The effect of grain size on the internal oxidation of Sm2Co17-type permanent magnets

This work has examined the influence of grain size in sintered Sm-2(Co,Fe,Cu,Zr)(17)-type permanent magnets on the growth of an internal oxidation zone (IOZ). Two magnets of similar composition but with average grain sizes of 46 mu m and 170 mu m respectively, were exposed to temperatures of 500 ?& nbsp;in air for times up to 1000 h. Overall, the smaller grained magnet showed a reduced growth rate of the IOZ (& nbsp;39.01 mu m(2)h(-1)) compared to the larger grained magnet (& nbsp;48.95 mu m(2)h(-1)) and a smaller irredeemable loss in B-r (3.11%) after 1000 h. However, growth rate of the IOZ is shown to depend on the growth direction, in respect to the crystallographic c-axis of the matrix phase. The smaller grain size was shown to be most effective at retarding the growth of IOZ under surfaces parallel to the c-axis. It is proposed therefore, that a smaller grained sintered magnet may be the microstructure of choice for use in demanding, high temperature applications, where a protective coating is not always a possibility, such as a high-performance electrical machine with a surface mount PM rotor. Orientation of the exposed face must also be considered to minimise the resultant magnetic induction loss caused by oxidation.