TABLE OF CONTENTS It is common practice to use solid lubricants such as graphite, tungsten disulphide or hexagonal boron nitride in sleeve-bearing materials to reduce friction in the 'boundary lubrication region'. The MA process has proved to be a powerful tool in producing an ultrafine microstructure containing dispersed solid lubricants such as graphite and tungsten disulphide [49,50], which has improved tribological properties.
Al-4.5%Cu-graphite has been produced by MA of the elemental mixture in an attritor mill in an inert atmosphere for a period of 3 hrs followed by consolidation by hot pressing. The MA results in a homogeneous distribution of graphite particles in the matrix, which leads to a dramatic decrease in the friction coefficient in the boundary region (Fig. 12.25) [49]. It is also interesting to note the slope of the curves in the boundary region, which is a measure of the rate of decrease in the friction coefficient f with respect to the 'bearing parameter' ZN/ P, where Z is the viscosity of the lubricating oil, N is the rpm of the shaft and P is the pressure on the bearing, which is faster in the case of MA materials. It is observed that an addition of approximately 3% graphite by MA is sufficient to improve the bearing performance of the alloy. Even in the presence of 3% graphite, the k-factor of the matrix remains unchanged, which is attributed to the refining of the grains and the strengthening effect...
