Mechanism of high-temperature lubrication of ceramics by deposits from organic vapors

Abstract

While lubrication of hot-pressed silicon nitride surfaces at temperatures between 350 and 650°C by carbons deposited from carbonaceous gases and vapors has been demonstrated to be very effective (friction coefficients of less than 0.02 and negligible wear), a chemical mechanism has not yet been established. Tribometer experiments followed by surface spectroscopies of Raman, fluorescence, Auger and XPS are consistent with a heterogeneous mechanism of vapor decomposition. Laser-induced photoluminescence determined with a Raman spectrometer using 514 nm argon ion excitation showed a nearly 1 ∶: 1 correspondence between the intensity of the broad maximum near 4000 cm^-1 with carbon adsorption and lubrication efficiency: surfaces of low fluorescence could not be lubricated by ethylene or similar gases. Carbon was found to adsorb preferentially on wear tracks, which also exhibited very high fluorescence. Wear track surfaces generally exhibited a lower N/Si surface composition ratio than the surrounding surfaces. The reduced nitrogen content could be responsible for dangling bonds and structural defects, which would result in more chemisorption and strong fluorescence.