The grain-boundary diffusion process in Nd–Fe–B sintered magnets based on the electrophoretic deposition of DyF3

In a novel process to enhance the coercivity we have electrophoretically deposited DyF3 powder onto the surface of an as-sintered Nd–Fe–B magnet as the initial step in the grain-boundary diffusion process. After a conventional heat treatment at 850 and 500 °C the coercivities were higher than in the case of simple dipping after a typical 10 h, with Hci values exceeding 1600 kA/m for a 200 μm-thick deposited layer. The electrophoretic deposition (EPD) process is quick, easily controllable in terms of thickness and can be used to deposit the rare earth fluoride powder on the surface of complex and irregularly shaped magnets. Since the amount of deposited powder can be tailored to maximise the coercivity while minimising the quantity of expensive heavy rare earth there is no wasted powder, making the diffusion process, which takes place after the sintering process, more environmentally friendly and potentially cheaper than conventional dipping.