Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si3N4/BN composites. In addition, the fracture toughness of Si3N4/micro BN composites was enhanced comparing to monolithic Si3N4 because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si3N4/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si3N4 matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.