Impact of Microstructure on MoS 2 Oxidation and Friction.

This work demonstrates the role of microstructure on the friction and oxidation behavior of the lamellar solid lubricant molybdenum disulphide (MoS2). We report on systematic investigations of oxidation and friction for two MoS2 films with distinctively different microstructures -- amorphous and planar/highly-ordered -- before and after exposure to atomic oxygen and high temperature (250°C) molecular oxygen. A combination of experimental tribology, molecular dynamics (MD) simulations, X-ray photoelectron spectroscopy (XPS) and high-sensitivity low-energy ion scattering (HS-LEIS) were used to reveal new insights about the links between structure and properties of these widely utilized low friction materials. Initially ordered MoS2 films showed a surprising resistance to both atomic and molecular oxygen (even at elevated temperature), retaining characteristic low friction after exposure to extreme oxidative environments. XPS shows comparable oxidation of both coatings via atomic oxygen; however, monolayer resolved compositional depth profiles from HS-LEIS reveals the microstructure of the ordered coatings limits oxidation to the first atomic layer.