Tribological characteristics of Si3N4–hBN ceramic materials sliding against stainless steel without lubrication

The main objective of this paper is to study the tribological behavior of Si3N4–hex.BN (hBN) composites sliding on stainless steel. Unlubricated wear tests were carried out on a MMU-5G type pin-on-disc wear tester. The results shows: (1) the addition of hBN to Si3N4 resulted in a severe decrease of the friction coefficient, from 0.95 for Si3N4 against stainless steel to 0.03 for Si3N4–30% hBN against stainless steel; (2) wear coefficients were below 10−4 mm3/Nm for all materials tested and decreased sharply with increase in hBN volume fraction, e.g. lower than 10−6 mm3 N−1 m−1 magnitude for Si3N4–20% hBN pin. The materials and worn surfaces as well as wear debris were characterized by means of SEM and XRD. The tribofilms were analyzed by using EPMA, laser scanning microscope (LSM) and XPS. The analysis indicates that, without lubrication, hBN in Si3N4–hBN composites was spalled off during the wearing tests and spalling pits were formed on the wearing surface of Si3N4–hBN composites firstly, and then the wear debris was embedded into the pits and oxidized, thus a tribofilm containing B2O3, SiO2 and Fe2O3 was formed on the wearing surface finally. This tribofilm can protect both Si3N4–hBN pin and stainless steel disc, and lubricate the wear surfaces.