Interaction mechanism between Sm–Co magnets and glass coatings doped with mixed adherent oxides

Abstract

To protect Sm–Co magnets against oxidation in air at elevated temperatures, many efforts were made. In this work, borosilicate glass coatings containing mixed adherent oxides have been prepared onto Sm–Co magnet substrates by brush painting and then were fired at 830 °C for 3min following by cooling in argon atmosphere. The compositions and morphologies near interface of the coatings and the substrates were analyzed by using scanning electron microscopy with energy dispersive X-ray spectroscopy. The results show that all glass coatings have good adherence with the substrates due to the formation of a silicate layer though they exhibit large difference in the thermal expansion coefficients. And a silicon diffusion layer forms over the internal oxidation zone of Sm–Co magnets. The appearance of precipitated reaction products in the glass coating area indicates that the glass coatings could react chemically with the magnet substrates. Transition metal cations of the adherence oxides are reduced by the ferrous ions that are selectively dissolved by the molten glass, and precipitate from the glass in sequence. The competitive reaction mechanism was discussed in detail.