Reliable low-friction, wear-resistant coatings that remain effective from room temperature to several hundred degrees are crucial for components such as foil air bearings, yet are difficult to realize because the dominant lubrication mechanism changes with temperature. Here we engineer a family of dual-layer MoAgN/MoS 2 -W composite coatings in which Ag diffusion and solid lubrication are jointly controlled by a W-alloyed MoS 2 top layer and a columnar Mo 2 N diffusion barrier. Under operando continuous-heating tests from 25 to 600 °C, the optimized coating containing a Mo 2 N barrier (S3) sustains a friction coefficient below 0.4 across the entire temperature range and exhibits the lowest high-temperature friction (∼0.23 at 600 °C). W alloying increases the hardness and elastic strain to failure of the MoS 2 top layer, enhancing its oxidation resistance and reducing the wear rate without compromising lubricity. Compared with the barrier-free MoAgN/MoS 2 -W coating, the wear rate of S3 at 600 °C is reduced by a factor of 3.6. By combining micro-area XRD, Raman spectroscopy, SEM/EDS, and TEM, we resolve the temperature-dependent evolution of the tribofilm. Lubrication transitions from basal-plane MoS 2 shear at low temperature, to a self-organized “sandwich” tribofilm composed of a WO 3 base layer, an intermediate Ag reservoir, and a MoO 3 /AgTM x O y top layer at intermediate temperature, and finally to a liquid-like silver molybdate film coexisting with Mo oxides at 600 °C. High-low temperature cycling tests (RT-350 °C-RT-500 °C-600 °C-RT) show that S3 maintains reproducible low friction up to 500 °C, while exposure to 600 °C eventually pulverizes the oxide-based tribofilm and increases friction upon cooling. These results establish a materials and architecture-based design strategy for wide-temperature, self-adaptive solid-lubricant coatings, and provide mechanistic guidelines for managing noble-metal diffusion and tribochemical film formation in severe tribological environments.
Cheng et al. (Mon,) studied this question.