The effects of anodic interlayer on the morphology and mechanical performances of electroless Ni–P coating on Al alloy

Abstract

We fabricate anodic film on Al alloy and deposit Ni–P coatings on the porous surface by electroless plating (EP) technique in comparison with those deposited directly on Al alloy. When Ni–P particles are directly deposited on the surface of Al alloy, a thicker film made by particles with bigger grain sizes is observed by SEM images, and the atom diffusions between their contacting interfaces may lead to the formation of loosely packed interlayer with thickness of around 4 μ as determined by EDS results. The porous anodic surface refines the size of Ni–P particles to form more compact Ni–P coating with smaller grain size and narrower size distributions; it also promotes particles embedding in the pores to form ‘interlock’ transient layer with depth of around 1 μ. Thus, the rougher surface and the interlocking function provided by the porous anodic surface enhance the compactness and stability of the composite coating, which in turn result in the great improvements in its microhardness and adhesion performances.