Properties improvement and structural optimization of sintered NdFeB magnets by non-rare earth compound grain boundary diffusion

Grain boundary diffusion using rare earth (RE)-containing compounds has recently become an effective approach for improving the coercivity and reducing the heavy RE content in sintered NdFeB magnets. Here we report the enhancement of magnetic properties and corrosion resistance of NdFeB magnets by a non-RE compound diffusion process. The Dy-free sintered NdFeB magnets were coated with an MgO layer by magnetron sputtering, followed by solid diffusion heat treatment. With the successful diffusion of MgO into the magnet, the coercivity increasing from 1094 to 1170 kA/m and the maximum energy product increasing from 240 to 261 kJ/m³, together with the enhanced temperature stability and corrosion resistance, have been demonstrated. The underlying mechanisms for these enhancements have been analyzed. Microstructural investigations show that MgO entered mainly into the intergranular regions and modified the composition and structure of the grain boundary phase. The intergranular Nd–O–Fe–Mg phases observed in the MgO diffused magnet contribute to the improved performance. The current non-RE compound grain boundary diffusion process has significance in further minimizing the use of rare earth (RE).