Preparation And High Temperature Oxidation Resistance Mechanism Of Stabilized Coating On Low Density Molybdenum Alloy

The development of aerospace vehicles is of great significance to the safety of the country and the nation.With the increase of flight Mach number and time,the key components of ultra-high speed aircraft are subjected to higher and higher temperatures.It is urgent to develop metal materials with excellent high temperature strength,oxidation resistance and low relative density to meet the needs of application in oxidation atmosphere at 1300°C and above.There are two problems in the development of this material:1)there is a contradiction between the low density component and the excellent high temperature strength;2)The high temperature strength and oxidation resistance of the material are difficult to guarantee at the same time.It is urgent to carry out basic research on material design,strengthening mechanism and the formation of dense oxide film tightly bonded to the substrate.The paper aims at the high temperature metal materials used in the high speed airflow above 1300°C,and selects the high melting point molybdenum with moderate density as the main alloy element to ensure the high temperature strength of the material.Low density elements or ceramic phases are reasonably added to meet both low density and high temperature strength requirements.The matrix alloy was prepared by powder metallurgy method,and then the silicide stable coating was prepared on the surface of the matrix alloy by embedding infiltration method,and it was evaluated at 1300°C oxidation atmosphere.The evolution of phase composition,microstructure and element distribution during the preparation of different molybdenum-based alloy stable coatings were revealed by XRD,SEM and EDS.The main conclusions are as follows:（1）The mechanism of high content alloying elements Cr and Al to improve the high temperature oxidation resistance of Mo matrix was clarified,and the influence model of Cr and Al content on oxidation rate was obtained.With the increase of Cr content（0～30 at.%）,the microhardness of Mo-Cr alloy increases,the solid solubility of Cr in Mo matrix increases,the defect concentration in the system increases,the diffusion rate of Si atoms accelerates,and the coating thickness increases from 75μmto 262μm.At the test temperature of 1300°C,the Mo-20Cr material still maintains a low oxidation weight gain rate after high temperature cyclic oxidation for 150 h,and the oxidation rate K_p in the later stage was only 0.003 mg²cm^-4h^-1.The K_p of Mo-30Cr was 966.628 mg²cm^-4h^-1 after cyclic oxidation at high temperature for 70 h,and it was completely invalid after 120 h.The Mo-30Cr-10Al material with a density of only 8.26 g/cm³ was prepared by adding high content of alloying elements Cr and Al.Compared with pure Mo material,the density decreased by 12.9%,the microhardness of the substrate at room temperature increased by 146.8%,and the bonding force between the coating and the substrate increased by 32.8%.The highest K_p value was only 0.756 mg²cm^-4h^-1 after 150 h cyclic oxidation at 1300°C.A dense Si O₂-Al₂O₃ and Cr₂O₃ composite oxide film was formed on the surface after high temperature oxidation,which reduced the fluidity of the single Si O₂ oxide film at high temperature and improved the high temperature oxidation resistance of the coating.（2）The effect of W content on the mechanical properties and high temperature oxidation resistance of Mo-based materials was studied.The results show that with the increase of W content from 0 at.%to 30 at.%,the solid solubility of W in the substrate increases,the microhardness of the substrate at room temperature increases by 46.9%,and the bonding force between the coating and the substrate increases by21.9%.The coating thickness increased from 89μm to 144μm.Under the test temperature of 1300°C,the Mo-2.5Zr-2.5Zr O₂ material without W failed completely after high temperature cyclic oxidation for 150 h,and the oxidation weight gain per unit area reached-937.19 mg/cm².The oxidation rate of Mo-2.5Zr-2.5Zr O₂-30W material is low,and the oxidation weight gain per unit area is only 2.86 mg/cm² after high temperature cyclic oxidation for 210 h.The reason is that the main phase（Mo,W）Si₂ of the coating degrades to low Si content（Mo,W）₅Si₃ at a lower rate than single-phase Mo Si₂ during oxidation,thus improving the oxidation resistance of the coating in high temperature oxidation atmosphere.The method of adding alloying element W to synergistically improve the mechanical properties and high temperature oxidation resistance of Mo-based materials was found.（3）The effects of Cr and Al on reducing the density of the system,improving the oxidation resistance and W on the synergistic improvement of mechanical properties and oxidation resistance were summarized.In order to coordinate the density of the system,Mo-30Cr-10Al-20W-10Ti alloy with density of 9.43 g/cm³ was prepared by adding W and Ti into Mo-Cr-Al.However,at the assessment temperature of 1300°C,its antioxidant stability is general.In order to improve the high temperature strength of Mo-Cr-Al and ensure the low density of the system,a small amount of W was added to Mo-Cr-Al,and a Mo-30Cr-10Al-5W alloy with a density of 8.94 g/cm³ was designed.The microhardness at room temperature was 116.2%higher than that of pure Mo,and the bonding force between the coating and the substrate was increased by 59.5%.By introducing rare earth Y-stabilized coating,Mo-30Cr-10Al-5W material containing Y-stabilized coating can be stably used for 280 h in oxidation atmosphere at 1300℃,and the oxidation weight gain per unit area is only 3.72mg/cm².At this temperature,the strength of Mo-30Cr-10Al-5W matrix isσ_b=156MPa,σ_0.2=144 MPa.Mo-30Cr-10Al-5W alloy coordinates the contradiction between low density composition and high temperature strength,so that the strength and oxidation resistance of the material can be guaranteed at the same time under high temperature conditions.It can be used for a long time in the oxidation atmosphere of1300°C,which provides theoretical and practical basis for the industrial application of the material under high temperature and high speed oxidation airflow.