SCANNING ELECTRON-MICROSCOPY OF TRIBOCHEMICAL FILMS ON SI3N4 TO H2O AT HIGH-TEMPERATURES

Polycrystalline silicon nitride has been abraded in a ball-on-flat apparatus in a controlled water environment at elevated temperatures up to 1100-C. It was found that thc friction coefficient and wear rate were significantly reduced at varying H2O partial pressures in the temperature range 300-800-degrees-C. The high friction and wear values were regained when the temperature was increased; the high temperature transition depended on the water partial pressure. Scanning electron microscopy of those wear tracks, when the friction coefficient was low, showed that the wear groove contained cylindrically shaped debris (rolls) oriented perpendicular to the stiding direction. The "rolls" support the load, lowcr thc friction coefricient and reduce the wear rate.