Metal incorporation in sputter-deposited MoS2 films studied by extended x-ray absorption fine structure

Abstract

Solid lubricant films produced by cosputtering metals with MoS2 and by forming metal/MoS2 multilayers are being planned for use in the next generation of solid lubricated devices on spacecraft, including gimbal and sensor bearings, actuators, and sliding electrical contacts. The films exhibit increased densities and wear lives compared to films without additives, but the mechanism of density enhancement is not well understood. The extended x-ray absorption fine structure (EXAFS) technique is ideal for elucidating the structure of these poorly crystalline films. We analyzed MoS2 films cosputtered with 0, 2, and 10% Ni, as well as Ni/MoS2 and Au(Pd)/MoS2 multilayer films. The results obtained at the Mo-K absorption edge showed that the metal-containing films comprised predominantly the same nanocrystalline phases present in similar films without added metals: pure MoS2 and a MoS2−xOx phase. MoS2−xOx is isostructural with MoS2, with O atoms substituting for S atoms in the MoS2 crystal lattice. For all Ni-containing films, EXAFS data obtained at the Ni-K absorption edge showed that the Ni had not chemically reacted with the MoS2−xOx and MoS2, but formed a disordered NiOx phase. However, Ni-cosputtered films showed decreasing Mo-Mo bond lengths in the MoS2−xOx phase with increasing Ni content, probably due to preferential oxidation of Ni compared to MoS2. EXAFS of these Ni-cosputtcred films showed only a small decrease in short-range order with Ni content, while x-ray diffraction showed a concurrent large decrease in long-range order. The results indicate that film densification in Ni-cosputtered films is caused by NiOx formation at the edges of nucleating MoS2−xOx/MoS2 crystallites, limiting the crystallite size attainable within the films.