Structure and Mechanical Properties in the Submicrovolumes of Sintered KS25 (Co-Sm) Permanent Magnets

The microstructure and the mechanical properties of the submicrovolumes in the rare-earth magnets KS25 sintered from a Co-25% Sm alloy is studied. The microstructure is studied by optical, electron, and atomic force microscopy. The microstructure is found to be characterized by the presence of SmCo5 dendrites, the interdendritic space consisting of a mixture of the SmCo5 and Sm2Co17 phases, individual Sm2Co17-phase grains, Zr5Co3FeSm compound crystals (1-5 mu m in size), and globular Sm2O3 samarium oxide inclusions (2-10 mu m in size). The hardness H (GPa) and Young's modulus E (GPa) of the main Sm2Co17 and SmCo5 phases are determined by nanoindentation. The machinability of the magnets is estimated using the nanoindentation data; the additional pressure and adhesion at the boundary of the Sm2Co17 and SmCo5 phases are calculated. The calculation shows that the additional pressure exceeds the external pressure by a factor of 300, and the adhesion of the phases (K-int = 0.543 MPa m(0.5)) is lower than that of the strengthening coatings by an order of magnitude. This finding can be the cause of magnet cracking along the boundary between the Sm2Co17 and SmCo5 phases during cutting and grinding.