MXenes in Tribology

Introduction

Over the past two decades there are extensive tribological research on various 2D materials, such as graphene and transition metal dichalcogenides (TMDCs), has been conducted with different conditions. These materials demonstrate excellent friction performance due to their lamellar structure with weak van der Waals forces, which gives them low shear strength. Additionally, 2D materials can adhere to different surfaces to reduce interfacial adhesion and enhance friction and wear characteristics. This occurs due to the formation of beneficial tribofilm and reorientation of basal planes. However, these materials face challenges like poor performance under high contact pressures and sliding velocities, and limited adhesion when applied via drop-casting or spray-coating, leading to increased wear and friction.

MXenes, like other 2D materials are used in tribology which feature a layered structure and better shear properties, mechanical strength, excellent tunability, and chemical reactivity. Their reactivity can be advantageous, especially under high pressure and temperature conditions typical in tribological contacts. This results to form a tribolayer which often adheres well to the substrate, leading to long-lasting low-friction and low-wear performance. In oxygen- and water-free environments, MXenes are more stable at elevated temperatures, maintaining their structure up to around 900°C in an inert atmosphere. Beyond this temperature, they transform into 3D crystalline bulk transition metal carbides, such as TiC, which are known for their hardness. Additionally, MXenes can be chemically modified or combined with other 2D materials like graphene or graphene oxide to enhance their compatibility with water- or oil-based lubricants.

MXene in Liquid lubrication

The lubricant additives are beneficial in different ways such as nanoparticles which can function like nano-roller bearings, shifting the friction mode from sliding to rolling, which reduces friction. Additionally, these nanoparticles can react with the rubbing surfaces to form beneficial tribofilms, resulting in lower friction and improved wear performance. However, the effectiveness of these nanomaterials in reducing friction and wear depends heavily on their phase compatibility with the base oil and the stability of their dispersion. Stable, homogeneous nanosheet/oil dispersions are essential for achieving consistent friction and wear reduction. In contrast, unstable dispersions can lead to agglomeration, which increases abrasion and resistance, thereby diminishing tribological performance. Hence with better shear and mechanical properties MXenes can be used as additives in liquid lubrication.

MXene in Solid lubrication

The effectiveness of solid lubricant coatings in tribology largely depends on the quality of the coating such as its homogeneity, surface roughness, thickness, and adhesion to the substrate. Only coatings that are uniform and well-adhered can significantly enhance tribological performance. Common deposition methods like drop-casting, spin-coating, and spray-coating rely on the creation of stable dispersions in various solvents. This highlights the importance of understanding MXene chemistry and its interactions with different solvents. MXenes surface chemistry and hydrophilicity enable good dispersibility in aqueous and polar organic solvents, which is essential for producing uniform solid lubricant coatings on solid substrates.

Conclusion

The field of MXene tribology is still in its early stages, yet these 2D materials have already shown promise in reducing friction and wear when used as lubricant additives, fillers in composites, or solid lubricant coatings, especially when combined with other 2D materials or nanoparticles.

Reference

[1] Rosenkranz, A., Righi, M.C., Sumant, A.V., Anasori, B. and Mochalin, V.N., 2023. Perspectives of 2D MXene tribology. Advanced Materials, 35(5), p.2207757.