Oxidation mechanism of rare-earth SmCo permanent magnet

Abstract

Given that there is a growing demand for magnets to perform well at temperature above ambient, high temperature magnets are subjected to irreversible loss of magnetic properties due to oxidation. The objective of this project is to determine the oxidation mechanism of one rare-earth SmCo permanent magnet (VACOMAX 262HR, purchased from VACUUMSCHMELZE). This is done by using SEM imaging to observe the physical changes of the SmCo hard magnets after subjecting the magnets to a high temperature of 500 °C with different time duration. The project found that SmCo hard magnet has oxidation patterns similar to the oxidation pattern of magnets found in literature review. The results suggest that the growth of IOZ follows a time-temperature dependent pattern and the growth rate differs between c axis (perpendicular and parallel) which is attributed to the anisotropic nature of the magnet which agrees with researches cited in this paper. The results seem to differ on the IOZ growth rate of SmCo hard magnets compared with magnets cited in literature review. This might be attributed to the inability of the precipitate formed within the IOZ to inhibit the IOZ growth. Possible mechanism has been proposed on the formation of the precipitate. The possible oxidation mechanism of the SmCo hard magnet is likely due to the influx of oxygen, which internally oxidizes the magnetic phase (SmCo) to form Sm oxide particle and releasing Co to form an CoFe rich IOZ matrix. The project was unable to draw a relation between the oxidation and the magnetic strength of SmCo magnet and more data points were required to further confirm the oxidation kinetics behavior.