Observation of delamination wear of lubricious tribofilm formed on Si 3N 4 during sliding against WC-Co in humidity air

Abstract

Hot pressed silicon nitride that was exposed to high (90%) and low (32%) relative humidity was examined in ball-on-disc geometry against cemented carbide ball at various normal loads. The study indicated that Si 3N 4 tested at high R.H. gave less specific wear rate compared with Si 3N 4 at low R.H. The friction coefficient of Si 3N 4−WC-6% Co tribopairs was found in the range of 0.32–0.39 and 0.05–0.17 at low humidity and high humidity respectively. It is suggested that adsorbed moisture markedly affected the wear and friction properties of silicon nitride. Following the tests, SEM was used to elucidate the wear mechanism and particularly to delineate the effects of relative humidity on the wear and friction. SEM micrographs showed that the main wear mechanism at low relative humidity (32%) was caused by mechanical wear including abrasive grooves, large holes and polishing, whereas at high relative humidity (90%) the main mechanism was highly influenced by a tribochemical reaction related to the moisture adsorption from the environment. It is concluded that the removal of lubricious tribolayer was occurred by delamination induced crack propagation.