Properties of nanocrystalline Ni/Al2O3 composites

Nanocrystalline Ni/Al2O3 composites were formed by electrolytic co-deposition. The co-deposition was achieved by adding Al2O3 nanopowder having a median particle diameter of 13 nm into different electrolytic baths followed by direct current (DC) plating with current densities j up to 8 A/dm(2). The structure and phase distribution of Ni/Al2O3 compound film were investigated after different heat treatments by light microscopy and in a transmission electron microscope (TEM) equipped with energy dispersive X-ray (EDX) unit. Beside that the incorporated ceramic nanoparticles are largely homogeneously distributed in the nickel matrix, it was found that they also act like an inhibitor for growth of nuclei resulting in significant increase of nucleation rate of nickel grains on the growing composite film. Therefore, the adding of alumina nanopowder to the electrolytic bath can promote the formation of a nanocrystalline nickel matrix with an average matrix grain size of less than 50 nm. whereas the same bath without nanopowder forms films of much larger grains even at larger j. Futhermore, it was found by TEM inspection that the nanocrystalline matrix structure is largely stabilised against grain growth even after long-term thermal treatments at 1050 degrees C. This was also confirmed by measurements of the coercivity and microhardness. Even though these properties decreased after thermal treatment due to hydrogen release from the composites, they showed constant enhanced values after further heat treatments, compared with pure nickel specimens.