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

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 pH5, 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 pH5, 6 and 7, while no dots were found in the coatings prepared at pH8, 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.