Solid lubricants that are effective over an extended range of operating temperatures are necessary for the development of high-temperature foil bearings. Unfortunately, there is no single known solid lubricant that can provide low-friction over the desired range, from sub-ambient to 800 ℃ in various harsh environments. Relative to liquid lubricants, solid lubricants generally have much lower vapor pressures, lower sensitivity to radiation effects and operate in wider temperature ranges. Also, a solid material can be tailored in crystal structure, stoichiometry, microstructure, surface morphology and physical form to achieve desired wear and friction properties. Materials whose lattices provide mechanically weak shear planes under required ambient conditions are often chosen for efficient solid lubrication applications. For example, in graphite, the hexagonal lattice with a large c-axis can provide good lubrication properties due to weak bonding force between adjacent hexagonal planes. However, graphite oxidizes in air at temperatures above 400℃ and, thereby, loses its lubricious properties. Most solid lubricant formulations presently used consist of molybdenum disulfide (MoS2) or graphite. MoS2 is an example of a class of compounds known as transition metal dichalcogenides. These materials typically have a hexagonal crystal structure. Each sheet is trilayered with a metal atom (e.g. Mo, W, Nb, or Ta) in the middle, covalently bonded to chalcogen atoms (e.g. S, Se or Te). The layered materials generally begin to oxidize at about 300 8C in air; thus limiting their application at relatively low temperatures. Tungsten disulfide (WS2) is sometimes used because it has been reported to have an upper operating temperature limit of about 100 ℃ higher than MoS2. In the last decade, in addition to these materials, various other phases such as NbSe2, NbS2, NbTe2,hBN, CFx, TiO2, PbO, CeF3, BaF2, and CaF2 in various configurations have been proposed for solid lubrication, particularly for aerospace applications. These coatings provide a low coefficient of friction as a result of the basal plane slip during sliding. Therefore, the crystallographic orientation of the deposited film becomes an important part for achieving good lubrication. However, some studies have suggested that this may not be a critical factor since crystallographic orientation can be favorably modified by sliding. A study of MoS2/metal coatings by Simmonds et al. concluded that a multilayer structure is not a prerequisite for an acceptable tribological performance and that it is not critical to promote the (002) orientation in MoS2 to achieve a low coefficient of friction. This finding is at variance with those of other researchers. Blau et al. and Bae et al. have studied a solid lubricant composition containing titanium nitride and molybdenum disulfide. Friction and wear tests for this material system at 300 and 700 8C produced encouraging results, but tests run at an intermediate temperature of 400 ℃ exhibited friction coefficients of 1.0 or greater. Oxidation and a change in the nature of the third-body debris layers formed during sliding were believed to be responsible for this behavior. Other studies of hard/soft multilayer coatings also examined coating properties using tribological tests however, they did not specifically attempt to develop a phenomenological understanding of the microstructure-properties- performance synergy in any particular application.
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