Size Effect of Al Particles on the Oxidation of Electrodeposited Ni–Al Composite Coatings

Abstract

Electrodeposited microparticle-dispersed composite coatings (EMCCs) and nanoparticle-dispersed composite coatings (ENCCs) of Ni–Al were developed by co-electrodeposition of Ni with various contents of Al particles. The Al microparticles were in a size range of 1 ~ 5μm, and the Al nanoparticles had an average size of 75 nm. TEM characterization showed that compared to EMCC, ENCC exhibited a more-homogeneous distribution of particles, a greatly narrowed interparticle spacing, and finer Ni grains. The oxidation performance at 950°C of the as-deposited composite coatings depended not only on the aluminum content but also on the particle size, which was concluded from two observations: (1) for a given Al content, the oxidation resistance of the ENCC was better than that of the EMCC counterpart; (2) the content for the formation of a continuous alumina layer on the ENCC was significantly reduced compared to the EMCC. Codeposition of the nanoscale Al particles instead of the micrometer-sized counterparts led to a great increase of the number per unit volume of the Al particles available for alumina nucleation, which favored the ENCC Ni–Al to thermally grow alumina scales during a short transient stage of oxidation. A mechanism is proposed to interpret the particle size effect on the transient oxidation.