Effect of surface etching on the magnetic properties and grain-boundary Dy-diffusion in DyH₂-dip-coated sintered Nd-Fe-B magnets

We investigated the effects of surface etching on the magnetic and microstructural properties of DyH₂-dip-coated, sintered Nd-Fe-B magnets. When the magnets were dip-coated with DyH₂ solution after their surfaces had been optimally etched in 1 wt% HNO₃ for 50 s, the Dy diffusion depth (~450 μm) improved compared with the values for unetched and DyH₂-dip-coated etched magnets (~350 μm). In addition, the core-shell microstructure of the dip-coated magnet was well developed by the optimal etching process. As a result, the coercivity of the dip-coated magnet increased by 4.6%. The optimal surface state of the Nd-Fe-B sintered magnet showed minimal surface damage after it was homogeneously etched along the grain boundaries. In contrast, overetched magnets showed kinks on the demagnetization curves because of surface damage. The relationship between the change in the etched magnet surface and the magnetic properties of dip-coated magnets was examined.