Research on cesium oxythiomolybdate, Cs2MoOS3, was initiated as a response to the requirement for a high temperature solid lubricant for lightweight single-use engines. These engines would most likely use ceramic bearings. Ceramics are a natural choice for engine components due to their lower density, higher melting points, and greater oxidation resistance than typical metals. In aerospace applications, silicon nitride became a leading candidate for ceramic bearings because of its excellent high temperature properties. Lubricants for these bearings were required for operation between -50 and +800℃. Cesium oxythiomolybdate has been shown to be an effective high temperature solid lubricant when used in a coating with a sodium silicate binder, giving a coefficient of friction of below 0.2 at 650℃. Ordinarily, thermal stability of a lubricant over the entire application temperature range is required. However, since Cs2MoOS3 performs well at elevated temperature, and understanding its oxidation has been the subject of research. In addition, by heating mixed powders of Si3N4 and Cs2MoOS3 in air and analyzing the products, it was found that the oxidation reactions were different in the presence of Si3N4. Cesium silicate glass also formed and may play an important role in lubrication. The silicate glass is produced as Si3N4 oxidizes. Silicon nitride reacts with oxygen to form a scale of SiO2. This protective layer is smooth, continuous, and amorphous. The reaction occurs at the nitrideyoxide interface with the formation of SiO2 and nitrogen gas: Si3N4(s)+3O2(g) 3SiO2(s)+ 2N2 (g). When kept at elevated temperature for long periods of time, glasses may transform to the lower free energy crystalline state via devitrification, which is thermodynamically irreversible. The amorphous silica formed on Si3N4 surface stends to crystallize at longer heating times, higher temperatures, and in the presence of impurities.
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