Design,Preparation And Properties Of High Temperature Resistant Wave Transmitting Coating On Porous Si₃N₄ Ceramics

Porous Si₃N₄ ceramics can be used in radome structure because of their good mechanical,thermophysical and electrical properties.However,when the radome is used as a high Mach missile radome,its porous structure is prone to the problems of moisture absorption and poor rain corrosion resistance,which inevitably leads to the weakening of the radome’s electrical signal transmission,the decrease of high temperature resistance and the decrease of structural strength.In order to solve the above problems,a layer of dense coating with high temperature resistance,rain corrosion resistance and excellent dielectric property should be prepared on the surface of the coating.Therefore,this paper aims to design and prepare a dense glass-ceramics coating to effectively seal the porous Si₃N₄ ceramics.By controlling the type and content of crystalline phase precipitated out of glass-ceramics,the thermophysical parameters and complex dielectric properties of the coating can be adjusted and controlled to ensure that it has good high temperature resistance,thermal shock resistance and wave permeability dielectric properties.According to the ternary phase diagram of MgO-Al₂O₃-SiO₂（MAS）system and Y₂O₃-Al₂O₃-SiO₂（YAS）system,the composition of Masy system glass-ceramics was designed.The organic solvent was mixed with the water-quenched MASY amorphous powder by slurry method.By means of scraping process and sintering system design,a glass-ceramics coating with a thickness of about 75μm was prepared on the surface of porous Si₃N₄ ceramics.The main crystal phases were Y₂Si₂O_7,Mg₂Al₄Si₅O₁₈ and Al₆Si₂O₁₃.It was found that the relative content of Y₂O₃/MgO increased,and the content of crystalline phase Y₂Si₂O_7,precipitated in the coating increased,while the content of Mg₂Al₄Si₅O₁₈ decreased gradually.In the absence of MgO in the system,the main precipitates are Y₂Si₂O_7,and Al₆Si₂O₁₃ phases.When there is no Y₂O₃ in the system,the main crystal phase precipitated from the system is Mg₂Al₄Si₅O₁₈.The results of dielectric properties show that the dielectric constant of Masy/p-Si₃N₄multiphase ceramics is between 2.5 and 4.8,and the tangent of dielectric loss Angle is between 5×10^-4 and 4×10^-3 at 2-18GHz and room temperature.The main crystal phases of Y20 glass-ceramics were Y₂Si₂O₇ and Mg₂Al₄Si₅O₁₈.The surface densification degree was high.The dielectric constant of the multiphase ceramics was between 4.3 and 4.4,and the dielectric loss Angle tangent was 1.3×10-3-3.0×10-3.After 30 minutes of air cold and hot shock at 1000℃,the coating did not crack or bubble,and the microstructure did not change after the thermal shock,and the waterproof rate was still above 99%.The Y25sample was sintered by two-step method（1100℃/30min+1350℃/30min）,and the crystalline phases of Y₂Si₂O_7,and Al₆Si₂O₁₃ were mainly precipitated.The surface coating was compact,the dielectric constant was between 4.7 and 4.8,and the loss Angle tangent was 5×10^-4 and 3.5×10^-3.Between;At the temperature of 1000℃,the surface and the structure of the cross section did not change.At the temperature of 1200℃,a slight bubble occurred on the surface,but only in the range of a few microns,and the type of crystalline phase remained unchanged before and after the thermal shock.The waterproof test results showed that the waterproof effect was more than 99%.The thermal coupling analysis of Masy/p-Si₃N₄ multiphase ceramics shows that there is a large thermal resistance in the core of porous Si₃N₄ ceramics and the bonding area of porous Si₃N₄ ceramics and glass-ceramics,and the cooling rate is slow and the heat flow is large.The thermal stress is mainly concentrated in the thin glass-ceramics layer,especially in the central area of glass-ceramics,and is mainly subjected to tensile stress,while the porous silicon nitride is mainly subjected to compressive stress.The farther away from the joint,the smaller the stress value is.The maximum equivalent stress is 12MPa,which is far lower than the breaking strength of the coating and the base material（130-150MPa）,and is not likely to fracture under the environment of air,cold and hot earthquakes.