Study of Graphite Action in Al2O3-Cu Matrix

Al_2O_3-Cu matrix composites doped with graphite of various mass fractions were prepared using powder metallurgy. The influence of graphite on the hardness and tribological behavior of the Al_2O_3-Cu cermet composite was investigated. The worn surface morphologies of the cermet composites were observed using a scanning electron microscope. Thus the friction and wear behaviors of the cermet composites sliding against 40Cr steel in a ring-on-block configuration and under discontinuous lubricated condition were evaluated on an MM-200 test rig. It was found that the incorporation of the graphite at a proper mass fraction contributed to effectively decrease the friction coefficients and wear volume losses of the cermet composites, which was attributed to the self-lubricity of the graphite lubricating film formed on the worn composite surface. At the same time, the lubricating film of the graphite also contributed to restrain the initiation of micro cracks on the worn surface of the cermet composite. It was imperative to keep the mass fraction of the graphite in the cermet composite as 3.0% so as to obtain the optimized mechanical and tribological properties of the composites. Otherwise, the hardness and wear-resistance of the cermet composites would be significantly worsened in the presence of excessive graphite as the filler. In other words, too high content of graphite was harmful to the bonding strength between the metallic matrix and the reinforcing ceramic particulates, which hence led to a significant damage to the mechanical strength and wear-resistance of the cermet composite, even though the graphite could function to reduce the friction to some extent.