Effect of Ni–Al intermetallic sublayers on tribological properties of crystallised Ni–P coatings on aluminium substrates

Abstract

The effect of Ni–Al intermetallic sublayers on the tribological properties of Ni–P coated aluminium substrates has been investigated. For this purpose an Ni–P coating was directly deposited on to an 1100 aluminium substrate and Al–Ni diffusion sublayers were produced by subsequent vacuum heat treatment. The diffusion layers were characterised by optical and scanning electron microscopy (SEM), X-ray diffrac tion, microanalysis, and tribological testing. The results show that after annealing for 10 h at 520°C, three layers are formed with a total thickness of 3–4 times that of the initial Ni layer thickness. The layers grown during diffusion annealing are the intermediate phases of the Ni–Al system where their formation is controlled by process kinetics. The outermost layer was identified as a crystallised Ni–P residue along with two distinct layers of Ni₂ Al₃ and NiAl₃ intermetallic compounds formed beneath it. The results of wear testing indicate much improved wear resistance after diffusion treatment, believed to result from a smooth hardness gradient and the formation of intermetallic phases at the interface, which acts as a firm sub strate beneath the hard crystallised Ni–P surface layer. From SEM studies of the wear debris and the worn specimens it can be concluded that delamination is the predominant wear mechanism of the surface layer.