Tribological behavior of aluminum/SiC/nickel-coated graphite hybrid composites

Tribological behavior of Al/SiC/Ni-coated graphite (Gr–Ni) hybrid composites with various amounts of Gr–Ni addition synthesized by the semi-solid powder densification method was studied. The SiC and nickel-coated graphite (Gr–Ni) particles are distributed quite uniformly in an aluminum matrix. The graphite agglomerates much less in composites with coated Gr–Ni particles than in composites with non-coated Gr particles. Coefficient of thermal expansion decreases significantly with the amount of Gr–Ni addition. The fracture energy decreases monotonously as the amount of Gr–Ni addition increases. Wear debris become smaller, which causes smaller electric contact resistance, as the amount of Gr–Ni addition increases. Seizure occurs for monolithic aluminum alloy, but no seizure occurs for Al/SiC and Al/SiC/Gr–Ni composites. The friction coefficient and its fluctuations decrease as the percentage of Gr–Ni addition increases. Graphite released from the composites bonds to the wear surfaces of the composites and counterparts. Due to the “composite effects”, wear rates of both the composite and counterpart increase as the amount of Gr–Ni addition increases up to 5% Gr–Ni addition, then drop for 8% Gr–Ni addition. The wear rates of all the composites with Gr–Ni additions are higher than the wear rate of base Al/SiC material with no Gr–Ni addition. Therefore, the composites incorporating graphite do not work for wear reduction, as the debit in mechanical properties is larger than the very small benefit in friction reduction.