Role of combination of hexagonal boron nitride and graphite in NAO friction material

Copper is added in non-asbestos organic (NAO) friction materials (FM) to increase the thermal conductivity and to enhance the lubricity at high temperature apart from the well-accepted fact that it influences tribological performance also. Copper is being phased out slowly in FM formulations because of its proven threat to the aquatic life. Nowadays, the research in this area is being focussed on finding alternatives to Cu. Hexa-boron nitride (hBN) also known as white graphite is a ceramic powdery filler, having crystalline structure similar to that of graphite along with various desirable properties (such as higher thermo-oxidative stability and thermal conductivity) which could be possibly suitable for developing Cu-free FMs. Hence, it was thought worthwhile to explore hBN as a potential ingredient in FM formulation for replacing copper. Five FMs in the form of brake shoes were developed, keeping all other ingredients identical and varying combinations of two solid lubricants viz. graphite and hBN systematically. The amount of these two solid lubricants was purposefully kept high (20%) so that the effect on performance properties could be visible clearly. Samples were tribo-evaluated using chase test rig as per IS 2742 standard. The paper discusses on the influence of these solid lubricants on performance properties of the series of five FMs.