Preparation And Anti-oxidation And Ablation Performance Of MoSi₂ Composite Coatin

Molybdenum and its alloys are a very important strategic resource and are widely used in glass smelting,nuclear,aerospace and military industries for their excellent high-temperature mechanical properties and corrosion resistance.However,molybdenum reacts violently with oxygen at high temperature and loses a lot of mass,which becomes a fatal defect in its application in complex environment.At present,the effective solution is to add high temperature protective coating.In this paper,in response to the problem of rapid oxidation failure of molybdenum and its alloys in high-temperature oxygen-containing environments,we determined that the MoSi₂composite coating was prepared using the powder-embedded infiltration method and atmospheric plasma spraying technology as the starting point for structural design and composition optimization,and designed a composite coating structure with Mo₂C or MoSi₂ transition layer,using Si C,Ti B₂,Zr B₂,Hf B₂ for MoSi₂ composition modification.The hardness,modulus,micromorphology,composition and element distribution of the coating were characterized and analyzed by using nano-indentation instrument,tungsten filament-scanning electron microscope（SEM）,X-ray diffractometer and electron probe microscope.The static oxidation resistance and dynamic ablation resistance of the coating at 1300℃and 1500℃were investigated,and the phase and structure transformation during oxidation were studied.The anti-oxidation protection mechanism and ablation resistance mechanism of the coating are discussed.The main research contents and conclusions are as follows:MoSi₂/Si C-Mo₂C composite coatings were successfully prepared on the pure molybdenum surface using a two-step embedding method.The MoSi₂/Si C-Mo₂C composite coatings increased in mass by 0.61 mg/cm² after oxidation for 30 h in an air environment at 1300℃,while the increment of the single MoSi₂ coating was only 0.38mg/cm².the average oxidation life of the composite coating was 56.67 h in multiple tests,while the average life of the single coating was only 46.67 h.Therefore,the MoSi₂/Si C-Mo₂C composite coatings has better oxidation resistance.MoSi₂/Si C-Mo₂C composite coating has better oxidation resistance.The formation of porous areas in the composite coating reduced the size and number of cracks in the coating,which improved the reliability of the coating.Meanwhile,the Mo₂C transition layer hinders the diffusion of Si into the substrate,which improves the effective service life of the coating.MoSi₂/40 wt.%boride fully encapsulated composite coatings were prepared using atmospheric plasma spraying technique.After oxidation at 1500℃for 20h,the quality of the single MoSi₂ coating decreased by 1.22%;the MoSi₂/Ti B₂ composite coatings had an effective lifetime of less than 20h due to its thin thickness and the decrease of Ti O₂ to Si O₂ use temperature;the MoSi₂/Zr B₂ composite coatings and MoSi₂/Hf B₂composite coatings had a quality change rate of+0.06%and+0.10%after oxidation at1500℃for 20h,respectively.The MoSi₂/Zr B₂ composite coatings and MoSi₂/Hf B₂composite coatings have better oxidation resistance performance.The addition of boride in the oxidation process can prevent the precipitation of quartz glass and keep the oxide layer dense,while Zr Si O₄（Hf Si O₄）plays a"riveting"role relative to the Si O₂ glass film,which improves the denseness of the coating and inhibits the crack expansion.MoSi₂-MoSi₂/40 wt.%boride composite coatings with MoSi₂ linkage layer were prepared.The single MoSi₂ coating,after 30 min ablation at 1800℃,showed a mass loss of 0.73%and the number of cracks per unit length in the coating was 1.08cracks/mm.The MoSi₂/Ti B₂ sample had the thinnest coating,resulting in the worst performance.the mass loss of the MoSi₂/Zr B₂ and MoSi₂/Hf B₂ composite coatings after 30 min ablation were 0.05%,0.03%,and 0.93 cracks per unit length/mm,respectively,0.03%and the number of cracks per unit is 0.93 cracks/mm and 0 cracks/mm,respectively.The MoSi₂/Zr B₂（Hf B₂）composite coatings has better reliability and oxidation resistance.The reason for this is the volatilization of gaseous products and the formation of oxide skeleton.Among them,Hf Si O₄ has better stability enhancement effect on Si O₂ than Zr Si O₄,so MoSi₂/Hf B₂ composite coatings has better oxidation resistance performance in short time.And MoSi₂/Zr B₂ composite coatings has better stability and is more suitable for long time ablation.