A facile method for fabrication of nano-structured Ni-Al2O3 graded coatings: Structural characterization

Powder charges of micron-size Ni and Al2O3 were utilized to deposit nano-structured Ni-Al2O3 composite coatings on an aluminum plate fixed at the top end of a milling vial using a planetary ball mill. Composite coatings were fabricated using powder mixtures with a wide range of Ni/Al2O3 mass ratio varying from 1:1 to plain Ni. XRD, SEM and TEM techniques were employed to study the structural characteristics of the coatings. It was found that the composition of the starting mixture strongly affects the Al2O3 content and the microstructure of the final coating. Mixtures containing higher contents of Al2O3 yield higher volume fractions of the Al2O3 particles in the coating. Though Ni-Al2O3 composite coatings with about 50% of Al2O3 particles were successfully deposited, well-compacted and free of cracks and/or voids coatings included less than 20% (volume fraction) of Al2O3 particles which were deposited from powder mixtures with Ni/Al2O3 mass ratios of 4:1 or higher. Moreover, mechanical and metallurgical bondings are the main mechanisms of the adhesion of the coating to the Al substrate. Finally, functionally graded composite coatings with noticeable compaction and integrity were produced by deposition of two separate layers under identical coating conditions.