Microstructural development and surface characterization of electrodeposited nickel/yttria composite coatings

The addition of yttria particles to a Watts nickel plating bath is shown to significantly change the surface morphology and preferred crystallographic orientation of the composite coatings deposited on Inconel 625 substrates. The typical morphology for nickel deposits from an additive-free Watts bath is pyramidal, however the addition of yttria particles in concentrations > 2 g dm-3 changes this morphology to hemispherical. The preferred growth direction was also influenced by yttria with a change from a < 100 > to a < 111 > preferred growth direction occurring when the particle loading exceeded 2 g dm-3. The results here show that both the nucleation and growth process of the nickel matrix was greatly influenced by the yttria additions. When yttria was absent from the electroplating bath the growth took place preferentially on (200) orientated planes. However, yttria acts to inhibit growth on grains of this orientation, leading to a requirement for additional nucleation, which takes place preferentially on (111) planes. This constant inhibition of growth and renucleation causes the changes to a hemispherical morphology observed.