pH Dependence of Tribochemical Wear of Silicon Nitride Sliding against Polycrystalline Diamond in Alkaline Solutions

The tribological behaviors of silicon nitride sliding upon sintered polycrystalline diamond (PCD) were explored via altering pH levels. High reductions in the friction and wear are achieved when sodium hydroxide solution is introduced at the contact interfaces. This reduction is found to depend on the pH value of the alkaline solution. The friction coefficient and wear loss of silicon nitride vary with pH level by more than half an order of magnitude and achieves the lowest values at pH 13. Moreover, the tribological behaviors reveal three stages versus the pH level, for which different tribochemical reactions occur due to the dissimilar hydroxyl concentrations. The dissolution of reaction products, such as Si(OH)(4) and dimers (OH)(3)Si-O-C(OH)(3), might lead to an increased proportion of hydrodynamic lubrication in the mixed lubrication regime and hence yield reduced friction and wear.