Annealed Al-Cr coating: A hard anti-corrosion coating with grain boundary modification effect for Nd-Fe-B magnets

•The surface protection and coercivity enhancement for high-performance Nd-Fe-B magnets can be greatly combined by using an annealed Al-Cr coating.•Cr addition can significantly improve the hardness of Al coating (~5 times higher than Al) and densify the coating by modifying the diffusion process of Al. A novel method is proposed for Nd-Fe-B magnets to achieve both surface protection and grain boundary modification by using annealed Al-Cr coating. In detail, Cr was introduced into Al coating of Nd-Fe-B magnets by sputtering deposition using Al-Cr targets. A follow-up annealing at 550 °C was carried out to diffuse Al atoms into the grain boundaries of the magnets. It was found that Cr addition can significantly improve the hardness of Al coating and modify the diffusion process of Al. The annealed Al50Cr50 coating exhibited a hardness of 1011 ± 1 HV, ~5 times as high as the Al coating. The great adhesion was also obtained between the Al-Cr coating and Nd-Fe-B substrate. Most importantly, the introduction of Cr can effectively enhance corrosion resistance of the annealed coating due to its densification effect on the coating by modifying Al diffusion process. The coercivity of the magnet increased 95 kA/m and 89 kA/m by diffusing the Al80Cr20 and Al70Cr30 coatings, respectively, due to the Al diffusion caused microstructure modification. The present work demonstrated that the surface treatment and grain boundary diffusion can be combined into one process by Al-Cr coatings for high-performance Nd-Fe-B magnets with high hardness, strong adhesion, sufficient corrosion resistance, good anti-oxidation and improved coercivity.