Study On Ion Carburizing Modification Of Tantalum Surface

As corrosion resistant materials and high temperature structural materials,Tantalum plays a very important role in aerospace,chemical,nuclear and other high-tech industries.But it must be done to prevent the further oxidation of tantalum at high temperature,which cause the decrease of mechanical properties and reliability of surface.To carburizing on the surface of tantalum,which forms tantalum carbides that have good strength and stability in high temperature,can protect the surface with low cost and no affect to substrate materials.In this work,first-principle calculations based on density function theory(DFT)were performed to investigate the mechanical properties,electronic structures and thermodynamic properties of TaC and Ta2C,which is the main production of tantalum carburization.Voigt-Reuss-Hill approximation and Chen's semi-empirical relation were used to work out the elastic modulus and hardness of them.Electron structure were calculated to discuss the cause of their differences in mechanical property.And thermodynamic properties were worked out by quasi harmonic approximation theory.Furthermore,we inferred the relationship between the properties of carbides and carburized layer,and the way of adjusting the composition of carburized layer.In experiments,we used ion carburizing technology based on hollow cathode effect to do the carburizing experiment at surface of tantalum,with the condition of 1300 ? and the atmosphere of methane and argon.XRD,SEM,XPS and AES were used to analysis the composition and structure of carburized layer.The results shows that the method of ion carburizing is feasible and has good effect on tantalum.In addition,The oxidation resistance of the carburized layer was proved by the oxidation experiments of pure tantalum and carburized samples.DFT calculation shows that both of TaC and Ta2C have high elastic modulus and hardness,Ta2C has lower strength but a certain degree of plasticity,while it's thermal expansion coefficient is lower than Ta and TaC.these calculations suggest that the presence of Ta2C can improve the ductility and impact resistance of carburized layer,but may reduce the thermal cycling stability because of the difference of thermal expansion capability between Ta2C and other phases.Based on the theoretical and experimental results,it is concluded that the formation of Ta2C can be improved by increasing the carburizing temperature or decreasing the partial pressure of CH4,and the composition of carburized layer is likely to change during the high temperature.In the experiment of ion carburizing,the carburized layer with good compactness and gradient composition was obtained on the surface of tantalum at relatively low temperature and voltage.Carburizing speed were increased by using hollow cathode effect which substitutes the external heating methods.The result shows that the composition of carburized layer was changed from TaC to Ta2C and then converted to Ta,and there is good bonding between different phases of carburized layers.Oxidation experiment shows that the carburized layer can play a good antioxidant role in 500 ? and a short time.