Crystallinity of rf-sputtered MoS 2 films
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INTRODUCTION
2H—MoS2
The current approach to optimizing the friction and wear characteristics of solid lubricant films is empirical. For example, parameters used in the radio-frequency (rf)-sputtering process to produce MoS2 films are varied until the "best" films are obtained, but the way in which changes in the basic chemical and physical properties of those films affect their wear behavior is not well understood. Better understanding of these properties will enable production of films that exhibit "absolute" rather than "relatively" optimum wear behavior with respect to variation of their chemical and physical structure parameters. The low coefficient of friction of MoS2 results directly from its hexagonal, two-dimensional crystal structure (see Fig. 1); hence the crystallinity of rf-sputtered MoS2 films should be highly correlated with their lubrication properties. Cleaving an MoS2 single crystal to produce a basal plane ( 0 0 1 ) surface does not result in the breaking of covalent bonds, because successive basal planes are bonded to each other by relatively weak van der Waals forces. Therefore cleavage results in the production of a highly inert surface with no dangling bonds. J. Mater. Res. 2 (6), Nov/Dec 1987
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Mo
(0 0 1)
FIG. 1. Molybdenum disulfide (MoS 2 ) crystal structure.
0003-6951 /87/060827-12$01.75
© 1987 Materials Research Society
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J. R. Lince and P. D. Fleischauer: Crystallinity of MoS2 films
The wear life of a solid lubricant film is dependent on the adhesion of the film to a lubricated device. The adhesion can be shown to be related to, among other factors, the stress within the film.' Films grown by highrate techniques, such as rf sputtering, generally exhibit significant amounts of stress.2 Therefore it is advantageous to develop methods of detecting compressive or tensile stress within rf-sputtered films in order to correlate the stress with the film's wear behavior. Until now there have been few studies of the crystallinity and stress within sputtered MoS2 films, although the basic morphology of the films has been well studied by electron microscopy. Spalvins3 used transmission electron microscopy and diffraction (TEM) and (TED) to show that very thin ( - 4 0 0 A) rf-sputtered films have crystallites oriented with their edge planes [i.e., (h k 0)-type planes] parallel to the substrate surface. He also suggested that a measurement of the widths of the diffraction rings gives a qualitative measure of the size of the crystallites. Dimigen and coworkers4 applied TED to examine material that had been removed from substrates deposited with MoS^ films. They demonstrated that the basic MoS2 crystal structure was retained, even for highly sulfur-deficient films, i.e., for 1.0Sx
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