Microstructure Control And Oxidation Resistance Of Si-Cr-Ti Silicide Coatings On Refractory High Entropy Alloy

Refractory high entropy alloys have the characteristics of high melting point,high thermal stability and excellent mechanical properties at high temperature,so it shows great application potential in hot end parts.However,the poor oxidation resistance of refractory high entropy alloy is the main obstacle to its application in high temperature aerobic environment,so the surface thermal protective coating technology becomes a necessary means to ensure its service.In this thesis,Si-Cr-Ti silicified material coating was prepared on the surface of Nb Mo Ta WTi refractory high entropy alloy by slurry melting method.The relationship between secondary melting process parameters and microstructure of coating was studied,and the influence of adding yttrium stabilized zirconia（YSZ）ceramic powder on the sintering process of coating was analyzed.The oxidation resistance of the coating at 1300-1500℃was investigated,and the oxidation resistance mechanism of the coating was explored.The main research results of this thesis are as follows:The Si-Cr-Ti silicide coating on the surface of Nb Mo Ta WTi high entropy alloy presents a multi-layer structure:The outer layer of low silicide（Si content 37-42 at.%）,the intermediate layer of high silicide（Si content 60-67 at.%）and the transition layer of low silicide（Si content 32-36 at.%）are all multicomponent silicide mixtures.The secondary melting process can effectively increase the coating thickness and improve the coating compactness.Too high sintering temperature and too long holding time will lead to the increase of the thickness of the outer layer and diffusion layer of the low silicide,and the decrease of the thickness of the middle layer of the high silicide.In this experiment,the optimized secondary melting process is as follows:sintering temperature 1450℃,holding time 30 min,dense coating microstructure,the least defects.The oxidation resistance of Si-Cr-Ti silicide coating at 1300℃was significantly improved by secondary melting process.The oxide layer is composed of Si O₂ and complex metal oxides.The oxidation kinetics of Si-Cr-Ti silicide coating followed parabolic law.However,the primary melting coating showed obvious failure after oxidation for 36 h,while the secondary melting coating remained compact and intact after oxidation for 50 h,and the mass gain per unit area was 8.12 mg/cm².The secondary melting coating began to fail after oxidation at 1500℃for 2 h.The defects of the coating increased at high temperature,and the oxygen reacted with the substrate along the penetrating holes and cracks.In the secondary melting process,different contents（10,15 and 20%）of YSZ ceramic powder were added to the coating to modify the silicide coating.YSZ powder reacts with Si to form Zr Si₂,which slows the diffusion of Si in the outer layer to the inner layer,and the outer layer of the coating is transformed into a composite layer composed of high silicide and low silicide.The addition of 20%YSZ will hinder the sintering process and lead to more defects such as holes in the outer layer of the coating.The 15%YSZ modified Si-Cr-Ti silicide coating shows good oxidation resistance at1500℃,and the unit area gain after 45 h oxidation is 7.83mg/cm².The surface oxide film is continuous and dense,with good adhesion.It is mainly composed of Zr O₂,Ti O₂and other metal oxide"skeleton"and glassy Si O₂.A small amount of Zr Si O₄ was nailed in Si O₂.At the later stage of oxidation,complex solid phase reaction occurs among the metal oxides,the volume increases,the oxygen resistance of the oxide film decreases,the internal oxidation phenomenon is serious,and the coating fails.