OXYGEN SUBSTITUTION IN SPUTTER-DEPOSITED MOS2 FILMS STUDIED BY EXTENDED X-RAY ABSORPTION FINE-STRUCTURE, X-RAY PHOTOELECTRON-SPECTROSCOPY AND X-RAY-DIFFRACTION

The morphology and crystallite structure/orientation of sputter-deposited MoS2 solid lubricant films have been shown to be strongly affected by oxygen-containing species in the sputtering chamber. We have studied the effect of oxygen incorporation within these films using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Films produced at The Aerospace Corporation, the National Centre of Tribology (U.K.), Hohman Plating and Manufacturing, and NASA Lewis Research Center were studied to determine common characteristics of different MoS2 films produced by sputter deposition. In agreement with previous results, XPS of the films indicated a bulk anion:cation ratio (i.e. (S + O):Mo) of about 2, and XRD revealed that the films consisted completely of MoS2-like phases that were compressed in the (h k 0) directions as compared with pure MoS2. This "edge plane compression" appeared to correlate with oxygen content. EXAFS confirmed the absence of MoO2 in the films and indicated that the films actually consist of two similar MoS2-like phases. Correlation of the EXAFS results with those obtained from XPS and XRD indicated that the two phases are MoS2 and an MoS(2-x)O(x) substitutional solid solution. The solid solution may be described as MoS2 with oxygen atoms that have substituted for sulfur atoms in the MoS2 crystal lattice. Relative peak height changes in the EXAFS radial distribution curves indicated that increasing amounts of oxygen cause a reduction in the size of crystallites in the films. The presence of the MoS2-xO(x) phase may explain the superior tribological performance of MoS2 films in some applications, which has been shown to correlate with oxygen impurities in the films.