Effect of temperature and vacuum on the magnetic properties and compositional changes in high temperature Sm–Co magnets

Electric propulsion systems for space exploration require materials and components that can perform reliably under a vacuum of 10−5 Torr and at temperatures of over 400 °C. Scanning electron microscopy analysis shows that when Sm–Co magnets are exposed to temperatures up to 550 °C for up to 240 h in a vacuum of 10−5 Torr, the surface alteration is morphologically similar to oxidation in air. However, the scale of the affected region is much smaller and consists of about 1–2 μm of a top outermost Sm-depleted oxide layer and about 10–20 μm of a more complex layer in which the constituent atoms redistribute with oxygen penetration. No excessive Sm depletion was observed in the second layer. For all Sm–Co magnet samples, the maximum energy product decreased by less than 10% even at temperatures of 100 °C above their specified maximum operating temperature TM. The residual induction of the magnets with TM=550 °C decreased by 8.5% after baking at 700 °C under vacuum for 240 h.