The Preparation And Characterization Of Anti-oxidation Coating On Pure Mo

Molybdenum is commonly used as standard and supporting materials in preheating impact experiment for its good high-temperature mechanical properties.However,the catastrophic oxidation of molybdenum lead to its embrittlement even to chalking,and its usage in hot air was also restricted.Based on user requirement and research findings of molybdenum oxidation protection technology at home and abroad,MoSi₂ coating was prepared on pure Mo by in-situ siliconizing under the protection of carbon.And the influence factors and mechanism of the coating preparation was discussed.At the same time,the isothermal oxidation test was used to study the oxidation resistance and failure mode of MoSi₂ coating.Under the failure mode,0.1Al₂O₃-ZrO₂ system was used to improve MoSi₂ coating.The composite coating 0.1Al₂O₃-ZrO₂-MoSi₂ was prepared by Non-hydrolytic sol-gel.Furthermore,the strengthening mechanism was also studied with isothermal oxidation tests and modern analytic determination methods.?1?MoSi₂ coating was prepared on Mo successfully under the protection of carbon instead of inert atmosphere or vacuum which is often used in the precious packing method.In this way,the coating process is easier with a lower cost.The phase and structure of the coating were analysised.Results indicate that the coating possess a perfect compact structure and shows good adhesion with the substrate.Further more,the specific looser structure in the coating surface was benificial to the formation of SiO₂ film during the oxidation protection.?2?The effect of annealing temperature,heat treatment time,NaF/Si ratio and different surface treatment of the substrate?roughness?to the coating structure was systematically studied.The result shows:a.The coating thickness increased obviously with the increasing annealing temperature?800¹200^oC?.And a higher temperature is good to the microstructure uniformity and bondability of the coating.However,when the temperature is 1300 ^oC,primary crack was found in the coating,the coating stucture was become unstable and the oxidation resistance was also weakening.b.The coating thickness increase first and after that it has an approximate constant with the increasing heat treatment time.And when the heat treatment time is 2 h,the coating thickness approximate the biggest.After that,coating thickness increase barely.c.NaF/Si ratio variation in a wide range?1/6¹/2?shows a small effect on coating structure.The roughness of the substrate surface has a great influence on growth rate.A rougher surface can provide more channels for silicon to penetrate,which facilitates the growth of the coating.?3?To simulate the working environment of Mo in impact experiment,the isothermal oxidation tests of the coated samples were performed at 1600 ^oC for 1 h in air.It turned out that the MoSi₂ coating prepared by packing method under the protection of carbon shows a good protection for the Mo substrate.In term of anti-oxidation property,it can meet the requirement of short-term oxidation protection in the preheating impact experiment.From the result of SEM,EDS and XRD,SiO₂ film was formed to prevent further oxygen corrosion.?4?The failure modes of MoSi₂ coating was analyzed.Due to the huge gap in thermal expansion coefficient between SiO₂ layer and substrate?over one order of magnitude?,the SiO₂ coating was shed from the substrate after oxidation and the anti-oxidation performance was also degraded.So the 0.1Al₂O₃-ZrO₂ coating was prepared on MoSi₂ to close the gap,and abtain a more stable 0.1Al₂O₃-ZrO₂-MoSi₂ composite coating.The effect of decomposition temperature,sol concentration and the mix of PVA to the 0.1Al₂O₃-ZrO₂coating was carefully studied.The optimal coating process was that the decomposition temperature is 150 ^oC,sol concentration is 0.6 mol/L and the PVA is 0%.?5?The composite coating was doing isothermal oxidation test in 1400 ^oC/5 h,1500 ^oC/5 h,1600 ^oC/1 h,1650 ^oC/1 h.SEM,EDS and XRD were used to analysis the sample after oxidation.Result indicate that the structure stability of 0.1Al₂O₃-ZrO₂-MoSi₂ composite coating is superior to MoSi₂ coating,and the shedding was greatly reduced.In addition,a large amount of ZrSiO₄ was formed during the oxidation,and the improve the high temperature resistance of the coating.Simultaneously,ZrSiO₄ and ZrO₂ particles form"zirconium nail"in SiO₂ glass phase to fix SiO₂ coating on Mo substrate.So that the stability of coating internal structure is strengthened.In this paper,we provided some new ideals for the preparation of Mo-based anti-oxidation coating,at the same time,it also can establish experimental reference and data support for the development of new coating systems.