Study On Fabrication And Properties Of Porous Silicon Nitride-based Ceramics With Gradient Porosity And Radome

With the development of hypersonic vehicles,radome must bear more load and thermal shock in higher working temperature and worse environment,and have better transmission characteristics and lower aiming error.It has all-weather working ability,which puts forward more stringent requirements for the selection of radome materials.In order to meet the urgent need of high temperature and high performance transmissive radome materials,the preparation process of composite materials was optimized,and the preparation and surface densification of pore gradient structure Si₃N₄-based ceramics were studied.Thermal,mechanical and electrical properties of pore gradient structure Si₃N₄-based ceramics were obtained.Pore-controllable Si₃N₄-based ceramics were prepared by adjusting the proportion of each component and sintering process.Porous graded Si₃N₄-based ceramics were designed and fabricated,and their surface densification was carried out.The mechanical,thermal and electrical properties of the composites were studied.Finally,porous graded Si₃N₄-based radome was fabricated.The temperature resistance,structural stability and wave transmission performance of radome materials are evaluated by loss detection,load-bearing assessment,heat-resistant assessment and dielectric performance assessment.The sample prepared by liquid-solid mixing method has little effect on the porosity and dielectric properties of the material,and can effectively improve the mechanical properties of porous silicon nitride,which can be increased by 21.7%.The sintering process of silicon nitride green body undergoes the formation of sintering additive liquid phase,the transformation fromα-Si₃N₄ toβ-Si₃N₄ and the sintering densification process.It was found that the crystallization of Yb ₂Si₂O₇could be effectively achieved by holding at 1650℃,and the high temperature strength of the sample after crystallization could be increased to 68.9%.The results show that the mechanical properties of materials deteriorate with the increase of porosity,and the increase of porosity is helpful to improve the electrical properties of materials.By changing the process parameters,the controllable adjustment of porosity between 30.1%and 80.3%can be achieved.The mathematical model of gradient Si₃N₄-based ceramics is established.The gradient structure material is selected as 5 layers and 5.8 mm thickness.The Si₃N₄-based ceramics with 30.1%porosity and 4.8 dielectric constant are selected as the high co MPact layer,and the Si₃N₄-based ceramics with 80.3%porosity and 1.3dielectric constant are selected as the inner layer.The gradient Si₃N₄-based ceramic plate specimens were prepared at-196℃.Further study on the transmittance of the material shows that the transmittance of the material is more than 70%in the test frequency range of 8-40 GHz.The dense layer on porous silicon nitride surface was prepared by plasma magnetron reactive sputtering,and the homogeneity of the deposited layer on the surface of the sample was favorable at 500℃.The mechanical and dielectric properties of the coated materials were studied.The Flexural strength and dielectric constant of the coated materials were 287.7MPa and4.97 respectively.Based on the gradient Si₃N₄-based ceramic plate process,the freezing reaction time was determined to be 15 minutes.By removing the positive die first and then removing the negative die demoulding control,the deformation and surface cracking phenomena caused by uneven shrinkage and uneven stress of the green body were effectively improved,and a larger size gradient Si₃N₄-based ceramic radome sample was prepared.Non-destructive testing technology was used to verify that the radome had complete shape,uniform wall thickness and no dangerous cracks or defects.The static load-bearing process of radome was studied.It was found that when loaded to 150%load,the deformation of standing point was3.155mm and that of root was 0.275mm,and no damage was found after unloading.The static and thermal process of the radome was studied.The maximum temperature of 0.15 section can reach 780℃,and the radome has not been damaged after the test.The dielectric property of the radome was studied.The lowest transmittance value was 71.5%.