Structure of electrodeposited graded composite coatings of Ni-Al-Al2O3

The structure of electrodeposited composite coatings of Ni-Al-Al2O3, with Ni as the matrix and Al and Al2O3 as second-phase particles, was investigated using light microscopy and scanning electron microscopy. Ni coatings with no particles, which were used as reference samples, had progressively coarser structures with increasing current density. Go-deposition with Al resulted in refinement of the Ni matrix structure at high (>10 A dm(-2)) current densities, For single-particle baths, the co-deposition of Al2O3 was more strongly affected by current density and bath particle content than was the co-deposition of Al. However, for baths containing both Al and Al2O3 the amount of incorporated Al2O3 no longer depended on current density. With the choice of appropriate conditions, coatings of Ni with up to 39 vol.% Al2O3 were made, Similar experiments with Al yielded a maximum of 17 . 5 vol.% only. Uniform and graded mixed-particle coatings were also produced. When coatings containing Al were annealed, the reaction of the two elements resulted in the formation of either single-phase gamma or two-phase gamma-gamma' alloys, in agreement with the equilibrium phase diagram.