Study On Process,Film Formation And Corrosion Resistance Of Tantalum Coating Deposited By Chemical Vapor Deposition

Refractory metal tantalum has become one of the key corrosion-resistant materials widely used in weapon equipment,biomedicine,nuclear industry and new energy due to its excellent corrosion resistance.However,there is a large texture preferred orientation in the preparation process of tantalum,which leads to local preferential corrosion and rapid failure of the coating.This is a potential safety hazard for tantalum in extremely corrosive environment.In order to solve this problem,the deposition behavior of chemical vapor deposition(CVD)Ta coating was studied by thermodynamic analysis and molecular dynamics calculation,and the first principles calculation was used to clarify the effect of crystal orientation on the surface activity of the coating.The internal mechanism of plasma etching parameters on fine structure,surface state and corrosion resistance of CVD Ta coating was obtained by experimental study.Finally,combined with the theoretical calculation and experimental results,the structural characteristics,electrochemical characteristics and atomic position interaction transport process of the coating were determined,and the surface multi-layer structure model was constructed to reveal the thermokinetic defect control mechanism of the surface microstructure of CVD Ta coating.The main conclusions of this paper are as follows:Thermodynamic calculation shows that the Gibbs free energy decreases with the increase of reaction temperature,and the critical reaction temperature is 881?.When the temperature is higher than 881?,the decomposition of TaCl₅occurs.Molecular dynamics simulation shows that with the increase of deposition temperature,the vacancy decreases and the coating quality improves;with the increase of chlorine gas flow,the surface roughness increases,but the vacancy increases and the film quality decreases.According to the results of molecular dynamics simulation,the suitable process parameters for Ta coating deposition were determined as follows:deposition temperature 1100-1300?,atomic incident energy 0.3-0.5ev,incident angle 0-10°.By analyzing the microstructure of Ta coating,it is found that the surface morphology of the coating is pyramid structure,the grain size of the coating increases with the increase of deposition temperature,and the range of grain size changes is smaller and smaller.When the deposition temperature is 1000?,the hardness and density of the coating are 245.02 HV and 99.85%respectively.The Ta coating prepared by CVD has obvious texture.When the deposition temperature is 1000?,the preferred orientation is not obvious;when the deposition temperature is1100?,the strong(200)preferred orientation is formed;when the deposition temperature is1200?,there are two kinds of preferred growth grains(200)and(110);when the deposition temperature is 1300?,the main preferred orientation is(200);the chlorine flow rate,hydrogen flow rate and chlorination temperature have no significant effect on the preferred orientation.In view of the problem of the grain selection and orientation of Ta coating,the relationship between the surface properties of the coating and the surface energy of the coating is studied by the first principle.It is found that the self diffusion activation energy of Ta is proportional to vacancy forming energy,vacancy transfer energy and bond energy.By introducing vacancies on Ta surface,the nucleation sites of Ta₂O₅film can be increased,the nucleation mode can be changed from single nucleation to multiple coexisting nucleation,the film reaction rate can be accelerated,and the growth density and thickness of Ta₂O₅film can be improved,which provides a theoretical basis for improving the stability of Ta₂O₅film by changing the surface characteristics of the coating.The corrosion resistance of Ta coating was studied by means of electrochemical test.Ta coating shows good corrosion resistance.The anodic oxidation process of Ta coating is beneficial to increase the thickness and uniformity of the oxide film on the surface of the coating.Compared with the deposited state,the E_corrof the anodic oxidation state is increased,and the j_corris decreased.The anodic oxidation improves the stability of Ta coating in corrosive medium.Combined with the theoretical calculation and experimental results,it is found that the etching treatment can change the nucleation mode of the oxide film in the anodic oxidation process from continuous nucleation mode to instantaneous nucleation mode,and improve the diffusion coefficient of defects in the anodic oxidation process,so as to promote the formation of Ta₂O₅oxide film,improve the thickness and uniformity of the oxide film,and make the surface of the coating uneven.The stable Ta⁴⁺is transformed into Ta⁵⁺,which improves the stability of the coating.The corrosion resistance of the coating is obviously improved compared with that of the as deposited coating.The self corrosion potential increases by 63.2%compared with that of the as deposited coating,while the self corrosion current density decreases by 50%.