TEM lattice imaging of the nanostructure of early-growth sputter-deposited MoS 2 solid lubricant films
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I. INTRODUCTION
2H-M0S,
Molybdenum disulfide (MoS2) is a useful solid lubricant because it deforms plastically more readily than the solid surfaces between which it is placed. On the macroscopic scale, the low shear strength of MoS2 reduces the friction between sliding surfaces. On the atomic scale, the low shear strength (which results in low friction) of MoS2 is explained by the material's anisotropic crystal structure and bonding: MoS2 is comprised of hexagonally packed planes (a,b = 0.274 nm; c = 1.23 nm) consisting of a layer of Mo bounded on each side by a layer of S (see Fig. 1). All effective strong bonding is within the "sandwich" planes, with weaker van der Waals bondings between adjacent sandwiches. MoS2 is strong in two dimensions and weak in thethird, which makes the material a two-dimensional mechanical analog to one-dimensional linear polymers. The low-shear-strength basal planess provide an atomic mechanism for single-crystal plastic deformation; this mechanism plays a role similar to that of dislocations in close-packed metals. However, on the microstructural scale, which relates atomistic phenomena to macroscopic phenomena, the mechanisms of MoS2 plastic deformation have not been fully explained. For example, MoS2 lubricants are polycrystalline, and while the mechanism of basal plane slip explains deformation within a single crystal, the mechanism of intercrystalline slip and its contribution to overall deformation are not understood at this time. 406
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J. Mater. Res., Vol. 5, No. 2, Feb 1990
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• Mo
Q S
FIG. 1. The MoS2 crystal structure.
Sputtering provides a method of applying MoS2 as a lubricant in thin film form. It avoids the use of organic binders used in powder applications, which can outgas in the vacuum of space. Studies have attempted to relate MoS2 sputter-deposition conditions to the ma11990 Materials Research Society
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M. R. Hilton and P. D. Fleischauer: TEM lattice imaging
terial's mechanical properties, such as friction and wear, and oxidation resistance.1"5 However, an increasing emphasis has been placed on elucidating the physical properties of films, such as composition, crystallinity, crystal orientation, and adhesion, in order to explain the effects of deposition conditions on film performance.6"24 These microstructural studies indicate that MoS2 films often possess a columnar-plate, zone 2 morphology (the Thornton model26'27), with the basal plates perpendicular to the substrate. The zone 2 morphology is indicative of active surface diffusion during film growth, which is favored by elevated substrate temperatures (these temperatures, in turn, being influenced by secondary-electron bombardment) and, in the case of MoS2, by a reduced water-vapor-pressure background during deposition.15'17'18 The zone models use the dominant diffusion mechanism operating during growth as a classification criterion. The zone 1 structure is the result of a low (or absent) adatom mobility that is insufficient to ov
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