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

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 MoS_2−xO_x phase. MoS_2−xO_x 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 MoS_2−xO_x and MoS₂, but formed a disordered NiO_x phase. However, Ni-cosputtered films showed decreasing Mo-Mo bond lengths in the MoS_2−xO_x 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 NiO_x formation at the edges of nucleating MoS_2−xO_x/MoS₂ crystallites, limiting the crystallite size attainable within the films.