Revealing the surface oxidation mechanism and performance evolution of Nd-Fe-B sintered magnets

Abstract

Achieving high magnetic properties and corrosion resistance simultaneously is a common goal for the community of the Nd–Fe–B permanent magnetic material, while remains challenging via traditional strategies. Here we conduct wide-range oxidation experiments to pave the approach of constructing a tunable surface oxidation layer. Temperature-dependent and time-dependent oxidation behaviors of the typical N50 commercial-grade Nd–Fe–B sintered magnets with the corresponding performance evolutions have been systematically unraveled. Results show that short-term low-temperature oxidation of 350 ℃-0.5 h or 250 ℃-3 h generates an excellent synergy of improved corrosion resistance and mechanical performance without compromising magnetic properties, due to the formation of thin and hydrophobic oxidation layer with less microscopic cracks under the low kinetic coefficients (1.1 × 10-17 ~ 9.5 × 10-16 m2/s). High oxidation temperatures of 450~650 ℃ with exponentially increased kinetic coefficients (1.5 × 10-14 ~ 3.2 × 10-12 m2/s) lower the anti-corrosion, mechanical and magnetic performance due to the thickening oxidation layer with macroscopic cracks, albeit of the superhydrophobic characteristics. With respective to the high-temperature oxidation mechanism, the formation of continuous and coarse grain boundary (GB) networks with multi-layered structure is identified in the internal oxidation zone for the first time. The multi-layered structure can be divided into four layers, with the first and second layers referring to continuous Nd/Pr/O-rich GBs with the maximum oxygen concentration (P3(_)m1 and Im3(_)m/Ia3(_) structured Nd2O3), the third layer referring to Fe-rich intermediate layer with extremely low concentrations of Nd/Pr and O (dominated by Im3(_)m structured α-Fe), the fourth layer referring to a mixture of Fe, Nd/Pr and O (coexisting Im3(_)m structured α-Fe and amorphous Nd2O3). In the external oxidation zone, the single crystalline α-Fe phase with the absence of amorphous Nd2O3 is observed. Both features accelerate the inward oxygen diffusion and explain the high oxidation kinetic of the 650 ℃ oxidized magnet. Above correlation between the tunable oxidation behaviors and performance of the oxidized Nd–Fe–B magnets, together with the temperature-dependent oxidation mechanisms provide new understandings for delicately controlling the corrosion-resistant oxidation coatings.