Tailoring the crystallinity and phase composition of MoS2 nanosheets for better lubrication performance

Abstract

Molybdenum disulfide (MoS2), a typical lubricant material in vacuum environments, encounters limitations in broader industrial applications due to its vulnerability to oxidative degradation. Adjusting crystal structure or improving crystallinity can effectively increase the oxidation resistance of MoS2, and then enhance its lubrication performance. Herein, a simple hydrothermal method combined with post-annealing treatments was used to synthesize MoS2 and tailor crystal structure and crystallinity. Structural and morphological analyses indicated that the crystal structure of MoS2 had been transformed from 1T′-MoS2 to 2H-MoS2, and the improved crystallinity with larger grain sizes reduced grain boundaries after annealing treatments. These structural variations were corroborated by in-situ Raman spectroscopy during laser annealing and subsequent tribological evaluation, which demonstrated 2H phase and higher crystallinity were beneficial for improving the oxidation resistance and longevity of MoS2 coating. These results were further validated by density functional theory calculation. The enhanced oxidation resistance could be ascribed to the protective role of the S atomic layer on Mo layer, coupled with high hydrophobicity and reduced grain boundaries by high crystallinity. Furthermore, the oxidation mechanism of MoS2 in the sliding process has been clarified, which can provide a novel pathway to improve its lubrication performance in atmospheric environment.