This work demonstrates the role of microstructure in the friction and oxidation behavior of the lamellar solid lubricant molybdenum disulfide (MoS). We report on systematic investigations of oxidation and friction for two MoS films with distinctively different microstructures-amorphous and planar/highly-ordered-before and after exposure to atomic oxygen (AO) and high-temperature (250 °C) molecular oxygen. A combination of experimental tribology, molecular dynamics simulations, X-ray photoelectron spectroscopy (XPS), and high-sensitivity low-energy ion scattering (HS-LEIS) was used to reveal new insights about the links between structure and properties of these widely utilized low-friction materials. Initially, ordered MoS films showed a surprising resistance to both atomic and molecular oxygens (even at elevated temperature), retaining characteristic low friction after exposure to extreme oxidative environments. XPS shows comparable oxidation of both coatings via AO; however, monolayer resolved compositional depth profiles from HS-LEIS reveal that the microstructure of the ordered coatings limits oxidation to the first atomic layer.
The
role of water in the tribochemical mechanisms of ultralow wear
polytetrafluoroethylene (PTFE) composites was investigated by studying
10 and 20 wt % polyether ether ketone (PEEK)-filled and 5 wt % αAl2O3-filled PTFE composites. These composites were
run against stainless-steel substrates in humidity, water, and dry
nitrogen environments. The results showed that the wear behavior of
both composites was significantly affected by the sliding environment.
Both composites achieved remarkably low wear rates in humidity because
of tribochemically generated carboxylate end groups that anchored
the polymer transfer films to the steel substrate. In nitrogen, PTFE–PEEK
outperformed PTFE−αAl2O3 because
of polar carbonyl groups on PEEK, which increased the surface energy
of PEEK, aiding it in adhering to the substrate and resulting in a
transfer film. Both composites in water exhibited high wear. The water
oversaturated the functional groups at the end of the polymer chains
and prevented the formation of a transfer film.
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