Microstructure and mechanical properties of sol-enhanced nanostructured Ni–Al2O3 composite coatings and the applications in WC-Co/steel joints under ultrasound

Abstract Reliable brazing of WC-Co to steel is crucial for the applications in shield tunneling and temping machines. However, the interface could not be effectively connected with high strength due to the large difference in coefficients of thermal expansion. We developed a sol-enhanced method to electroplate Ni–Al2O3 nanocomposite coatings on steel in order to improve the bonding and strength of the joints. Al2O3 sol was fabricated with mean size of 59 nm, which was identified through TEM analysis. The change of Al2O3 concentration of the coatings with different Al2O3 sol addition was investigated through LA-ICP-MS test. The phase, microstructure, mechanical properties and corrosion performance of the coating were strongly affected by the sol concentration through the XRD and TEM analysis. At very high sol concentration, cracks appeared on the coating surface, which deteriorated the properties. It was found that the Ni–Al2O3 coating improved the micro-hardness and corrosion resistance by polarization Tafel tests and EIS analysis. Specifically, 35CrMo with a Ni–Al2O3 coating was joined with tungsten carbide by ultrasound-assisted brazing in air. The shear strength of the joints was increased by 31% (approximately 427 MPa) than conventional method.