High-coercivity Nd-Fe-B magnets obtained with the electrophoretic deposition of submicron TbF3 followed by the grain-boundary diffusion process

• Published in: Acta materialia 2016-08, Vol.115, p.278-284
• Main Authors: Soderžnik, Marko, Korent, Matic, Žagar Soderžnik, Kristina, Katter, Matthias, Üstüner, Kaan, Kobe, Spomenka
• Format: Article
• Language: English
• Subjects: Coercive force; Coercivity; Core-shell-type microstructure; Diffusion; Electrophoretic deposition; Ferrous alloys; High-coercivity; Induction coupled plasma with optical electron spectroscope; Low heavy-rare-earth concentration; Magnetic materials; Magnets; Nd-Fe-B magnets; Sintering
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2016.06.003

Using a grain-boundary diffusion process (GBDP) involving the electrophoretic deposition (EPD) of submicron TbF3 powder, we substantially increased the coercivity of sintered Nd-Fe-B permanent magnets. The experiments used magnets with low heavy-rare-earth (HRE) content (HRE = 1.2 wt%) and a coercivity of 790 kA/m (at 75 °C). After experiencing optimized conditions at 875 °C for 10 h and subsequent annealing at 500 °C for 1 h, the coercivity was increased to 1536 kA/m (at 75 °C). This value is 1.94 times higher than that for a sintered magnet, without post-sintering heat treatment. Furthermore, a vibration test revealed satisfactory adhesion of the TbF3 powder to the surface of the magnet with no detected reduction in coercivity. Using field emission gun scanning electron microscopy (FEG-SEM) with an energy dispersive spectroscope (EDS), we confirmed the formation of various secondary intergranular phases and the core-shell-type microstructure, which increases the coercivity. The Tb content in the magnet, exposed to the EPD-based GBDP, was controlled by inductively coupled plasma optical electron spectroscopy (ICP-OES). The additional Tb detected in the magnet due to the described technology was only 0.12 wt%. [Display omitted]