Evaluation of advanced solid lubricant coatings for foil air bearings operating at 25° and 500°C

The tribological properties of one chrome oxide and one chrome carbide based solid lubricant coating were evaluated in a partial-arc foil bearing at 25 degrees and 500 degrees C. Start/stop bearing operation up to 20,000 cycles were run under 20 kPa (1.5 psi) static deadweight load. Bearing friction (torque) was measured during the test. Specimen wear and SEM/EDS surface analyses were conducted after testing to understand and elucidate the tribological characteristics observed The chrome oxide coating which contains both Ag and BaF2CaF2 for low and high temperature lubrication, exhibited low friction in sliding against Al2O3 coated foils at 25 degrees and 500 degrees C. The chrome carbide coating, which lacked a low temperature lubricant but contained BaF2/Caf(2), as a high temperature lubricant, exhibited high friction at 25 degrees C and low friction at 500 degrees C against both bare and Al2O3 coated superalloy foil surfaces. Post test surface analyses suggest that improved tribological performance is exhibited when a lubricant film from the coating transfers to the foil surface.