Pulsed laser-ablated MoS2-Al films: friction and wear in humid conditions

The application of MoS2 as a solid lubricant in dry conditions is well established, but the presence of humidity degrades the tribological behavior of MoS2 films. There has been much progress toward increasing the efficiency of MoS2 films in humid environments by using various additives. In our work, aluminum is evaluated as an additive because of its affinity for O2 and (OH) compared to Mo and its potential to modify transfer film formation. The MoS2 films examined here were deposited on 440C steel substrates by laser ablation along with simultaneous magnetron sputtering of Al. The friction and wear studies were made on a ball on disc tribometer. Films containing Al outperformed pure MoS2. Under optimal conditions, 1–2-μm-thick films with ∼8% aluminum had a wear life of 150 to 250 K cycles in 25% humidity. Micro-Raman and X-ray photoelectron spectroscopy (XPS) were used to analyze chemistry and scanning electron microscopy (SEM) was used to study the morphology and film cross-section. Nanoindentation showed that Al addition did not increase film hardness. The role of aluminum in increasing the wear life of laser-ablated MoS2 films in humid conditions is discussed. The mechanism controlling the increased wear life is partially explained by showing that Al is gradually turned into alumina, creating an adhesive and lubricating mix of wear debris under tribostress in humid surroundings.