Tribological performance of Ti/MoS₂–Ti coatings deposited by pulsed-DC closed-field unbalanced magnetron sputtering under atmospheric and vacuum conditions

Many engineering components are required to operate under both atmospheric and vacuum environments, where friction and wear mechanisms differ significantly. Therefore, solid lubricant coatings designed for such applications must maintain stable tribological performance across multiple environments rather than being optimized for a single condition. In this study, Ti/MoS₂–Ti composite coatings were deposited on AISI 52100 bearing steel using pulsed-DC closed-field unbalanced magnetron sputtering. A Taguchi L9 experimental design was employed to systematically investigate the influence of key deposition parameters, including working pressure, substrate bias voltage, and target currents, on coating composition, microstructure, mechanical properties, and tribological behavior. The coatings were characterized using SEM/EDS, XRD, and nanoindentation analyses, while friction and wear behavior were evaluated by ball-on-disc tests under atmospheric, low-vacuum (5 ×10⁻³ mbar), and high-vacuum (8 ×10⁻⁶ mbar) conditions. The coatings exhibited sulfur-to-metal ratios S/(Mo+Ti) ranging from 0.28 to 0.48, with hardness and elastic modulus values between 9.9–32.4 GPa and 150–398 GPa, respectively. Tribological performance strongly depended on both environmental conditions and coating composition. Under vacuum conditions, coatings with higher sulfur retention showed significantly improved wear resistance due to the formation of a stable MoS₂-based tribofilm, whereas sulfur-deficient coatings exhibited severe wear and tribofilm instability. The results demonstrate that high hardness alone does not guarantee improved wear resistance in vacuum environments, and coatings combining sufficient sulfur retention with balanced mechanical properties provide superior tribological stability across different environments. © 2026 Elsevier Ltd.