Microstructure of cosputter-deposited metal- and oxide-MoS 2 solid lubricant thin films
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Microstructure of cosputter-deposited metal- and oxide-MoS2 solid lubricant thin films M. R. Hiltona) Mechanics and Materials Technology Center, The Aerospace Corporation, El Segundo, California 90245
G. Jayaram and L. D. Marks Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 10 March 1997; accepted 15 July 1997)
The effect of cosputtering small amounts of Ni (3%, 9%) and SbOx (20%) on the final microstructure of MoS2 lubricant thin films has been studied using a combination of scanning and transmission electron microscopy imaging, and electron and x-ray diffraction techniques. The early-growth, near-interface microstructure of both MoS2 and 3% Ni–MoS2 cosputtered films is revealed to be a mixture of (002) basal and elongated, large-size (100) and (110) edge islands. Cosputtering with 9% Ni induces a dramatic change in the microstructure, i.e., primarily basal domains with very small isolated regions of edge islands, while cosputtering with 20% SbOx produces films having no long-range order. The results are compared with and are consistent with previously published x-ray absorption fine structure data. The impact of film morphology on tribological performance is discussed.
I. INTRODUCTION
Sputter-deposited films of MoS2 have been used as solid lubricants on a variety of spacecraft mechanisms.1 Most of the early applications included low-cycle devices such as release and deployment mechanisms. The earliest generation of these films had strong (100) and (110) crystal orientation parallel to the substrate, i.e., the low-shear-strength (001) orientation was perpendicular to the substrate, with a columnar plate zone 2 morphology (Thornton model2 ) having significant porosity.3,4 [The zone models use the dominant diffusion mechanism operating during growth as a classification criterion. Zone 2 films result when surface diffusion dominates; this structure consists of well-defined columnar grains that are generally crystalline. MoS2 zone 2 have a columnar-plate structure. A zone 1 structure is the result of limited (or absent) adatom mobility that is insufficient to overcome the effect of shadowing favored at low TyTm (deposition and melting temperatures, respectively) and high gas pressures. The morphology of the zone 1 films consists of large dome-shaped grains that have poorly defined fibrous interiors. These films can be amorphous or crystalline. There is also a zone T (transition) morphology, favored by lower gas pressures and a higher TyTm that has a structure between the zone 1 and 2 structures. The zone T material has the zone 1 fibrous interior but with flat tops and it is not porous.] In MoS2 films, the zone 2 morphology has several disadvantages. Most of the film is in the wrong oriena)
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J. Mater. Res., Vol. 13, No. 4, Apr 1998
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tation for lubrication, since the basal planes are perpendicular
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