Wear of aluminium matrix composites reinforced with nickel-coated carbon fibres

The dry sliding wear of an A356 aluminium alloy reinforced with nickel-coated non-graphitic carbon fibres (A356-4%C-f) was studied. Wear tests were carried out at constant load levels within the range of 5 to 300 N using a constant sliding velocity of 0.5 m s(-1). In addition, the effect of incremental loading on wear rates was also studied. Wear behaviour of A356-4%C-f was compared with that of an unreinforced A356 alloy, a particle reinforced composite A356-20%SiCp as well as an A30 grade grey cast iron which is the conventional tribological material for automotive applications for which the composites are considered for. Results indicated that the A356-4%C-f had higher wear resistance than the unreinforced A356 alloy in the mild wear regime. This was attributed to the improvement in the load carrying capacity of the composite due to the presence of the carbon fibres as well as to Al3Ni intermetallic precipitates formed during the fabrication process. The carbon fibres and Al3Ni phase were also responsible for delaying mild to severe wear transition to higher loads. At high loads reinforcing particles fractured, and a surface layer consisting of comminuted particles was formed. This layer had a high hardness and provided protection against seizure. Wear rates of carbon fibre reinforced composites were at least equivalent or lower than those of the SiC particulate reinforced materials. The better wear resistance of A356-4%C-f compared with A356-20%SiCp is shown to be due to the formation of iron oxide-rich transfer layers on the contact surfaces of the former. Compared with SiC particulate reinforced composite, carbon fibre reinforced composites inflicted less wear damage to the counterface made of a SAE 52100 type bearing steel. Within the mild wear regime, the wear rates of the grey cast iron were considerably higher with respect to those of the composites possibly because of the surface and subsurface fracture initialed at graphite flakes. Surfaces before and after wear tests were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffractometry, and by nanohardness tests. Roles of microstructure, formation of iron oxide-rich transfer layers on the contact surfaces and subsurface zones consisting of comminuted fibres and particles on the wear resistance of composites were discussed.