Study On Surface Modificative Technology Of Sintered NdFeB Permanent Magnetic Material

NdFeB permanent magnetic material as the third-era permanent magnet is widely used in many industries because of its outstanding properties, such as high saturation magnetization, coercive force, magnetic energy product (BH)_max, machinability, and comparatively low price. Unfortunately, it is prone to oxidation in common environments, especially in damp heat atmosphere, severe electrochemical corrosion will occur. So poor corrosion resistance of NdFeB permanent magnetic material is the main obstacle to its widely applications. As relation to the life-span of the magnetic material, the corrosion resistance of NdFeB permanent magnetic material was the focus of the world. Thus it is of great significance to investigate the surface modification technology of NdFeB permanent magnetic material.In this thesis, different surface modification technologies of sintered NdFeB permanent magnetic material were investigated, including sol-gel coating, phosphating coating, n-TiO₂/Ni nano-composite electroplating coating, based on the corrosion mechanism and anticorrosion treatment research of the papers in the recent years.The MgO thin films were first successfully prepared by sol-gel method taken Mg(NO₃)₂·6H₂O as precursor, anhydrous enthanol as solvent and proper surfactant. The different technique conditions on the sol stability were investigated and the surface properties were analyzed by DSC-TGA, XRD, immersion test. The results indicated that (1) transparent steady sol was achieved by keeping the concentration of Mg～(2+) at a range of 0.03～0.15mol·L^-1 and the volume ratio of solvent and surfactant at 6:4～8:2; (2) density, anti-corrosion MgO thin films were obtained on sintered NdFeB permanent magnetic material surface after optimal heat treatment technique, and the crystal face structure of the thin film will be transformed to main (200) and (220), the average grain size of crystalline MgO is 23.43nm after 450℃heat treatment.The nonpoisonous and friendly phosphating conversion coatings have been successfully prepared on sintered NdFeB permanent magnet material to improve its corrosion resistance. The effect factors on the preparation such as the pretreatment technique, the components and concentration of the solution, temperature and duration of the coatings formation were investigated. The preparation and anticorrosion properties of the conversion coatings on NdFeB surface were analysed by OM, SEM, potentiodynamic polarisation curve test, immersion test. And the optimum technological parameters were settled. The results show that the phosphating conversion coatings prepared under optimum technological parameters, were much more compact and anti-corrosive.n-TiO₂/Ni composite coatings were prepared on sintered NdFeB permanent magnet by composite electroplating method to improve anticorrosion property of the substrate. n-TiO₂/Ni composite coatings were electrodeposited from a traditional plating Ni bath containing nano-TiO₂ particles of different phase. The properties of n-TiO₂/Ni composite coatings were analysed by XRD, potentiodynamic polarization curve test, immersing test, porosity test and adhesion strength test. The results show that the structure of the matrix nickel was not changed with adding of nano-TiO₂ particles. XRD patterns of the gradient deposit showed that the Ni and TiO₂ particle diffraction peaks at their own given angles respectively, independent of each other. n-TiO₂/Ni composite coatings were no porosity, with a good adhesion to the NdFeB permanent magnet. The anticorrosionproperties of the n-TiO₂/Ni composite coatings were improved greatly.The composite films were prepared on sintered NdFeB permanent magnet, in order to obtain most effective protective coatings. Substrates were immersed in phosphating solution to form phosphating coatings before n-TiO₂/Ni composite electroplating. The properties of coatings were analysed by XRD, XPS, potentiodynamic polarization curve test, immersion test, porosity test and adhesion strength test. The anticorrosion properties of the composite films were improved greatly.