Oxidation protection of Sm2Co17-based alloys

SmCo alloys form the basis of excellent permanent magnets with potential service applications up to 550 °C. However, the rapid growth of an internal oxidation zone limits their usefulness because the accompanying microstructural changes destroy the alloys’ high-temperature magnetic properties within this zone. In this work, a high-temperature grade of a Sm2(Co,Fe,Cu,Zr)17-based alloy was treated with a variety of potentially protective coating systems and then oxidised in air at temperatures of 450 and 550 °C to assess their performance. It was found that a diffused Pt coating and a paint-like overlay coating, containing Ti and Mg oxides, performed best at both temperatures. Sputtered SiO2 was effective at 450 °C but less so at 550 °C. Additionally, an alumina-based overlay coating was effective at 550 °C but less so at 450 °C. Specimen cracking and deformation during high-temperature exposure is also discussed.

► Assessment of seven types of coating for oxidation protection of SmCo magnets.
► Platinum based, silica and a proprietary paint coatings protect at 450 °C.
► Platinum based and a proprietary paint coatings protect at 550 °C.
► Magnets deform and crack under differential oxidation conditions.