Surface tension of intergranular regions of NdFeB nanocomposite magnets

The surface tension and contact angle of some Nd-rich compounds specific to the stoichiometry of intergranular regions of NdFeB permanent magnets were investigated using the sessile drop method. The surface tension of the molten Nd90.9Fe9.1, Nd88Co12, and Nd77.5Ga22.5 wt.% alloy drops resting on BN substrate was calculated under the Dorsey formalism. As an overall trend, the contact angle of the three investigated alloys, corresponding, respectively to the Nd7Fe3, Nd3Co, and Nd5Ga3 phases, in liquid state, decreases with increasing temperature, whereas, the surface tension generally increases with temperature raise above the melting point indicating a capillarity effect. From the experimental data, it can be inferred that the highest value of the surface tension at temperatures just above the melting point corresponds to Nd5Ga3, γ≅400 mN/m, whereas Nd7Fe3 and Nd3Co presents a surface tension of about γ≅200 mN/m. Wetting angles around 150–160° are specific to all investigated compositions near the melting point.