Microstructure And High Temperature Property Tailoring Of ZrB₂-based Ultra-high Temperature Ceramic Coating

ZrB₂ is one of the most important candidate materials in Ultra-high temperature ceramics?UHTCs?family due to its high melting point,high thermal conductivity and relatively low density.ZrB₂-SiC system and ZrB₂-MoSi₂ system have been extensively studied in the past.However,the serious volatilization of the liquid at high temperature,the active oxidation of SiC at low oxygen pressure,and the loose ZrO₂ structure limited their application in extreme environments.Based on the above background,this work focused on improving the oxidation resistance and ablation resistance of ZrB₂-based coating.The oxidation behavior of ultra-high temperature ceramics and silicon-containing materials at different temperatures and oxygen environment was analyzed by thermodynamic calculation,which provided theoretical guidance for the selection of ultra-high temperature ceramic materials and added phases.Tungsten-containing substances were introduced to ZrB₂-SiC to stabilize the liquid layer and to decrease the active oxidation of SiC.Yb₂O₃ was added to ZrB₂-SiC and ZrB₂-MoSi₂ to make the solid layer denser.In this study,the modified phases were introduced to ZrB₂-based coating which were prepared by vacuum plasma spraying.The aim of this study is to develop new ultra-high temperature ceramic coating with excellent oxidation resistance and ablation resistance.The main results obtained are described as follow:1.The thermodynamic behavior of boride,carbide,nitride and silicon-containing substances were calculated and analyzed.The Gibbs free energy of these substances are negative between 1000 to 3000�C,indicating that oxidation can occur.By calculating the volatilization diagrams of these materials,the required content of oxygen for the oxidation at 1500,2000 and 2500 ? is clarified.The viscosity and volatilization of liquid phases?B₂O₃,SiO₂,borosilicate glass?were calculated.It was found that the viscosity of borosilicate glass decreased rapidly with the increase of B₂O₃.2.The ZrB₂-SiC-WB,ZrB₂-SiC-WSi₂ and ZrB₂-SiC-W coatings were tested by static oxidation and plasma ablation.It was found that tungsten-containing additives improved the oxidation resistance and ablation resistance of ZrB₂-SiC coating by stabilizing the liquid layer after static oxidation.Through plasma ablation,WB could form W at 10^-6-10^-1212 atm to avoid SiC depleted layer and WSi₂ could form W at10^-6-10^-1414 atm.In addition,W could enhance the thermal conductivity of the material to decrease the surface temperature.The structure evolution and failure mechanism of ZrB₂-SiC-WB under different temperature control modes were studied as well.3.The ZrB₂-SiC-Yb₂O₃ coatings were tested by static oxidation and plasma ablation.It was found that Yb₂O₃ led to the formation of solid phase on the surface to destroy the integrity of the liquid phase.Yb₂O₃ did not significantly reduce the surface temperature and mass ablation rate.Many pits were observed on the surface after ablation and no dense solid layer was formed.The thickness of the oxide layer increased with the addition of Yb₂O₃,indicating that Yb₂O₃ accelerated the penetration of oxygen in ZrB₂-SiC based coatings.4.The ZrB₂-MoSi₂-Yb₂O₃ coatings were tested by static oxidation,plasma ablation and laser ablation.It was found that Yb₂O₃ stabilized the liquid layer at high temperature and it would react with ZrO₂ to form Yb₂Zr₈O₁₉9 to densify the solid layer.However,the excess of Yb₂O₃ would react with SiO₂ to form Yb₂Si₂O₇,which decreased the oxidation resistance.