying interlayer interactions, adhesion, and shear strength. By reviewing the most relevant results and clearly addressing the existing knowledge gaps, this article aims at summarizing the key design principles and outlining the remaining challenges to develop MXene-based solid lubricants and lubricant additives. Graphical abstract

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