Electrodeposition Of Refractory Metal Carbide Coating In Molten Salt

Refractory metal carbides including niobium carbide and tantalum carbide are attractive materials that exhibit unique properties,such as high melting point,high hardness,good wear resistance and chemical inertness.Due to these excellent properties,the carbides are promising materials for wide applications in many fields.A coating of refractory metal carbide(NbC or TaC)can not only maintain properties of the substrate,but also endorse unique properties of the carbide for a composite material.The most frequently utilized methods for deposition of niobium and tantalum carbide coatings are physical vapor deposition and chemical vapor deposition.However,the complicate operation and high temperature are usually adopted in most of these methods.So achieving good adhesion of niobium carbide coatings to metal substrates is always a big challenge because of the residual stress derived from the high temperature,which can induce delamination of the carbide film.So it is necessary to develop a novel method to produce niobium and tantalum carbide coatings in a simple,efficient and economical way.In this study,a novel process for preparation of refractory metal carbide coatings has been proposed on the surface of Nb and Ta substrates based on electrochemical reduction of carbonate ions in LiCl-KCl and CaCl2 molten salts.The surface of metallic substrates was purified and activated using electro-deoxidation,and oxidation in combination with subsequent electro-deoxidation of the substrates.Electrochemical mechanism for carbide deposition was investigated.Some factors involving working temperature,concentration of carbonate ions,deposition voltage and duration were discussed.The main results are summarized as follows:NbC and TaC coatings have been prepared successfully on niobium and tantalum substrates respectively by electrochemical reduction of carbonate ions in LiCl-KCl and CaCl2 molten salts.The existence of oxides or the derivates of tantalates in molten salts on the substrates is the most outstanding challenge for synthesis of carbides,which would separate the contact of the reduced carbon and the substrates.Electrochemical deoxidation of niobium and tantalum substrates with and without oxide scale is an effective method to remove the oxides present on the substrates,and also is an efficient way to activate the substrates,which promotes the generation of NbC and TaC.Increasing working temperature is available for formation of the carbides.The relationship between carbide formation and carbonate concentration in molten salt,as well as deposition voltage is not obvious if the deposited carbon is sufficient on the substrates.With the increase of deposition duration,the carbides nucleated and grew to fine crtstalline grains,experiencing the intermediates phases of Nb2C and Ta2C.Electrodeposition in molten CaCl2 is available for slowing down the diffusion and electro-reduction of carbonate ions in comparison with that in LiCl-KCl molten salt.It is benefit to eliminate the aggregation of the deposited carbon on the carbide coatings.The carbide coatings enhance the hardness of the substrates effectively.