Microstructure And Properties Of Modified Mo-Si Functionally Gradient Coatings On Molybdenum Surface

Molybdenum metal has high melting point,excellent electrical conductivity,thermal conductivity and corrosion resistance,and is widely used.But the poor oxidation resistance of molybdenum at high temperature limits the application range of molybdenum.MoSi₂ with high melting point can form continuous and protective Si O₂ film in high temperature oxidation environment,which is often used as anti-oxidation coating.However,the thermal expansion coefficient of the single MoSi₂ coating is mismatched with the Mo substrate,which leads to cracks and degradation of oxidation resistance of the coating.The oxidation of B to B₂O₃ can improve the fluidity of Si O₂ at high temperature,and can effectively healing the cracks generated in the process of high temperature oxidation.However,the B-modified MoSi₂ coating prepared by co-deposition method has low density,and the structure gradient is not obvious,so that the bonding between the coatings is not firm.It is found that the functionally gradient coatings is a multi-layer composite structure with obvious structural gradient and high bonding strength.The addition of rare earth or its oxides can fine the grain,make the coating more compact,and also enhance the bonding strength between the coating and the substrate.However,the research on the rare earth oxides modified functionally gradient coatings on molybdenum surface is still insufficient.Therefore,the B modified Mo-Si functionally gradient coatings,CeO₂ modified Mo-Si-B functionally gradient coatings and Y₂O₃ and CeO₂ co-modified functionally gradient coatings were prepared on the surface of molybdenum substrate by two-step pack cementation methods.The effects of single modification and co-modification of B,CeO₂ and Y₂O₃ on the microstructure of composite coatings before and after oxidation were analyzed.The oxidation behavior and action mechanism of Mo-Si-B functionally gradient coatings modified by different elements were studied.The main research results are as follows:(1)B modified Mo-Si functionally gradient coating formed MoSi₂ and Mo B two layer structure,and MoSi₂ layer contained dispersion Mo B phase,and the composite coating formed metallurgical bonding with the Mo substrate.In the oxidation process,B₂O₃ could improve the high temperature fluidity of Si O₂ film.The weight gain(0.85 mg/cm²)and oxidation rate constant(1.76×10^-2 mg²/(cm⁴·h))of the composite coating modified by 0.50 wt.%B after oxidation at 1150?was significantly lower than that of the without B doping.The composite coating was uniform and complete after oxidation at 600?,and the weight loss was 0.008mg/cm².(2)The appropriate amount of CeO₂ doping had permeation effect on Si element in pack cementation process,and refine the grain.The composite coatings with thick,uniform and dense structure were obtained when the content of CeO₂ is 1.00 wt.%.After oxidation at 1150?for 255 h,the weight gain was 0.41 mg/cm²,and the oxidation rate constant was 1.70×10^-3mg²/(cm⁴·h),which were smaller than that of the other modified composite coatings with CeO₂content.In the process of low temperature oxidation,CeO₂ could promote the diffusion of Si and B to form an oxide film,and the weight gain was 0.18 mg/cm² oxidation at 600?for 255h.The modified composite coating has good oxidation resistance.(3)The synergistic effect of Y₂O₃ and CeO₂ had better permeation effect on Si and B element in pack cementation process,and refine the grain.The modified composite coatings with thickest(58.69?m),uniform and dense structure were obtained when 1.50 wt.%Y₂O₃ and1.00 wt.%CeO₂ are co-modified.However,the thickness of modified composite coatings decreased with the further increase of Y₂O₃ and CeO₂ content.In the oxidation process,Y₂O₃and CeO₂ was beneficial to form the complete and compact of Si O₂,and inhibited the further diffusion of Si to the substrate,reduced the formation of Mo₅Si₃ layer.The weight gain of co-modified composite coating by 1.50 wt.%Y₂O₃ and 1.00 wt.%CeO₂ was only 0.29 mg/cm² after oxidation at 1150?for 255 h,the oxidation rate constant was 6.68×10^-4 mg²/(cm⁴·h),and the weight gain was only 0.07 mg/cm² after oxidation at 600?for 255 h.The co-modified composite coating has better oxidation resistance.