Structure Modulation and Self-Lubricating Properties of Porous TiN–MoS2 Composite Coating Under Humidity–Fluctuating Conditions

To improve the friction performance and service life of protective coatings in humidity-fluctuating environments, porous hard titanium nitride (TiN)–molybdenum disulfide (MoS2) composite coatings were prepared by using direct current magnetron sputtering (DCMS) with the mode of oblique angle deposition (OAD) and chemical vapor deposition (CVD) technologies. The structure and chemical component were characterized by field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), grazing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The tribological properties of these TiN–MoS2 composite coatings were investigated. The results indicate that the porous TiN–MoS2 composite coating exhibited outstanding friction performance and long service life under humidity-fluctuating environments. At the initial 20% relative humidity (RH) stage, the MoS2 on the porous TiN–MoS2 composite coating surface worked as an effective lubricant; thus, the coating demonstrated excellent lubrication performance, and the friction coefficient (COF) was about 0.05. As the humidity was alternated to 70% RH, the lubrication effect diminished due to the production of molybdenum oxide (MoO3), and the COF was about 0.2, which was attributed to the degradation of MoS2 on the wear track and the release of fresh MoS2 from the porous TiN matrix. After the environmental conditions shifted from 70% to 20% RH, the MoO3 was removed, and the lubrication effect was restored. In summary, TiN–MoS2 porous composite coating offers a promising approach for lubrication in humidity-fluctuating environments.