Wear resistance of Ni/nano-Al₂O₃ composite coatings by brush electroplating

In this study, Ni/nano-Al₂O₃ composite coatings and the Ni/Ni–Al₂O₃ multilayer coatings have been prepared on the 304 stainless steel substrate by adding the nano-Al₂O₃ particle in Ni plating solution and by alternately brush electroplating, respectively. The microstructure, hardness and wear resistance of the composite and multilayer coatings were studied. Results showed that adding Al₂O₃ nanoparticle in coatings significantly influences the size and growth direction of grain. The Al₂O₃ content and hardness of coatings first increased, reached a maximum value of 603 HV and then decreased with the increase in the addition of Al₂O₃ nanoparticle in Ni plating solution. The friction coefficient, wear loss and wear rate showed an opposite trend. In terms of mass loss and wear rate, the wear resistance of coating increases with increasing Al₂O₃ content in coatings. An appropriate addition (20–25 g L⁻¹) of Al₂O₃ nanoparticles can effectively enhance the wear resistance of the brush electroplating Ni coating. The minimum wear rate is 2.2–2.3 10⁻⁵ mm³ N⁻¹ m⁻¹. The multilayer structure coatings can prevent or delay microcrack initiation and growth by layer interface and grain boundary and increase the thickness of coatings without cracks. The multilayer coatings possessed the approximately same hardness (595 HV) and tribological properties compared with monolayer coatings.