Processes，Microstructures and Properties of Niobium Coatings Prepared by Molten Salt Pulsed Electrodeposition AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23110014

Chinese Library Classification Number:TG146.4+16

Citation Information:

Niobium （Nb） has been widely used in aerospace，medical，and superconducting fields in recent years due to its advantages of high melting point，excellent high temperature strength，good biocompatibility，and high critical temperature of superconductivity. The molten salt electrodeposition method can efficiently，controllably，and economically prepare Nb coatings，which have high purity，compactness，and good adhesion to the substrate. In the selection of molten salt system，fluoride system is toxic and corrosive to equipment，which is not conducive to engineering application. The coatings prepared by traditional binary chloride systems have poor compactness and poor adhesion with the substrate. By adding CsCl to the NaCl-KCl molten salt，a continuous and dense Nb coating can be prepared，while the introduction of Cs ions helps to stabilize the Nb complex ions in the active salt，K₂NbF₇，and at the same time reduces the tendency of the co-deposition of oxygen-containing impurities in the molten salt. However，this system is susceptible to deterioration of the quality of the coating，such as nodules，due to concentration polarization when direct current （DC） is used for long-time electrodeposition. To improve the surface quality and mechanical properties of Nb coatings prepared by the traditional DC electrodeposition process，this paper investigated the influence of the pulsed electrodeposition process on the organization and properties of Nb coatings，revealed the mechanism of the influence of the pulsed electrodeposition process parameters on the organization and properties of Nb coatings，and realized the optimization of the preparation process of Nb coatings. The used support salt composition in this study was 17.6% NaCl-22.4% KCl-60% CsCl，and the active salt composition in the mixed molten salt was K₂NbF₇ with a Nb ion concentration of 2.4%. The cathode used for electrodeposition was a Mo plate and the anode was a Nb crucible. The effects of pulse electrodeposition process parameters such as average current density，frequency，and duty ratio on the organization and microhardness of Nb coatings were investigated separately. The results showed that with the increase of average current density，the cathodic polarization was enhanced，then the surface morphology of the coating changed as "hexagonal star-rock sugar-globular clusters" . The coating consisted of a fine equiaxed crystal nucleation layer and a continuous columnar crystal growth layer，and the thickness of the continuous layer decreased due to nodulation. The pulse frequency affected the ion mass transfer and subsequently the coating surface quality by regulating the pulse loading/termination time and the bilayer charging/discharging time. Too small pulse frequency led to concentration polarization，too high frequency would lead to the actual current close to DC，either of which led to the deterioration of the surface quality of the coating. A moderate pulse frequency allowed the active ions to be replenished timely and avoided concentration polarization，thus optimizing the surface quality of the coating. At a constant average current density，the peak current density during pulse loading could be influenced by adjusting the duty ratio，leading to different cathodic polarizations and ultimately affecting the coating surface quality. When the duty ratio was low，the peak current density was much higher than the cathodic limit current density which would rapidly trigger the concentration polarization，and when the duty ratio was high，the single pulse deposition time became longer，and the active ions were difficult to recover in time，which also caused the concentration polarization and led to the deterioration of the coating surface quality. When the duty ratio was moderate，the appropriate peak current density and deposition time could make the cathodic polarization moderate，and the active ions were replenished in time to optimize the coating surface quality. The microstructure of the coating was related to cathodic polarization. When electrochemical polarization or concentration polarization was not strong，the pulsed electrodeposited Nb coating grew in an upward growth mode with a typical columnar crystal structure，and the optimal growth crystal surface was （211）. When the deposition conditions corresponded to a significant increase in cathodic polarization，the optimal growth crystallographic of Nb coatings transitioned to （110） or （200）. The microhardness of the coating was little affected by the change of process parameters and was positively correlated with the content of oxygen impurities in the coating. When the cathodic polarization corresponding to the pulse electrodeposition condition was weak，no oxide was formed in the Nb coating，and the microhardness of the coating was between 260 and 300 kg·mm^-2. When the cathodic polarization corresponding to the pulse electrodeposition condition was strong，oxide impurities were formed in the Nb coating，which led to a significant increase in the microhardness of the coating，up to 927 kg·mm^-2. In all，the pulsed electrodeposition process affected the coating surface quality mainly by influencing the degree of cathodic polarization. Electrodeposition processes that resulted in higher cathodic current densities （higher average current densities or lower duty ratios） or longer continuous deposition times （higher/lower frequencies or higher duty ratios） all increased cathodic polarization and led to coating surface nodules. Using the optimized pulsed electrodeposition process （average current density of 10~50 mA·cm^-2，frequency of 0.5~25 Hz，and duty ratio of 50%），Nb coatings that were smooth，dense，with a typical columnar structure，with a preferred growth plane of （211），and with microhardness in the range of 260~300 kg·mm^-2，could be obtained.