Synergistic water–oxygen degradation pathways in PVD-MoS ₂ solid lubricant coatings

Ambient exposure to water vapour and oxygen undermines the long-term lubricity of magnetron-sputtered molybdenum disulphide (MoS 2 ) coatings, yet the individual and synergistic contributions of the two species remain poorly quantified. Here we decouple their effects by ageing 1 µm PVD-MoS 2 films for 500 h in six precisely controlled gas/humidity atmospheres (dry, 25% RH and 50% RH; N 2 or air) followed by micro-tribological testing in dry N 2 . Complementary “unaged” samples were slid directly in identical environments to probe instantaneous reactivity. Coefficient-of-friction data, Raman spectroscopy, SEM-EDS mapping and cross-sectional TEM reveal three key findings: (i) humidity alone raises steady-state friction by up to 2.5-fold, whereas dry oxygen has little effect; (ii) concurrent H 2 O and O 2 during storage triggers sub-surface penetration of O-species, formation of MoO 2 /MoO 3 within wear scars and inhibition of basal-plane realignment, yielding a persistent friction penalty (µ ≈ 0.12) independent of run-in; (iii) ageing in dry or humid N 2 preserves lamellar re-orientation and maintains low friction (µ ≈ 0.05) after only 50 cycles. A mechanistic model is proposed in which adsorbed water lowers the activation barrier for oxygen insertion at defect and edge sites, corroborated by a 2–3 cm −1 blue shift of the E 1 2 g Raman band. By isolating the water-assisted oxidation pathway at ambient temperature, this study provides the first quantitative guideline for MoS 2 component storage: moisture-free inert gas environments effectively arrest chemical degradation, whereas even moderate humidity in air accelerates tribo-oxidative damage. The insights inform design tools for precision-mechanical assemblies where pre-deployment shelf life is critical.