posted on 2023-01-26, 19:35authored byTimothy MacLucas, Andreas Klemenz, Patrick Grünewald, Volker Presser, Leonhard Mayrhofer, Gianpietro Moras, Sebastian Suarez, Martin Dienwiebel, Frank Mücklich, Michael Moseler
When lubrication of rolling bearings with oil or grease
is not
possible, for example because the lubricant evaporates in vacuum,
solid lubrication by multiwall carbon nanotubes (MWCNT) is a viable
alternative. To understand the mechanisms underlying MWCNT lubrication
of highly loaded contacts, we combine an experimental approach with
large-scale molecular dynamics (MD) simulations. Tribometry is performed
on ground iron plates coated with two different types of MWCNTs by
electrophoretic deposition. Although structural differences in the
MWCNT materials result in slightly different running-in behavior,
most of the tests converge to a steady-state coefficient of friction
of 0.18. The resulting wear tracks and tribolayers are subjected to
structural and chemical characterization and suggest a tribo-induced
phase transformation resulting in tribolayers that consist of MWCNT
fragments, iron oxide, and iron carbide nanoparticles embedded in
an amorphous carbon matrix. Covalent bonding of the tribolayer to
the iron surface and low carbon transfer to the alumina counter body
indicate sliding at the tribolayer/ball interface as the dominant
mechanism underlying MWCNT solid lubrication. MD simulations of nascent
a-C tribofilms lubricated by MWCNT bundles and stacks of crossed MWCNTs
reveal two different sliding regimes: a low-load regime that leaves
the MWCNTs intact and a high-load regime with partial collapse of
the tube structure and formation of a-C regions. The critical load
for this transition increases with the filling ratio of the MWCNT
and the packing density of the stacks. The former determines the stability
of the MWCNT, while the latter controls the local stresses at the
MWCNT crossings. For both MWCNT materials, the high-load regime is
predicted for the experimental loads. This is confirmed by a remarkable
agreement between transmission electron microscopy (TEM) and atomistic
simulation images. Based on the findings of this work, a multistep
lubrication mechanism is formulated for MWCNT coatings rubbing against
alumina on an iron substrate.