Densification and anticorrosion performances of vacuum evaporated aluminium coatings on NdFeB magnets

Abstract

Al coated NdFeB magnets obtained by vacuum evaporation technique were densified by high energy ball milling method. The surface morphology, metal composition and microstructure of the coatings were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The anticorrosive properties were investigated by potentiodynamic polarization curves and neutral salt spray test. The pores in the Al coatings of columnar crystals (Al) induced by the evaporation technique, were apparently filled in the following ball milling process, leading to the densification of Al coatings and the evident improvement of the anticorrosive performances. When treated with ball milling for 30 min, the sample achieves the best anticorrosive performances with the self-corrosion potential of −0.87 V, self-corrosion current density of 1.65 μA/cm² and the neutral salt spray (NSS) time of 144 h (red rust). The improvement of the anticorrosive performances of vacuum evaporated Al coating mainly lies in the densification effect of the coating, which depends on different loading conditions of ball milling process.

Introduction

As the third generation permanent magnet materials, NdFeB exhibits excellent magnetic properties, and has been widely used in various fields, such as electronics, acoustics, communications, automation, magnetic resonance imaging and biomedical applications.^1,2 However, poor anticorrosive performance and thermal stability in various environments extremely limit its further applications, which are related to the existence of multiple phases in the microstructure,3, 4, 5 including Nd₂Fe₁₄B main phase and Nd-rich phases in the grain boundaries.

At present, many studies have been devoted to improve the anticorrosive performance of NdFeB magnets, such as alloying technology,^6,7 metal coatings,8, 9, 10, 11 ceramic coatings12, 13, 14 and organic coatings.15, 16, 17 By adding alloy elements Co and Ce to the magnets in the casting process, the potential difference among the phases in the magnets can be reduced or even eliminated to a certain extent, thus reducing the corrosion electrochemical activities of the Nd-rich phases and improving the corrosion resistance of the magnets. However, the alloying process also increases the production cost, and reduces the magnetic properties (remanence, magnetic energy product) in the meanwhile.⁷ Ceramic coatings have excellent corrosion resistance, wear resistance, oxidation resistance and biocompatibility. However, the bonding force between the ceramic coating and the substrate is poor, and the addition of the coating results in the decrease of the remanence and the maximum magnetic energy product of the sintered NdFeB magnets.¹⁸ At present, the commonly used organic coatings are epoxy resin coatings, fluorocarbon organic coatings and polyurethane coatings, which can be obtained by spray and electrophoresis.^19,20 Most organic coatings will cause serious environmental pollution, and the organic coating prepared by cathodic electrophoresis shows poor moisture and low heat resistance. Therefore, the magnet surface protection coating is dominated by metal coatings including electroplating and electroless plating with Ni, Zn and Al for a long time.

There are many methods used for preparing metal coatings, such as electroplating,21, 22, 23 chemical plating¹⁰ and physical vapor deposition.^24,25 At present, electroplating has become the most commonly used method for NdFeB magnets due to the low productive cost. However, the electrolyte will penetrate into the pores on the surface, resulting in corrosion of the magnet when immersed in the electrolyte for a long time. On the other hand, low adhesion between the coating and substrate will cause the crack and peeling-off of coatings, resulting in the failure of the magnets.^22,23 Electroless plating is usually used to meet the need for higher corrosion resistance. The chemical nickel-plated phosphorus layer, acting as a novel coating, shows excellent corrosion resistance and mechanical performances compared with the electroplating coatings. However, only in acidic solution, the Ni–P coating with high phosphorus content could be obtained. Under this condition, hydrogen generation occurs between the Ni–P coating and the substrate, leading to poor bonding strength between the Ni–P coating and NdFeB substrate, further restricting crack development.¹⁰ With the development of science and technology, the traditional surface anticorrosive coating of NdFeB has to meet the changeable requirements of high corrosion and other working environment. Therefore, as an environmentally friendly method, physical vapor deposition has attracted increasing attention for improving corrosion resistance, such as TiN coating,²⁶ Zn–Mg coating²⁷ and Al coatings.^28,29

Among metal coatings, Al coating has been the preferred one for sintered NdFeB magnets due to its unique advantages, such as the negative corrosion potential as a sacrificial anode compared with the substrate, and dense Al₂O₃ film forming on coating surface with high anticorrosion performances.^30,31 Gao et al.³² deposited pure Al coating on sintered NdFeB by multi-arc ion plating, and studied the excellent corrosion resistance by using electrochemical impedance spectroscopy and neutral salt spray test. Chen et al.³⁰ prepared pure Al coating on sintered NdFeB magnets by direct current magnetron sputtering. It was found that the coating did not affect the magnetic properties of the sintered NdFeB magnets while improving the corrosion resistance. However, columnar crystal growth of vapor depositing Al coating led to the poor density of the coating and high porosity, resulting in the rapid diffusion of the corrosive medium.

There have been many studies on solving the problem of densification of coatings. Zhang et al.²⁵ prepared Al coatings on the surface of NdFeB by plasma assisted physical vapor deposition. It was found that the holes in the Al coating were filled with a complete passivation film after trivalent chromium passivation, which improved the anticorrosive performances of the Al coating. However, owing to the high harmfulness and carcinogenicity of trivalent chromium ions, this passivation process was not a environmentally friendly method. Mao et al.³³ prepared the IBAD-Al thin films by DC magnetron sputtering and ion beam-assisted deposition (IBAD). Due to the bombardment of the Ar ion beam, IBAD-Al layer in the multi-layer film had fine crystal grains without the columnar structure, which effectively improved the anticorrosive performances of the Al-based coating.

Here, Al coatings were deposited on NdFeB magnets by vacuum evaporation, and were densified by a convenient ball-milling method, so as to improve the anticorrosive performances of Al coated NdFeB magnets. Compared with other methods mentioned above, the two-step method of physical vapor deposition combined with high-energy ball milling used in this paper, not only obtains densified Al coating, but also avoids the problems of complicated process and environmental pollution. It has a long-term development prospect in the day when green environmental protection is increasingly advocated.