Of Si <sub> 3 </ Sub> N <sub> 4 </ Sub>-based Ceramic Missile Radome Materials

The fundamental goal of the present study is to develop a process of silicon nitride based ceramics for radome application. The composite materials for a broad brand high-speed missile radome have been studied based on the superior dielectric properties and mechanical performance of Si3N4 BN, A1N materials.The gas-pressure-sintering(GPS) technology of silicon nitride ceramics has been studied in this paper. The properties of GPS-Si3N4 ceramics with the temperature , atmosphere , etc. have been discussed.The system of Si-B-N-O compound and Si-B-N-O-Al compound have been designed in this paper. The properties of the porous content, the mechanical performance and the dielectric properties with any component varieties have been discussed.The experimental results show that: the addition of Y2O3 can promote the sintering of the matetial effectively, and its best addition amount is 5wt%; the addition of the power material of BN can reduce the dielectric constant, and improve the thermal shock resistance of the materials effectively, but it will also block the GPS process, so its best addition amount should be 25~35wt%; the addition of nanometer SiO2 power can improve the densification and the flexural strength of the materials obviously, and the best addition amount is 5wt%; the addition of A1N will improve the flexural strength and fracture toughness of the materials greatly for the formation of Sialon ceramics. But if too much A1N is added, the mechanical performance of the materials will be reduced for the formation of big pores in the course of A1N decomposition; the dielectric constant and the dielectric loss deduce with the increasing of the porous content, and the mechanical performance deduces too.SEM, XRD, etc. has been applied to analyze the microstructure andcrystalline phase composition of two systems.By XRD, it shows that the main crystalline phases of the Si-B-N-0 system are 3 -Si3N4> BN andSi2ON2, and the main crystalline phases of the Si-B-N-O-Al are Si5A10N6, BN and a few quantity of SiO2 and A12O3.By SEM, it shows that the Si-B-N-0 system has a cellular structure, the pores are distributed evenly, and the post P -Si3N4 and the samdwich BN distribute crossly. There are a lot of pores in the body of Si-B-N-O-Al system, which distribute unevenly, and thick crystalline grains exist.