An analytical observation of the tribochemical reaction of silicon nitride sliding with low friction in aqueous solutions

Friction and wear of silicon nitride sliding against the same material is investigated in aqueous solutions: distilled water, 1% H2O2 and 3% NaCl. The friction coefficient (mu) of silicon nitride in water decreases from 0.8 to 0.01 during sliding. In a 1% H2O2 solution, the friction coefficient decreases at an earlier stage of sliding than in pure water. But in 3% NaCl solution, the friction coefficient keeps a high value (mu=0.8) during sliding over 1000 m, Analysis of the wear debris of silicon nitride in 3% NaCl solution shows that the composition of the debris corresponds to Na:Si:O = 1:2-3:4-5, which supposedly suppresses the lubricating surface friction force on the sliding solid surfaces, The analytical results support the theory that the silicon nitride surface sliding with a low friction coefficient is composed of a reactive site substitutable by a Na+ ion. The results indicate that the lubricant film which supports the high contact pressure between silicon nitride surfaces includes the hydroxide of silicon. A structural model of the sliding surface is proposed. (C) 1997 Elsevier Science S.A.