Fe₄N is expected to be used as a superior soft or hard magnetic material mainly because of its high saturation magnetization (186emu/g). The production of its dense compact by using pressureless sintering is said to be difficult, because the diffusivity of Fe atoms is poor even at temperatures just below its decomposition temperature of 940K. Therefore, we investigated whether the dense compact can be obtained by pressureless liquid phase sintering of Fe₄N and Zn mixed powder; the melting point of Zn (693K) is lower by 247K than the above decomposition temperature.
It was found that a usual simple method, i.e., sintering of Fe₄N and Zn mixed powder compact resulted in the expansion of the compact. We, however, succeeded in enhancing the densification of the sintered compact from 54% of theoretical density to 84% by applying the following unique process: Fe₄N and 10mass%Zn mixed powder was heat-treated in flowing H₂ gas at 703K for 10.8ks to form Fe-Zn compound layer on the surface of Fe₄N powder particles, and then the product was mixed with 20mass%Zn powder, compacted and sintered in H₂-NH₃(2:1) flowing gas at 823K for 3.6ks. The enhanced densification was attributed to good wetting characteristics between Zn liquid phase and the Fe-Zn compound layer.