Study On The Preparation Of High Temperature Oxidation Resistant Coating For Molybdenum Alloy

Molybdenum alloy,as a typical superalloy,has high strength,vermicular resistance and corrosion resistance,which makes molybdenum alloy as a high-temperature fittings and electrode material used widely.However,Mo is very easy to oxidize during the heating process.When the temperature is 400?,the oxidation rate of molybdenum alloy is very slow and the surface of the alloy produces a volatile MoO₂.At 400⁷50?centigrade generates volatile MoO3;when the temperature is higher than 750?,and the MoO₃ volatilization makes the weight increase sharply down,the mass loss is serious and restricts its loss.The possibility of its used as heat-resistant structural materials was limited.Aiming at the problem of molybdenum oxidation at high temperature,the aim of this study is to develop a high temperature oxidation resistant coating for molybdenum substrate.The modified coating system is a ZrB₂-SiC thermal barrier coating with MCrAlY as a bonding layer.The coating with a thickness of 200³00um on the surface of molybdenum substrate is prepared by supersonic plasma spraying.Compared to the vacuum or inert gas processing technology,this method has the advantages of simple technology,known as local and obvious protection effect.The high temperature oxidation resistance of the coating was studied from two aspects of NiCoCrAlTa coating and ZrB₂-SiC coating.First,on the basis of comprehensive analysis of literature work,the MCrAlY coating system was selected and the appropriate components were calculated according to the thermal expansion coefficient.The effect of Ta elements on the oxidation resistance and oxidation behavior of the coating was studied.The microstructure changes of the coating surface during the oxidation process were analyzed.The standard free energy of formation indicates that the components of the coating system are inert to molybdenum matrix in the range of oxidation temperature.After studying the oxidation behavior of MCrAlY coating,using NiCoCrAlY as a bonding layer,the Zr B₂-SiC layer was prepared by supersonic plasma spraying on the bonding layer.The oxidation behavior of each layer during the oxidation process at different temperatures was studied.The effect of thermal growth oxide on the failure of the coating at the interface,and the interface between the element diffusion and the interface were analyzed.Protect the influence of the film.During the entire service period of the coating,the bonding strength of the coating and the substrate has an important influence on the stability of the coating.According to the different testing conditions for various testing methods,the bonding strength of the coating is tested by scratch method,and the bonding strength of the membrane based on supersonic plasma spraying is studied.Combined with the test results,the protective mechanism of NiCoCrAlTa coating and Zr B₂-SiC coating was discussed.According to the scanning electron microscopy?SEM?analysis,it is found that the surface of the coating is slightly compact glass phase.By the analysis of X-ray?XRD?diffraction and energy spectrum?EDS?,it is known that the Al₂O₃-Ta₂O₅ composite protective film with self healing properties is formed on the surface of the coating,and the refractory particles can play a supporting role in the coating structure.A protective layer with self healing properties is formed on the surface of the ZrB₂-SiC coating with NiCoCrAlY as a bonding layer.The existence of the protective layer can effectively prevent the diffusion of oxidizing gases from the molybdenum matrix.