Influence of Heat Treatment on the Coercivity and Morphology of the Magnet Powders Prepared by Mechanical Crushing Sintered NdFeB Magnets

Heat treatment experiments were carried out to magnet powders prepared by crushing the sinteped NdFeB hard magnets, within the annealing temperature range from 700 to 1050℃. The results have shown tha the annealing treatment can effectively enhance the coercivity of the crushed powders, which is very low after the communtion, to relative high values. The microstructural changes of the magnet powders during the annealing were checked using optical and scanning electronic microscopes. It is found out that the magnet powders have many cracks in their surface layer caused by the mechanical crush, while their center parts remain free from crack. On the other side, the magnet powders after annealing treatment possess have few cracks in the surface layer. This change is considered as the result of the repairing effect of the annealing treatment to the damaged surface, which should be the reason for the enhanement of the coercivity of the magnet powders. Meanwhile, in the powders after annealing, more Nd-rich prticles can be observed evidently,some of which are determined as Nd-oxide. It indicates the ocurrence of oxidation is an other process happened during the annealing treatment,which should affect the recovery of the coercivity of the manget powders negatively.