Friction and wear properties of Si3N4-hBN ceramic composites using different synthetic lubricants

This paper presents a tribological investigation of Si3N4-hBN composite ceramics using synthetic lubricants. The friction and wear properties of Si3N4-hBN ceramic composites sliding against TC4 titanium alloy (Ti6Al4V) were investigated via pin-on-disc tests. An axial compressive load of 10 N was applied with a sliding speed of 0.73 m/s. Three different lubrication conditions including simulated body fluid (SBF), physiological saline (PS) and bovine serum (BS) were used. For SBF lubrication, the friction coefficients and wear rates of Si3N4-hBN/Ti6Al4V pairs were varying with the increase of hBN contents. When using 20 vol% hBN, the average friction coefficient and wear rate of Si3N4 (0.28 and 3.5 × 10−4 mm3 N−1 m−1) were as good as that of the pure Si3N4 (0.34 and 3.69 × 10−4 mm3 N−1 m−1). Meanwhile, the processability of the Si3N4 material would be improved by adding hBN. It was worth to mention that when using 30 vol% hBN, the tribological performance of bearing combination deteriorated with extensive wear from the ceramic pin. This may due to the reduction of mechanical property caused by adding hBN and the occurring of tribochemical reaction. According to the worn surface examination and characterization, the main wear mechanism was abrasive and adhesion wear. Scratch grooves were observed on the metal disc, and metallic transform layers were seen on the ceramic pin. Moreover, surface lubrication film consisting of TiO2, SiO2·nH2O, Mg(OH)2, and H3BO3 were formed on the metal disc when using SBF lubrication and 20 vol% hBN content. Among the three lubrication conditions, SBF generally led to the best tribological performance. No surface lubrication film was found during BS and PS lubrications. This may be resulted from the absence of essential ions to promote the formation of surface lubrication film (PS lubrication) and the formation of a protein barrier on the surface of the metal disc (BS lubrication).