LUBRICATION OF SILICON-NITRIDE IN A SIMULATED TURBINE EXHAUST-GAS ENVIRONMENT

Testing has been conducted on silicon nitride (Si3N4) in simulated eng ine exhaust gas environments in four-ball rolling contacts and pin-on- disk sliding contacts to 590 degrees C. Utilizing a steel race, the de pth of the wear track formed at the Si3N4/Si3N4 rolling contact in the presence of exhaust gas was roughly one-half that formed in the prese nce of N-2 alone. Deposition of lubricous microcrystalline graphite ca rbon from CO within the exhaust was confirmed by Raman spectroscopy. R emoval of H2O from the exhaust gas further reduced rolling wear. Exhau st gas alone provided no benefit to Si3N4/Si3N4 sliding contacts, wher e the rate of wear greatly exceeds the rate at which lubricous carbon may deposit from the exhaust environment. As compared to that measured in the presence of N-2 alone, the directed admixture of hydrocarbon ( C2H2) to the exhaust gas enhanced the lubrication of Si3N4 with reduct ions of up to 25-fold in rolling wear, 650-fold in steady-state slidin g wear rate and 20-fold in sliding friction.