| Nitrides ceramics, with excellent mechanical properties and ablation resistance properties, proper dielectric properties and thermal insulation performance, gradually show unique advantage on supersonic and hypersonic missile radome. Half-wavelength wall thickness radome was analyzed including performance evaluation, electricity and porous structure design, materials and manufacturing process. Over a wide frequency range, series of high wave-transmissivity Si-B-N-O ceramics radomes with adjustable mechanical, physical, and dielectric properties were prepared, which could satisfy the operating requirement of high velocity. The preparation of nitride ceramics radomes with thermal insulation, load-bearing and wave-transparent properties realizes structural and functional properties integral requirement for wave-transparent materials.In this paper, the 4-Pin network and pore structure control theory were used to build mathematical physics model for half-wavelength wall-thickness radome. The dielectric properties, radome thickness and coating thickness of wave-transparent materials were optimium designed, conbining materials preparation technique. And then the Si-B-N-O radomes were desiged and prepared with excellent mechanical properties and ablation resistance properties, proper dielectric properties and thermal insulation performance. It provided an important way for radome preparation. The results showed that the wave-transmissivity was more than 80% in the given wave frequency when the dielectric constant was less than 3.5. It was little effect on the wave-transmissivity when the dielectric constant of coating and the ratio of coating thickness to radome wall thichness were less than 3.2 and 1:15 respectively.The pore-formed mechanism and the effect of porosity, pore distribution on the mechanical and dielectric properties of porous Si3N4 were studied systematically. The gradient porous ceramics with different porosity and pore distribution were prepared by adding pore-formed agent. The results showed that the porosity and size ofβ-Si3N4 grains, adding 15wt% PF resin, were larger than those without pore-formed agent. The ratio of length to diameter ofβ-Si3N4 is from 5 to 7. With the content of corn starch increasing from 5wt% to 20wt%, the porosity of pore Si3N4 ceramics changed from 42% to 63%, the flexural strength was from 37.8MPa to 78.2MPa, and the dielectric constant was from 2.3 to 3.3. The Si-N-O ceramics were porosity gradient distribution from inner layer to outer space, using SiO2 hollow balls as pore-formed agent. The Si3N4 composites with low density, high porosity and high strength were prepared by adding rare earth oxides and using suitable sintering process. The columniation grains growth mechanism was studied. The results showed that Y2O3 as the simple sintering addition could promote theβ-Si3N4 columniation grains growth. And when the content of Y2O3 was 9wt%, the porosity, flexural strength, fracture toughness of Si3N4 ceramics was 42.1%, 126MPa and 2.21 MPa·m1/2 respectivly. Both the theoretical value and the measured value decreased firstly and then increased as the content of Y2O3 increasing. Series of porous material and pore gradient materials were prepared for broad band radome with the sintering temperature of 1700℃and holding time of less than 2h.In this paper, the effect of BN particles and nano-SiO2 on the performance of SiO2-BN-Si3N4 composites was studied systematically. With the content of BN particles increasing, the porosity of SiO2-BN-Si3N4 composties also increased and the density and the flexural strength decreased. But adding BN particles was helpful to make lower elastic ratio and improve the thermal shock resistance. When the content of BN particles was 20wt%, the porosity of SiO2-BN-Si3N4 composites was 23%, the flexural strength was 226MPa, and the Young's modulus was 114GPa. Moreover, BN particles could improve the dielectric property of composites. SiO2-BN-Si3N4 composites with controllable dielectric constant from 4.0～7.0 and the dielectric loss less than 10×10-3, could be fabricated by adjusting the content of BN particles. Nano-SiO2 was added to decrease sintering temperature of SiO2-BN-Si3N4 ceramics , and promote the growth ofβ-Si3N4 grains. The results showed that when the nano-SiO2 content was 5wt%, the porosity was the minimum, the flexural strength and elastic ratio was the maximum. With the increasing of nano-SiO2, the content of Si2N2O phase increased, and theβ-Si3N4 grains had abnormal size, the porosity also increased, the flexural strength decreased gradually. Moreover, nano-SiO2 could improve the dielectric properties of SiO2-BN-Si3N4 composites effectively. A series of Si-B-N-O ceramics radome with lower dielectric constant from 4.0 to 6.0 and lower dielectric loss were prepared by adjusting the content of nano-SiO2.The three-dimensional chopped BN fiber reinforced Si3N4 was firstly prepared. Preparation method of BN fiber/Si3N4 and three-dimensional chopped BNf fiber reinforced Si3N4 was systematically studied, and its mechanics and dielectrical properties were analyzed. The results showed that by preceramic polymer pyrolysis, coating formed in the surface of fiber Si3N4, acting as the boundary layer, could make good combination between the fiber and the base. With the increasing of unidirectional distribution BN fiber, the porosity of the composite increased, strength reduced, but the fracture toughness and thermal shock resistance increased obviously. The fracture toughness of BN fiber/Si3N4 composites was 3.7MPa·m1/2. when the content of short BN fiber was up to 10w/%. As to the continuous BN fiber woven, The fracture toughness of BN fiber/Si3N4 composites was 5.1MPa·m1/2 A new kind of high performance wave transparent material BN fiber /Si3N4 with transmissivity above 85% in band 3～6GHz and 16～18GHz was prepared.Moisture-proof coating can effectively decrease the hygroscopic coefficient of radome. SiO2 coating and Si3N4 coating on the porous Si-B-N-O material were prepared through insuccation-coating method. It showed that a uniform, smooth and compact film of SiO2 with thickness of 115μm could be obtained after spray coating-solidification and pyrolysis. Then the flexural strength of porous Si-B-N-O material increases from 79.5MPa to 116.3MPa. The dielectric constant, dielectric dissipation and transparent ratio of specimens with dam-proof coating are unconverted after immersed in water for 24h. Excellent amorphous state Si3N4 film was prepared from SiCl4 and NH3 through intermolecular chemical reaction on porous Si-B-N-O material under H2 atmosphere . Growth mechanics of Si3N4 film was also studied, and growing model was established.Super large dimension ofΦ490×900mm Si-B-N-O radome with 90% transparent ratio in band 4～6GHz and 13～14GHz was prepared by cold isostatic compaction, gas-pressure sintering (GPS) and accurate cold working technology. Ablating resistance property, load-bearing property and wave-transparent property were evaluated through analyzing high temperature oxidation mechanism of Si-B-N-O system. It showed that the samples was not destroyed with smooth surface and ablating rate was only 0.042mm/s when heat flow was 60007000kW/m2. Si-B-N-O radome was destroyed when the loading was up to 220%. The wave-transmissivity was 90～92% when the special wave band was 4～6GHz and 13～14GHz. So it could completely satisfy the reqirement of high match numbers radomes.
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