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Studies in ultra-high vacuum have shown that a number of inorganic layer lattice solids can perform as solid lubricants in environments where adsorbed surface films of gases cannot form. Most notable of these is molybdenite (MoS2) which has better frictional properties in ultra-high vacuum (<10−9 torr) than in ordinary high vacuum (10−6 torr). In lightly loaded friction experiments of this sort, sliding appears to be largely confined to crystallite surfaces already present in the specimen. Then it is the adhesive forces between these surfaces that should determine friction levels. Hence adhesive forces between “clean” MoS2 surfaces appear to be substantially less than for surfaces contaminated with active gases (i.e., water vapor commonly present in unbaked high vacuum systems). This behavior contrasts sharply with that reported for many metals.
Haltner, A. J.