Overcoming Thickness Limits in Grain Boundary Diffusion Magnets via a Bonding‐Diffusion Process

ABSTRACT Sintered Nd‐Fe‐B magnets are essential for high‐efficiency energy conversion (e.g., in motors and generators), while ultra‐thick magnets (>10 mm) further expand their applicability to high‐power‐density systems such as wind turbines and maglev transportation. However, the coercivity enhancement offered by conventional grain boundary diffusion (GBD) is significantly limited in thick magnets due to restricted penetration depth. This study proposes a scalable terbium hydride (TbH ₂ )‐assisted bonding‐diffusion process that simultaneously achieves diffusion and bonding through spray coating and stacking of magnets, overcoming the thickness constraint of standard GBD. The coercivity of a 15 mm‐thick magnet assembly is increased by 10 kOe, reaching 27.20 kOe. Electron probe microanalysis and transmission electron microscopy confirm the formation of a rare‐earth‐rich bonding interface and a core–shell structure induced by the process, along with an improvement in the shear strength of the magnets. Micromagnetic simulations further clarify the mechanism behind coercivity enhancement via multilayer diffusion. This work provides a scalable strategy for fabricating ultra‐thick, high‐performance Nd‐Fe‐B magnets through integrated diffusion bonding, validating their feasibility for high‐power applications.