Metal DiSulfide thin films synthesized by pulsed ion beam ablation: Self-assembled structures with improved mechanical properties in humid conditions

Thin films composed of transition metal dichalcogenides such as molybdenum disulfide (MoS2) or tungsten disulfide (WS2) are well known for their solid lubricating behavior in dry environments such as space, but deteriorate rapidly in environments where oxygen is present. There have been many attempts over recent decades to improve MoS2 lubricating performance in humid environments by addition of metals such as titanium to the film. The additions have come in the form of co-sputtering with the MoS2, or formation of nano multilayer coatings. We have previously reported on the synthesis of MoS2 films by high-power ion beam ablation [Prasad et al, Matls. Lett. 65 (2010) 4-6]. The resulting films are observed to contain spheres of pure Mo of 10-100 nm size evenly dispersed throughout the similar to 1 mu m-thick film, a form of self-assembly not before seen in films produced by sputtering or by pulsed laser deposition (PLD). The remaining MoS matrix was observed to crystalize in situ under frictional contact into MoS2 with the basal planes orienting themselves in the direction of sliding underneath the wear surface. The present study concerns metal additions in the form of alternating layers with MoS2 deposited in between from an ablation target. The addition of Ti resulted in the extension of good tribological performance to humid air conditions, but the path to good performance appears to be relatively narrow. The same distributed Mo spheres were present in the new film, and similar self-assembled morphologies when V was substituted for the Ti, and when multilayers were formed of WS2 with Zr additions. It appears that addition of Ti resulted in improvements in mechanical film strength that contributed to the wear ability in humid conditions.