Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate - Part II - Thermo-oxidative stability of Cs2MoOS3/Si3N4 mixtures

Cesium oxythiomolybdate (Cs2MoOS3) may be an excellent high temperature lub ricant, providing a friction coefficient below 0.2 at 650 degreesC. However , oxidation products provide the lubrication above 400 degreesC. Lubricant effectiveness depends strongly on the composition of the substrate material s in contact, such as Si3N4, suggesting that tribochemical and/or thermal r eactions at the interface produce new compounds. The thermo-oxidative stabi lity of Cs2MoOS3/Si3N4 and Cs2MoOS3/SiO2 mixtures have been evaluated betwe en room temperature and 1000 degreesC in air. The transition temperatures a nd oxidation products were identified. The thermal chemistry of Cs2MoOS3/Si 3N4 mixtures was significantly different than that of Cs2MoOS3 alone, large ly due to the oxidation of Si3N4 to glassy SiO2. Cesium oxythiomolybdate fo rmed cesium oxides, which melted below 600 degreesC. As SiO2 is formed, the cesium oxides diffused into it, creating a cesium silicate glass. Also, Cs 2MoO4 was preferentially formed over complex cesium molybdates and molybden um oxides. In a tribological application, Cs2MoO4, oxides, and cesium silic ate glass may be formed at contacting interfaces from Cs2MoOS3 films deposi ted on Si3N4 substrates. Lubrication would be provided as the shear strengt h of these compounds decreases with increasing temperature. (C) 2001 Kluwer Academic Publishers.