Tribological behavior and lubricating mechanism of Si3N4 in artificial seawater

Abstract Tribological behavior and lubrication mechanism studies on Si3N4 sliding against two different materials (stainless steel/PEEK) under different normal loads in artificial seawater condition was conducted by pin-on-disc wear test, SEM, EDS and XPS techniques. With increasing load from 10 N to 30 N, the friction coefficient of Si3N4/stainless steel sliding pair increased from 0.48 to 0.72. Whereas, the friction coefficient of Si3N4/PEEK sliding pair decreased from 0.27 to 0.07. However, the wear rates of these two sliding pairs were consistent. When Si3N4 slid against stainless steel, the formation of tribochemical film contributed to a reduced friction and wear at a low load of 10 N. Si3N4 sliding surface was protected by the tribofilm and smoothed. As a result, friction coefficient of the sliding pair decreased. However, with increasing load, the film could not withstand the pressure. Hence, the direct contact of Si3N4/stainless steel pair was developed and furrows on their solid surfaces occurred, leading to the increase of friction coefficient. When Si3N4 slid against PEEK, a good tribological performance was achieved in seawater condition, which was possibly attributed to the viscoelasticity of PEEK, seawater cooling process, water adsorption of PEEK, and tribochemical reaction of Si3N4. Besides, the relation between friction coefficient and normal load could be expressed by a classical equation f = K N ( n − 1 ) .