Effects of Zn coating on the microstructure and magnetic properties of Nd–Fe–B magnets

Abstract

Recent work at Birmingham has shown that the use of a low-pressure pack sublimation (LPPS) process produces Zn-coatings on Nd–Fe–B magnets, which result in dramatic improvements in corrosion resistance. In the present work we have studied the microstructures of the LPPS coatings and their effects on the magnetic properties of the magnets. It was found that the coating process had a small detrimental effect upon the magnetic properties of the magnets. For example, in one case, production of a 12 μm LPPS Zn coating led to a decrease of ∼3% in both coercivity and remanence. Microstructural changes were, however, confined to the near-surface regions with three microstructural ‘layers’ identified: A surface region saturated in Zn consisting mainly of Fe–Zn compounds which resemble those observed previously for galvanised steel (layer 1). A transition region in which grain boundary films of the Fe–Zn compounds surround the hard magnetic Nd 2Fe 14B grains (layer 2). An unaffected substrate with no ingress of Zn (layer 3). It can be inferred from these observations that there are microstructural differences between the LPPS coatings on Nd–Fe–B magnets and those for galvanised steel. Moreover, it is shown that the change in magnetic properties is consistent with an effective reduction in the volume fraction of the Nd 2Fe 14B phase, rather than with any change in the inherent characteristics of the phase.