WEAR MECHANISMS IN HYBRID COMPOSITES OF GRAPHITE-20 PCT SIC IN A356 ALUMINUM-ALLOY (AL-7 PCT SI-0.3 PCT MG)

The wear behavior of A356 aluminum alloy (Al-7 pet Si-0.3 pet Mg) matrix composites reinforced with 20 vol pet SiC particles and 3 or 10 vol pet graphite was investigated. These hybrid composites represent the merging,of two philosophies in tribological material design: soft-particle lubrication by graphite and hard-particle reinforcement by carbide particles. The wear tests were performed using a block-on-ring (SAE 52100 steel) wear machine under dry sliding conditions within a load range of 1 to 441 N. The microstructural and compositional changes that took place during wear were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), energy-dispersive X-ray spectroscopy (EDXA), and X-ray diffractometry (XRD). The wear resistance of 3 pet graphite 20 pet SiC-A356 hybrid composite was comparable to 20 pet SiC-A356 without graphite at low and medium loads. At loads below 20 N, both hybrid and 20 pet SiC-A356 composites without graphite demonstrated wear rates up to 10 times lower than the unreinforced A356 alloy due to the load-carrying capacity of SiC particles. The wear resistance of 3 pet graphite 20 pet SiC-A356 was 1 to 2 times higher than 10 pct graphite-containing hybrid composites at high loads. However, graphite addition reduced the counterface wear. The unreinforced A356 and 20 pet SiC-A356 showed a transition from mild to severe wear at 95 N and 225 N, respectively. Hybrid composites with 3 pet and 10 pet graphite did not show such a transition over the entire load range, indicating that graphite improved the seizure resistance of the composites. Tribolayers, mainly consisting of a compacted mixture of graphite, iron oxides, and aluminum, were generated on the surfaces of the hybrid composites. In the hybrid composites, the elimination of the severe wear (and hence the improvement in seizure resistance) was attributed to the reduction in friction-induced surface heating due to the presence of graphite- and iron-oxide-containing tribolayers.