Improving adhesion of electroless Ni–P coating on sintered NdFeB magnet

The poor adhesion of Ni–P coatings on sintered NdFeB magnets imposes a great challenge in industrial applications. In this work, a kind of Zn phosphate film was fabricated on NdFeB sintered magnets surface to enhance the adhesion strength between the Ni–P coating and the substrate. The results showed that the adhesion strength between magnet surface and Ni–P coating was enhanced from 2.3 MPa to 10.1 MPa after Zn phosphating treatment. The addition of the phosphate created a protective layer over the substrate and inhibited the dissolving of the substrate and generation of H2, which deteriorated the mechanical strength of the substrate and led to lower adhesion strength. On the other hand, the pores and micro-cracks on the phosphate film enhanced the interfacial contact area as well as reinforced the anchoring of Ni–P grains through the pinning effect, thus leading to high adhesion strength between Ni–P and magnet substrate. Phosphating solution composition and process were also optimized by orthogonal experiments in this work.