Influence of Spray Coating Deposition on Magnetic Properties and Microstructure of Grain Boundary Diffusion Nd-Fe-B Magnets

Abstract:

A novel method of spray coating deposition (SCD) was proposed to deposit TbF₃ powder solution uniformly onto the surface of Nd-Fe-B magnets, and then the Tb element was introduced into the magnets through the grain boundary diffusion process (GBDP). Nd-Fe-B magnets with thickness up to 5 mm were treated with this method (SCD+GBDP). The effects of mass gain ratio (w) of TbF₃ coatings, diffusion time and the diffusion temperature on the microstructure and magnetic properties of the sintered magnets were investigated. The samples were diffused at 940 °C for 10 h and then annealed at 480 °C for 5 h. Results show that when the TbF₃ mass gain ratio increases from 0% to 0.8%, the coercivity of the magnets increases from 1201 to 1930 kA/m and the remanence is only decreased by 0.01 T. With increasing the mass gain ratio of TbF₃, the coercivity of the magnets increases firstly and then decreases obviously. SEM results show that the (Nd, Tb)₂Fe₁₄B core-shell phases can be formed by Tb which replaces the Nd in the boundary region among Nd₂Fe₁₄B grains. The improved decoupling effect by the continuous grain boundary phase and the higher magnetocrystalline anisotropy of the core-shell phase plays a positive role in the coercivity enhancement. The distribution and concentration of core-shell phase have a close influence on the coercivity. When the TbF₃ mass gain ratio is more than 2.4%, the grain boundary diffusion is obviously enhanced in the region close to the magnet surface. The SEM image of the element shows that the more Tb enters the inside of the grain, the higher the Tb concentration in the core than in the shell. Furthermore, the formation of a large amount of Nd-F/Nd-O-F phase causes the grain boundary phase not being as continuous as that of the sample with w=0.8%, which may be the main reason for the decrease in coercivity.