Enhanced corrosion resistance of NdFeB type permanent magnet coated by a dual layer of either Ti/Al or Ni/Al intermetallics

NdFeB magnets were coated with a dual layer of either Ti/Al or Ni/Al by thermal evaporation. When the coated samples were annealed at 400°C, intermetallic reactions occurred within these coatings, between Ti and Al, and between Ni and Al. When the samples were annealed at 500°C, the reaction were extended to the matrix of the magnet forming Al₃₆Ti₄₃Fe₁₇Nd₄ in the sample coated with Ti/Al, but forming Ni₂AlFe_0.1 in the Ni/Al coated sample. The presence of these intermetallics were confirmed by leaching the coated magnets in NaOH. We had observed that the annealed Ti/Al coated sample had a smooth surface, while that of the Ni/Al was porous with microcracks as a result of the volume contraction of the Ni/Al layer. It was also found that Nd in the Nd-rich grain boundaries of the magnet tended to react actively with the dual layers. The areas involved would be extended as the annealing temperature and time increased. This shows that the quality of the resultant intermetallic coating depends upon the microstructure of the NdFeB magnet. Furthermore, the adhesion between the coating and the magnet can be improved greatly by the appropriate heat treatment