Magnetic properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Abstract Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering. The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity, namely diffusion treatment and annealing treatment. The coercivity increases significantly from 18.3 to 28.0 kOe, the remanence decreases slightly from 14.1 to 14.0 kGs, and the comprehensive magnetic properties are than 75 (Hcj+(BH)max=76.7). SEM results indicate that, on the one hand, 950 °C is the optimal diffusion temperature. Lower diffusion temperature resulted in insufficient diffusion of Tb element. Higher diffusion temperature can lead to the main phase grain growth, the decrease of Nd-rich phase, and forming holes in the magnet. On the other hand, 500 °C is the optimal annealing temperature. Lower annealing temperature can resulte in the reduction of Nd-rich phase. Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.