Fluoride Solid Lubricants for Extreme Temperatures and Corrosive Environments

Fluorides of the alkali metals and the alkaline earth metals have an interesting combination of properties that make them promising candidates as solid lubricant materials for use at high temperatures and in corrosive environments. They are chemically inert in strong oxidizing or reducing environments; they are relatively soft and nonabrasive; and some of them, such as CaF2, have planes of perfect cleavage in their crystal structure suggesting low-shear strength and good friction properties. Thin, fused fluoride coatings (0.001 inch) were applied to nickel-chromium alloys by spraying water slurries of the fluorides on the bearing surface, drying it, then firing it in a hydrogen atmosphere. Coatings of CaF2-LiF mixtures and of CaF2-BaF2 mixtures were effective as solid lubricants in hydrogen to 1500 F and in air to 1200 F. The CaF2-BaF2 coatings were effective solid lubricants in liquid sodium at 1000 F and a sliding velocity of 2000 ft/min.