Testing has been conducted on silicon nitride (Si₃N₄) in simulated engine exhaust gas environments in four-ball rolling contacts and pin-on-disk sliding contacts to 590°C. Utilizing a steel race, the depth 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 presence of N₂ alone. Deposition of lubricous microcrystalline graphite carbon from CO within the exhaust was confirmed by Raman spectroscopy. Removal of H₂O from the exhaust gas further reduced rolling wear. Exhaust gas alone provided no benefit to Si₃N₄/Si₃N₄ sliding contacts, where the rate of wear greatly exceeds the rate at which lubricous carbon may deposit from the exhaust environment. As compare to that measured in the presence of N₂ alone, the directed admixture of hydrocarbon (C₂H₂) to the exhaust gas enhanced the lubrication of Si₃N₄ with reductions of up to 25-fold in rolling wear, 650-fold in steady state sliding wear rate and 20-fold in sliding friction.

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Published in the Journal TRIBOLOGY TRANSACTIONS
Volume 40 (1997) pages 374-380