MXenes, i.e., two-dimensional (2D) transition metal carbides, carbonitrides, or nitrides, have a layered structure that can be easy to shear apart, which could make them useful as solid lubricants. Solid-phase alternatives to oil-based, liquid lubricants can be useful, e.g., at elevated temperatures or under vacuum. However, solid lubricants based on other 2D layered nanomaterials—such as graphene or molybdenum disulfide—are often hampered by their wear performance, which shortens their lifetime.

Philipp G. Grützmacher, TU Vienna (Vienna University of Technology), Austria, Andreas Rosenkranz, University of Chile, Santiago, and colleagues have studied the long-term solid lubrication ability and wear performance of MXenes of the type Ti₃C₂T_x. Such MXenes can be synthesized by selectively removing the Al atoms from Ti₃AlC₂ and replacing them with −O, −OH, −F, and/or −Cl surface groups, which are represented by the “T_x“. The team prepared Ti₃C₂T_x by etching Ti₃AlC₂ powder with hydrofluoric acid. Then they deposited 100-nm-thick multilayer Ti₃C₂T_x coatings on stainless-steel substrates by electrospraying.

The researchers assessed the lubrication properties and wear performance of the MXene by measuring the coefficient of friction (COF) over time during up to 100,000 back-and-forth sliding cycles and observing the wear tracks in scanning electron micrographs. They found that the MXene-coated sample showed low friction that was maintained well over time. Compared with an uncoated reference sample, they observed a six-fold friction reduction. According to the team, MXene flake “reservoirs” form at the ends of the sliding track, where the direction is reversed, and continuously resupply the contact zone with fresh lubricant—leading to improved long-term performance.

• Superior Wear-Resistance of Ti₃C₂T_x Multilayer Coatings,
  Philipp G. Grützmacher, Sebastian Suarez, Aura Tolosa, Carsten Gachot, Guichen Song, Bo Wang, Volker Presser, Frank Mücklich, Babak Anasori, Andreas Rosenkranz,
  ACS Nano 2021.
  https://doi.org/10.1021/acsnano.1c01555