Research On The Preparation And Property Of Ni/TiO₂ Coatings Electrodeposited On NdFeB Surface

The Nd Fe B is widely used in medical devices,electronics,automotive industry and electric machinery owing to its excellent physical properties,such as high remanence,excellent magnetic energy product,outstanding intrinsic coercivity and good mechanical property.However,it is vulnerable to be oxidized and corroded due to the porous internal structure of the sintered Nd Fe B material and its high chemical activity,which greatly limits their wide application.In this paper,a layer of Ni/Ti O₂coating was deposited on the surface of sintered Nd Fe B by nano-composite electrodeposition in order to address the issues of high porosity and poor corrosion resistance.The micro-morphology of the coating was investigated by SEM.The porosity and bonding force of the composite coatings were determined by filter paper sticking method and scratching method.Hardness of the samples before and after electroplating were obtained by Vickers hardness test and the results were compared and analyzed.The corrosion resistance of the composite coating was characterized by static immersion test,polarization curve and impedance spectroscopy.The effects of electroplating voltage,bath temperature,nano-Ti O₂content,p H value and Sodium Dodecyl Sulfonate?SDS?content on the surface morphology,hardness,porosity,adhesion and corrosion resistance of the coatings were studied by single factor characterization.The results indicate that when the plating voltage was a single variable,the coating with voltage of 3V exhibited the best micro-morphology,highest hardness and best corrosion resistance in salt solution.While the bath temperature is the single variable and the optimum temperature was 50?,the coating revealed the best corrosion resistance,greatest microhardness,lowest porosity and best adhesion between the coating and the matrix.When the concentration of Ti O₂was 10 g/L,the coating had the best compactness and corrosion resistance.When the p H value of the bath equaled to 4.5,the coating presented uniform and smooth surface,no obvious defects,the minimum porosity,the best adhesion between the substrate and the coating,the highest hardness and the best corrosion resistance.When the SDS concentration in the bath was 1 g/L,the grains were arranged regularly in the coating,all other properties of the coating were uniform and the corrosion resistance was excellent.The orthogonal test results of five factors and four levels implied that the coating adhesion was greatest when the plating voltage was 2 V,the temperature was 40?,the concentration of Ti O₂was 1g/L,the p H value was 4 and the concentration of SDS was 0.5 g/L.When the voltage was 2 V,the temperature was 60?,the concentration of Ti O₂added was 10 g/L,the p H value was 5 and the amount of SDS added was 2g/L,the microhardness of the coating was the greatest.The corrosion potential of the coating was optimal when the temperature was 50?,the concentration of Ti O₂was 1 g/L,the p H value was5.5 and the concentration of SDS was 2 g/L.When the voltage was 4 V,the temperature was 65?,the concentration of Ti O₂is 5 g/L,the p H value was 4and the concentration of SDS was 2 g/L,the porosity of the coating was minimal.The results of range analysis show that when the voltage was 2 V,the temperature was 65?,the concentration of Ti O₂as 1 g/L,the p H value was 4.5and the concentration of SDS was 2 g/L,the comprehension property of the coating was excellent.Based on the analysis and comparison result of the above optimum samples,the best combination of electrodeposition parameters was obtained.According to the obtained coating micro-morphology,hardness,porosity,binding force,average corrosion rate and electrochemical curves,the optimum combination of electrodeposition parameters were as follows:plating voltage:3 V,bath temperature:65?,concentration of Ti O₂:1 g/L,bath p H:4.5,concentration of SDS:2 g/L.