Deposition Of Al/Ni Thin Films On Sintered NdFeB Magnets And Their Anti-corrosion Properties

NdFe B permanent magnet materials are widely used in various fields for their outstanding magnetic properties. However, in practical application, NdFeB magnets have to be surface protected due to their poor corrosion resistance. Currently the common technique for surface treatment of NdFeB magnets is to electroplate a layer of Ni or other metals. Compared with electroplating, physical vapor deposition technique has shown advantages, such as environmental friendliness and the high quality of coating. In this thesis work, the Al films, Ni films and Al/Ni multilayers were deposited on the surface of N35 type sintered NdFeB magnet by DC magnetron sputtering. The deposition process, morphology, and structure of the films were investigated systematically. The effects of film compsition and structure on the anti-corrision properties and other properties were studied. The main research contents and conclusions are as follows:(1) The effects of sputtering process on the Al, Ni, and Al/Ni composite multilayers films depositing on the NdFeB substrate were analyzed. The results show that Al and Ni single-layer coatings all grow as columnar crystals on the substrate. The film thickness increases with the increase of the sputtering power and time. Negative bias turns out to be effective in interrupting the columnar crystal growth, therefore improving the quality of coatings. In the Al/Ni multilayers coating, Al and Ni deposited in separated phase structure. but the columnar structure of Al film can be interrupted by Ni film. With the layer increased from 3 to 13, the surface became dense and uniform. In addition, the step coverage and uniformity of magnetron sputtered film were also investaigated. It was found that the film thickness at lateral surface is less than that on the vertical surface. The deposition rate is affected by the substrate cleanliness, roughness and sputtering process.(2) The electrochemical measurement indicates that the magnets with Al film deposited at 100 W sputtering power and the magnet with Ni film deposited at 120 W with 150 V negative bias of Ni coatings show an order of magnitude lower corrosion current density than the bare Nd FeB, indicating both Al and Ni films can protect the magnet from corrosion.(3) The Al/Ni multilayers coating can also improve the corrosion resistance performance of NdFeB magnets. The corrosion current density decreases when the thickness increases, a negative bias is employed or the coating number increases form 3 to 13. The EIS test also confirms that the coating magnets have larger capacitive arc radius than bare NdFe B. Especially the NdFeB magnet with Al/Ni multilayers in the coating numbers of 13 and prepared under 150 V negative bias shows about 90 times larger radius than bare Nd FeB magnet, indicating that this film system has excellent magnet protective effect.(4) The oxidation test of the bare NdFeB, industrialNdFeB with chemical plated Ni film and the Nd Fe B with magnetron sputtered Ni film showed that the magnetron sputtered coating has the best oxidation resistance.(5) The current experiments demonstrated that the magnetron sputtered film coating can improve the corrosion resistance of sintered NdFeB magnets without damaging their magnetic performance.