Preparation And Properties Of Porous Si₃N₄-based Ceramics With High Porosity And Their Dense Coatings

With the rapid development of aerospace and aviation technology,the aircraft’s speed,firing range and strike accuracy have increased rapidly,and the requirements for radome materials have also increased.Si₃N₄ ceramics meet the requirements of missile radome materials due to its high temperature resistance,thermal properties,mechanical properties and processing properties.Porous Si₃N₄ ceramics combine the advantages of both porous materials and Si₃N₄ ceramics.The porous structure effectively solves the problem of high dielectric constant of Si₃N₄ ceramics,and has been widely used in the aerospace industry.However,porous Si₃N₄ ceramics tend to absorb moisture,which affects the stability of dielectric properties.It is expected that the preparation of dense coating on the surface of porous Si₃N₄ ceramics can effectively solve this problem.Porous Si₃N₄ ceramics were prepared by gel-casting combined with gas pressure sintering method.By introducing h-BN as the second phase,the sintering shrinkage ratio is lowered.When 5 vol%h-BN is added,the sintering shrinkage rate of the porous Si₃N₄ ceramic is remarkably lowered,and the porosity is improved.With the increase of the content of sintering aids Y₂O₃ and Al₂O₃,the sintering shrinkage rate is further increased,and the density is continuously increased.The solid content is in the range of15-25 vol%,and a porous Si₃N₄/h-BN composite having a porosity of 65%-85%,good wave transmission performance,and excellent mechanical properties can be obtained.A mixed dense coating of SiO₂/CAS,SiO₂/CBS and Y₂Si₂O₇/BAS systems was prepared on the surface of the Si₃N₄/h-BN composite ceramics by slurry spraying method.The high temperature wetting behavior shows that the composite coating material has good wettability and the wetting angle is between 5°and 20°.For the SiO₂/CAS and SiO₂/CBS systems,the stability of the SiO₂ transition layer slurry was studied.By adjusting the coating process,solid content,sintering aids and sintering temperature,a controlled thickness and low porosity transition was prepared.The surface of the SiO₂ layer is coated with a sealing ceramic coating.The effect of sintering temperature on the compactness and penetration thickness of the coating is explored.It is concluded that the optimum sintering temperature of CAS and CBS glass is 900°C and 1000°C,respectively.The mixed slurry of Y₂O₃ and SiO₂ is used as the transition layer,and the BAS glass is the sealing layer.At 1350°C,BAS promotes the reaction of Y₂O₃ and SiO₂ to form Y₂Si₂O₇,forming a composite coating structure of Y₂Si₂O₇/BAS.After the optimal temperature treatment,the three composite coatings are smooth and dense,the coating thickness is uniform,and the coating is tightly combined with the matrix to form a strong bonding structure.By comparing the water absorption changes of Si₃N₄/h-BN composite ceramic substrates before and after sealing,the water absorption rate of the matrix is nearly 70%due to the high porosity,but the water absorption rate of the porous Si₃N₄/h-BN composite ceramic with coating is greatly reduced,the water absorption of the SiO₂/CAS,SiO₂/CBS and Y₂Si₂O₇/BAS coating is 0.25%,0.21%and 0.19%,respectively.Dielectric properties change little,the dielectric constant of the substrate before and after coating is about 2.15,and the dielectric loss is less than 5×10^-3.The thermal shock resistance of the materials coated with different systems is quite different.The material coated with SiO₂/CAS undergoes a thermal shock temperature difference of 775°C,which causes obvious cracking.The Si₃N₄/h-BN multiphase ceramic substrate coated with SiO₂/CBS and Y₂Si₂O₇/BAS coatings only showed microcracks on the surface when the temperature difference was 875°C.The number of cracks increased and the size increased when the temperature difference was 975°C.Due to the presence of Y₂Si₂O₇ rod-like grains,the crack in the coating can be obviously deflected,and it is obviously cracked after 20 cycles of 1000°C thermal shock temperature difference.The thickness of the coating strongly affects the thermal shock resistance of the material.The results show that for the Y₂Si₂O₇/BAS system,the coating thickness of the material in the range of 30-80μm has good thermal shock resistance,and the thermal shock temperature difference of 975°C cyclic thermal shock5 times,no obvious thermal shock cracks.