Solid lubricants for applications at elevated temperatures

Relative motion between mating surfaces at elevated temperatures often causes substantial material degradation due to friction and wear. Conventionally, solid lubricants have been used to reduce wear damage and friction drag under extreme conditions where liquid lubricants do not function properly. The recent trend towards higher operating temperatures in advanced power generating systems, i.e. turbomachinery, gas turbines, and hot adiabatic diesel engines, has imposed severe limitations on the currently available solid lubricants. The unusually aggressive conditions in these systems phased out most conventional solid lubricants and gave impetus to the search for more efficient materials. This paper discusses the lubricating characteristics of four different groups of materials known to provide lubricity under elevated temperature conditions. These groups are polymers, laminar solids, metal fluorides and metal oxides. Polymer lubricants are efficient lubricants within the range from room temperature to about 300 °C. Laminar solids extend that range to about 450 °C. Graphite, also a laminar solid, is an exception since it can offer excellent lubricity beyond 450 °C in the form of gaseous oxidation products. Stable fluorides and metal oxides are useful lubricants between 500 and 1000 °C, though their performance is rather poor at lower temperatures.