Research Of Particles Reinforce Silicon Nitride Composite By CVI Method

Silicon nitride has many advantages, such as low density, high strength, high hardness, resistance to erosion, oxidation, high temperature and thermal shock, low dielectric constant, and is expected to be the most promising candidate of radome material for next generation, using as radome for hypersonic aerocraft.Preparation of a kind of particle reinforce silicon nitride composite (Si₃N_<sub>4p/Si₃N₄ by chemical vapor infiltration (CVI) method was researched in this paper. Silicon nitride composite was characterized and its' properties were measured. The influence of preporm preparation technics and CVI technics on composite's microstructure and properties has been investigated. The main contents and results are as follows:1. The impact of preparation technics on preporm properties was studied. Granulating binders decide aperture distributing inside granulas, while binders and granula size decide aperture distributing among granulas. Preporms' density increases sharply while moulding pressure increases. Granula size distribution has little contribution on the density of preporms.2. The impact of CVI technics on infiltration effect was investigated. Weight gain speed decreases while infiltration time increases. Infiltration temperature decides reacting speed and deposit type. The flux ratio of reactant gases and aperture distributing of perform decide transport mode and infiltration effect. Preforms can be infiltrated well and waste of reactant gases can be reduced when CVI parameters are as follows: 900℃, H₂ flux of100ml/min, NH₃ flux of 40ml/min, Ar flux of 200ml/min.3. The contents and phases of the composite were measured. The samples mainly have Si, N, O three kind of elements, and consists of α-Si₃N₄, amorphous contents, small amount of β-Si₃N₄ and Si. Crystal contents derive from raw material Si₃N₄ powder. Noncrystalline contents are deposit and amorphous SiO₂. Amorphous deposit can be converted into α-Si₃N₄ and β-Si₃N₄ by high temperature heat treatment.4. The realationship between mechanical properties and microstructures of the composite was analyzed. For composite prepared with phenolic resin binder, weak combination and fine pores exist between particles and matrix, and residual coarse pores exist among granulas, so maximum flexural strength of samples is about 71MPa.For composite prepared with silica sol binder, strong binding exists between particles and matrix, and residual pores are very small, which leading to higher flexural stress, at best 94Mpa. High temperature heat treatment will decrease the composite's stress.5. Dielectric constants of silicon nitride composite decrease while testing frequency increases, finally tend to be stable. Heat treatment can decrease the values of permittivity. Dielectric constants of the composite are decided by permittivity and volume fraction of each phases in the samples. While replacing SiaN4 powder by SiC>2 powder in granulation process, silicon nitride volume fraction could be decreased, so the composite's permittivity could be decreased.Silicon nitride composite prepared by CVI method has good performances near to the requirements of radome materials, and is expected to be next generation radome material.