Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS2 upon Aging

Molybdenumdisulfide (MoS2) coatings have attractedwidespread industrial interest owing to their excellent lubricatingproperties under vacuum and inert conditions. Unfortunately, the increasein MoS2 interfacial shear strength following prolongedexposure to ambient conditions (a process referred to as "aging")has resulted in reliability issues when MoS2 is employedas solid lubricant. While aging of MoS2 is generally attributedto physical and chemical changes caused by adsorbed water and/or oxygen,a mechanistic understanding of the relative role of these two gaseousspecies in the evolution of the surface chemistry of MoS2 is still elusive. Additionally, remarkably little is known aboutthe effect of thermally- and tribologically-induced microstructuralvariations in MoS2 on the aging processes occurring inthe near-surface region of the coating. Here, we employed three analyticaltechniques, namely, X-ray photoelectron spectroscopy (XPS), time-of-flightsecondary ion mass spectrometry (ToF-SIMS), and grazing-incidenceX-ray diffraction (GIXRD), to gain insights into the aging phenomenaoccurring in sputtered MoS2 coatings before and after tribologicaltesting, while also evaluating the impact of thermally-induced variationsin the coating structure on aging. The outcomes of XPS analyses provideevidence that a substantial surface oxidation of MoS2 onlytakes place under humid conditions. Furthermore, the correlation ofXPS, ToF-SIMS, and GIXRD results allowed for the development of aqualitative model for the impact of shear-induced microstructuralvariations in MoS2 on the transport of water in the near-surfaceregion of this material and on the extent of surface oxidation. Theseresults add significantly to our understanding of the aging mechanismsof MoS2 coatings used in tribological applications andtheir dependence on environmental conditions.