Surface reaction and sm depletion at 550 degrees C for high temperature Sm-TM magnets

Sm-2 (Co, Fe, Cu, Zr)(17) type magnets are unique high energy product magne t materials for applications at temperatures higher than 200 degreesC. Thes e magnets have (BH)(max) values up to 32 MGOe at 25 degreesC and up to 26.5 MGOe at 200 degreesC. A new, recently introduced class of these Sm-TM 2:17 type magnets can be used at temperatures as high as 550 degreesC. However, exposure to such high temperatures presents new challenges because of the reactions between the magnets and the environment. An investigation has bee n conducted using SEM/EDXA and EPMA/WDXA to study the microstructure formed in Sm-2 (Co, Fe, Cu, Zr)(17) magnets after long-term exposure to air at 55 0 degreesC. It was anticipated that the permanent magnetic loss observed wa s due mostly to surface oxidation. Surprisingly, the investigation shows th at the major cause for the magnetic loss is Sm depletion. It was found that at the surface layer, Sm is lost by vaporization, leaving an oxide of Fe-C o. But Sm depletion also occurs in a zone between the surface layer and the matrix of the samples. In this affected zone, many Sm free Fe-Co-Cu stripe s are formed as part of the process of Sm atoms migrating toward the surfac e layer and eventually evaporating. Coating can minimize Sm depletion and r educe the magnetic loss. Research for improved coating systems is in progre ss.