EXAFS OF SPUTTER-DEPOSITED MOS2 FILMS

The composition and structure of sputter-deposited MoS2 solid lubricant films are difficult to analyze by diffraction techniques because the films typically exhibit poor crystallinity. The extended X-ray absorption fine structure (EXAFS) technique is ideal for analyzing these properties because it probes short-range (local) order parameters such as interatomic bond lengths. EXAFS data for the Mo K X-ray absorption edge were analyzed, including least-squares fitting, to investigate films produced under a number of conditions, as well as those produced with varied amounts of oxygen incorporated in the films. The results were compared with X-ray photoelectron spectroscopy and X-ray diffraction data of the same films. Analysis showed that the films, which contained 9-40 at.% oxygen, consisted predominantly of a MoS2-xOx phase, with x continuously variable. The MoS2-xOx phase exhibited a MoS2-like structure, where oxygen atoms had substituted for sulfur atoms in the MoS2 crystal lattice. In addition, smaller quantities of pure MoS2 (10-25 times lower) were detected in the films. Increasing oxygen content correlated with increasing values of x in the MoS2-xOx phase, as well as with decreasing relative amounts of the pure MoS2 phase. The presence of the MoS2-xOx phase may explain the decrease in friction with increasing oxygen content that has been observed for MoS2 films. In addition, the poor order (high defect level) in this phase may explain the density enhancement caused by crystallite size reduction; higher densities can be correlated with beneficial wear behavior of the films.