Effects of electrolyte pH on morphologies and mechanical properties of α-Al2O3/Ni composite coatings and role of zeta potentials in co-deposition process

• Composite coatings are prepared by means of brush plating at different PH.
• Morphologies of composite coatings vary with the change of zeta potential.
• The content of α-Al2O3 in coatings is varied with electrolyte pH.
• The hardness and wear resistance are promoted by the increase of α-Al2O3 particle.
• Different zeta potentials lead to different co-deposition processes.

Zeta potential of α-Al2O3 particles in electrolytes was measured and used to describe the electrical properties of nano-particles. α-Al2O3 reinforced composite coatings were prepared from electrolytes respectively from pH 5, 6, 7, 8, 9 and 10 by electro-brush plating. Morphologies of the six coatings were observed in a scanning electron microscope (SEM). Some bright dots were found in the coatings prepared at pH 5, 6 and 7, while no dots were found in the coatings prepared at pH 8, 9 and 10. To investigate the differences, energy dispersive spectroscopy (EDS) was performed to determine the composition of bright dots in the coatings. And it was found that the bright dots were α-Al2O3 depositing in the coatings. The result indicated that α-Al2O3 particles with positive zeta potentials deposit in a different way from particles with negative zeta potentials. A new co-deposition process of nano-particles and metallic ions was introduced from the perspective of nano-particle electrical behaviors. In general, the higher the absolute value of zeta potential is, the more the nano-particles will deposit in the coatings, and the better performance the coatings will have in micro-hardness and wear resistance because of the lower size of grains. But more negative zeta potential will prevent nano-particles from depositing.