Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS upon Aging.

Molybdenum disulfide (MoS) coatings have attracted widespread industrial interest owing to their excellent lubricating properties under vacuum and inert conditions. Unfortunately, the increase in MoS interfacial shear strength following prolonged exposure to ambient conditions (a process referred to as "aging") has resulted in reliability issues when MoS is employed as solid lubricant. While aging of MoS is generally attributed to physical and chemical changes caused by adsorbed water and/or oxygen, a mechanistic understanding of the relative role of these two gaseous species in the evolution of the surface chemistry of MoS is still elusive. Additionally, remarkably little is known about the effect of thermally- and tribologically-induced microstructural variations in MoS on the aging processes occurring in the near-surface region of the coating. Here, we employed three analytical techniques, namely, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and grazing-incidence X-ray diffraction (GIXRD), to gain insights into the aging phenomena occurring in sputtered MoS coatings before and after tribological testing, while also evaluating the impact of thermally-induced variations in the coating structure on aging. The outcomes of XPS analyses provide evidence that a substantial surface oxidation of MoS only takes place under humid conditions. Furthermore, the correlation of XPS, ToF-SIMS, and GIXRD results allowed for the development of a qualitative model for the impact of shear-induced microstructural variations in MoS on the transport of water in the near-surface region of this material and on the extent of surface oxidation. These results add significantly to our understanding of the aging mechanisms of MoS coatings used in tribological applications and their dependence on environmental conditions.