Tribological characteristics of Si 3N 4–hBN ceramic materials sliding against stainless steel without lubrication

Abstract

The main objective of this paper is to study the tribological behavior of Si 3N 4–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 Si 3N 4 resulted in a severe decrease of the friction coefficient, from 0.95 for Si 3N 4 against stainless steel to 0.03 for Si 3N 4–30% hBN against stainless steel; (2) wear coefficients were below 10 −4 mm 3/Nm for all materials tested and decreased sharply with increase in hBN volume fraction, e.g. lower than 10 −6 mm 3 N −1 m −1 magnitude for Si 3N 4–20% 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 Si 3N 4–hBN composites was spalled off during the wearing tests and spalling pits were formed on the wearing surface of Si 3N 4–hBN composites firstly, and then the wear debris was embedded into the pits and oxidized, thus a tribofilm containing B 2O 3, SiO 2 and Fe 2O 3 was formed on the wearing surface finally. This tribofilm can protect both Si 3N 4–hBN pin and stainless steel disc, and lubricate the wear surfaces.