Increase of Coercivity and Composition Distribution in Fluoride-Diffused NdFeB Sintered Magnets Treated by Fluoride Solutions

Rare-earth and fluorine were diffused along grain boundaries in NdFeB sintered magnets during heat treatment after rare-earth fluoride liquid coating. In the case of Dy fluoride coating, the coercivity for NdFeB sintered magnets with a thickness of 1 mm after the diffusion was increased from 0.80 MA/m to 1.13 MA/m (41%) with remanence reduction of 0.6%. Segregations for Dy atoms near the grain boundaries and fluorine atoms at the grain boundary were found by TEM observations. The increase of coercivity per the weight of the rare-earth fluorides has a correlation with the anisotropy field of the RE₂Fe₁₄B (RE is rare-earth elements). Furthermore, the Dy quantity required for the observed increase of coercivity can be reduced to 1/3 compared to the previous NdFeB sintered magnets using the fluoride coating and diffusion process.