On high-temperature oxidation and protection of 2:17-type SmCo-based magnets

Abstract

AbstractSm2(Co,Cu,Fe,Zr)17 are the best high-temperature permanent magnets because of their high Curie temperature (800°C–850°C). However, irreversible and unacceptable coercivity losses retard their use in applications at temperatures over 550°C. The coercivity loss has been correlated with poor oxidation resistance at high temperatures. The current research progress on the effect of oxidation and its prevention, for 2:17-type magnets, is reviewed. Oxidation in air at 500°C–700°C causes the magnets to form three regions: (1) an external oxide scale mainly consisting of (CoxFe1-x)3O4, (2) a thicker internal oxidation zone where the typical cellular precipitation (2:17R cell and 1:5H cell boundary) structure has been completely collapsed due to the Sm oxidation into Sm2O3, and (3) an oxidation-free zone where the cellular precipitates remain unchanged in lattice structure. No unacceptable coercivity loss is seen in the oxidation-free zone. Its thickness can be impressively increased within the magnets at high temperature, when they are covered with surface diffusion barriers for oxygen from the atmosphere, such as thin films of Cr2O3, Al2O3, and the metals with the ability to thermally grow these oxides.