  Multi-layer Ti3C2Tx coatings have demonstrated an outstanding wear
performance with excellent durability due to beneficial tribo-layers formed.
However, the involved formation processes dependent on the tribological
conditions and coating thickness are yet to be fully explored. Therefore, we
spray-coated Ti3C2Tx multi-layer particles onto stainless steel substrates to
create coatings with two different thicknesses and tested their solid
lubrication performance with different normal loads (100 and 200 mN) and
sliding frequencies (1 and 2.4 Hz) using linear-reciprocating ball-on-disk
tribometry. We demonstrate that MXenes' tribological performance depends on
their initial state (delaminated few-layer vs. multi-layer particles), coating
thickness, applied load and sliding frequency. Specifically, the best behavior
is observed for thinner multi-layer coatings tested at the lower frequency. In
contrast, coatings made of delaminated few-layer MXene are not as effective as
their multi-layer counterparts. Our high-resolution interface characterization
by transmission electron microscopy revealed unambiguous differences regarding
the uniformity and chemistry of the formed tribo-layers as well as the degree
of tribo-induced MXenes' exfoliation. Atomistic insights into the exfoliation
process and molecular dynamic simulations quantitatively backed up our
experimental results regarding coating thickness and velocity dependency. This
ultimately demonstrates that MXenes' tribological performance is governed by
the underlying tribo-chemistry and their exfoliation ability during rubbing.
