Wear behaviour of steel-on-Si₃N₄ and Si₃N₄-on-Si₃N₄ systems with vapor phase lubrication of oleic acid and TCP

The wear property of Si₃N₄ under vapor phase lubrication of oleic acid and tricresyl phosphate (TCP) in both steel-on-Si₃N₄ and Si₃N₄-On-Si₃N₄ systems were investigated. The used oleic acid and TCP were fed into the evaporator simultaneously with two syringe pumps, and the vaporized lubricants were delivered by the carrier gas of nitrogen to the ball pot of a modified four-ball wear tester. Tests were made using M50 tool steel ball or Si₃N₄ ball against Si₃N₄ discs at a load of 98 N and temperature of 370°C. Oleic acid, TCP and the combination of oleic acid with TCP as the vapor lubricants were evaluated for test time from 30 min to 3 h. Results indicate that the oleic acid provides effective lubrication for steel-on-Si₃N₄ system, and for the combination of oleic acid with TCP, the wear can be even reduced. The wear of Si₃N₄ disc in steel-on-Si₃N₄ system is always lower as compared to that of in Si₃N₄-On-Si₃N₄ system. X-ray photoelectron spectroscopy (XPS) analyses of the rubbed surfaces suggest the formation of a film containing carbonaceous deposits, iron phosphate, N-containing compounds and silicon oxides due to the oxidation of Si₃N₄.