Silicon nitride and graphite ceramic powders with a supposed composition of 50% of each by weight were mechanically alloyed in laboratory ball mill with different milling times (12.5, 25, 50, 75 and 100 hours). The investigation was carried out on the microstructure and density of ball-milled powder. It was observed that with milling time density increased initially and then started decreasing. The density was 2.83 at 0 hours and 2.81 g/cm3 at 100 hours. The results showed that ball milling is a very dominant method and a very dynamic process for preparation of combined powder with homogeneous microstructure having appropriate morphology and homogenized density.

Silicon nitride and graphite are ceramic materials and find application in the fabrication of particulate metal matrix composites because they are hard and brittle having well wear resistance, and used in tribological applications. Silicon nitride is a very hard material (HV = 35.31 GPa) with a combination of high mechanical strength, good wear and corrosion resistance (Han et al., 2008). Graphite, on the contrary, is a soft material having low hardness (HV = 0.25 GPa) and acts as a self-lubricating material and reduces the wear in case of MMCs by forming a thin lubricating film between the sliding surfaces (Liu et al., 1997; and Lin et al., 1998). In case of composite material, when more than one reinforcement is added with metal matrix, the composite manufactured is known as hybrid composite. So the ball milled Si3N4/Gr ceramic powders can be used in making of Hybrid Aluminum Matrix Composites (HAMCs), and these HAMCs have found increased application than single reinforced AMCs because of better mechanical and tribological properties and can substitute single reinforced AMCs. HAMCs possess superior properties like high strength, high stiffness and hardness, high wear resistance and high temperature stability than unreinforced alloy and these properties can be tailored to some specific requirements (Mbaya, 2005). These HAMCs have found application in the areas of aerospace, defense, automotive, sports good and marine industry (Wang et al., 1995; and Garcia et al., 1996).

Mechanical Engineering Journal, Scanning Electron Microscopy (SEM), Silicon Nitride (Si3N4), Graphite (Gr), Mechanical Alloying (MA), Hybrid Aluminum Matrix Composites (HAMCs).