Environment-dependent tribological behavior of W—S—Si nanocomposite films induced by silicon incorporation

Layered WS2 is a promising solid lubricant, but its tribological behavior is highly sensitive to the environment, and the role of Si in regulating this sensitivity remains unclear. In this work, W—S–Si nanocomposite films were deposited by magnetron co-sputtering, and the effects of Si incorporation on microstructure, mechanical, and tribological behavior in vacuum and humid air were systematically examined. Si incorporation suppressed the growth of columnar WS2 platelets and promoted a denser, largely amorphous structure with improved load-bearing capacity. More importantly, increasing Si content induced an environment-dependent reversal in friction and wear behavior. Under vacuum, friction life decreased and wear increased progressively with Si addition, although the films became mechanically stronger. Interfacial analyses showed that Si accumulated at the sliding interface and hindered the reorientation and reconstruction of lubricious WS2 tribofilms. In contrast, under humid air, increasing Si content led to lower wear and stable low-friction sliding. This improvement was associated with tribo-oxidation and hydration of Si, which generated silanol-rich transfer films and a hydrated low-shear interfacial layer that acted synergistically with WS2 tribofilms. These results demonstrate that Si plays a dual, environment-sensitive role in WS2-based coatings: it is detrimental in vacuum because it suppresses tribofilm ordering, but beneficial in humid air because it activates tribochemical lubrication. The study provides a mechanistic basis and practical guidance for designing solid lubricating films for humid and vacuum service conditions.