STRUCTURE OF ELECTRODEPOSITED GRADED COMPOSITE COATINGS OF NI-AL-AL2O3

The structure of electrodeposited composite coatings of Ni-Al-Al2O3, w ith Ni as the matrix and Al and Al2O3 as second-phase particles, was i nvestigated using light microscopy and scanning electron microscopy. N i coatings with no particles, which were used as reference samples, ha d 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, t he co-deposition of Al2O3 was more strongly affected by current densit y and bath particle content than was the co-deposition of Al. However, for baths containing both Al and Al2O3 the amount of incorporated Al2 O3 no longer depended on current density. With the choice of appropria te conditions, coatings of Ni with up to 39 vol.% Al2O3 were made, Sim ilar experiments with Al yielded a maximum of 17 . 5 vol.% only. Unifo rm and graded mixed-particle coatings were also produced. When coating s containing Al were annealed, the reaction of the two elements result ed in the formation of either single-phase gamma or two-phase gamma-ga mma' alloys, in agreement with the equilibrium phase diagram.