Preparation Process And Property Of A Niobium Coating By Molten Salt Electrodeposition

Due to its excellent comprehensive properties,niobium（Nb）has attracted increasing attention recently in medical,aerospace and superconducting fields.In the aerospace field,Nb can be used as the transition connecting material between rhenium/iridium combustion chamber and titanium alloy injector to solve the metallurgical incompatibility between rhenium and titanium alloy.However,the CVD Nb used at present has high deposition temperature（1200℃）,long process cycle,high cost,difficult quality consistency to control,and is difficult to implement large-scale industrial production.The molten salt electrodeposition method can prepare uniform and dense Nb coating on the surface of complex surface substrate with high efficiency and low cost,which is expected to test the engineering application of Nb coating.Therefore,it is necessary to systematically study the process and properties of molten salt electrodeposition Nb.The traditional fluoride supporting electrolyte is beneficial for coating quality but highly toxic.The commonly used binary Na Cl-KCl supporting electrolyte is nontoxic but difficult to obtain a dense coating.Therefore,it is necessary to find a supporting electrolyte system with low toxicity and good coating quality.In this paper,a low-toxicity Na Cl-KCl-Cs Cl-K₂Nb F₇ system was used to prepare Nb coatings on Mo substrates.Through the preparation of molten salt,the physical properties and electrochemical properties of molten salt,the formula of molten salt was optimized.The process and properties of Nb coating prepared by direct current and pulse electrodeposition were studied to reveal the regulation mechanism of electrodeposition process parameters on the microstructure and properties of the coating.On this basis,the process of electroforming thick wall Nb was preliminarily explored by optimizing the deposition process.The molten salt system for electrodeposition of Nb was prepared by the study of molten salt preparation process.The thermophysical properties of Na Cl-KCl-Cs Cl supporting electrolyte with different Cs Cl content were studied by TG-DSC and molten salt physical property analyzer.The molten salt formula was optimized according to the deposition quality of electrodeposited Nb coating in different components of molten salt,and its electrochemical characteristics were analyzed by cyclic voltammetry.The results showed that Na Cl-KCl-Cs Cl supporting electrolyte could be effectively purified by high temperature melting（750℃/30 min）,mechanical impurity removal and Cl₂ chlorination bubbling(100 ml·min^-1/10 min)in Ar gas atmosphere.The eutectic temperature of Na Cl-KCl-Cs Cl molten salt ternary mixed molten salt was about 485℃.With the increase of Cs Cl content,the initial crystal temperature of molten salt decreases first and then increases,the density increases,and the conductivity and surface tension decrease.In the mixed molten salt for electrodeposition,the Nb coatings prepared with the Cs Cl content in the range of 60～80wt.%were smooth and compact without tumor.During molten salt electrodeposition of Nb coating,Nb⁵⁺is first reduced to Nb⁴⁺and then to elemental Nb.The addition of Cs Cl can negatively shift the reduction potential of Nb⁴⁺ions and oxygen-containing Nb complex in molten salt,which is helpful to increase the reduction potential and obtain Nb coating without oxygen impurities.The effects of direct current deposition processes such as temperature,current density and deposition time on the microstructure and properties of Nb coating were studied by single variable method.The results showed that the coating consisted of nucleation layer and growth layer.When the coating growth layer was columnar crystal not affected by epitaxial growth,its preferred orientation was<211>,and the orientation had weak correlation with process parameters.With the increase of polarization degree（i.e.increasing current density or decreasing temperature）,the particle morphology of Nb coating changed from hexagonal star to cone.In the process range studied,the cathodic polarization was weak,no oxide was generated in the Nb coating,and the microhardness of the coating was less affected by the deposition process parameters,which was roughly in the range of 220～250 kg/mm².The effects of pulse electrodeposition processes such as average current density,frequency and duty cycle on the microstructure and properties of Nb coating were studied by single variable method and the feasibility of electroforming thick wall Nb layer was explored.The results showed that excessive peak current density（i.e.high average current density or low duty cycle）or too long electrodeposition time（i.e.too high or too low frequency and too high duty cycle）would result in the formation of spherical particles on the coating surface.Non-spherical particles Nb coating surface particles were polygonal or six-pointed star-shaped.If the coating growth mode is"upward growth",the preferred orientation of the coating changes in the order of<110>→<211>→<200>with the increase of cathodic polarization（i.e.the average current density increases,the duty cycle increases,and the frequency increases or decreases）.When the cathodic polarization was weak（i.e.low current density,high duty cycle or high frequency）,no oxide was generated in the Nb coating,and the microhardness of the coating was less affected by the change of process parameters,which was between 260～300 kg/mm².When the cathodic polarization was strong,oxide inclusions were generated in the Nb coating,resulting in a significant increase in the microhardness of the coating,which could reach about 842kg/mm².The quality of Nb thick coating prepared by long-time pulse electrodeposition（i.e.electroforming）was better than that of direct current electrodeposition.Under the conditions of low average current density and high active ion concentration,the combination of multiple electroforming and polishing could reduce the influence of concentration polarization,thereby increasing the density of electroformed Nb coating.The density of the prepared coating reached 94.9%of the theoretical density of Nb.