With the development of hypersonic aircraft, radomes must be used in harsher environments and have stronger electromagnetic wave transmission characteristics. As a strong covalent compound, silicon nitride ceramics have the advantages of high specific strength, high thermal shock resistance, low dielectric constant, etc., making them promising candidates as missile radome materials. Dense silicon nitride ceramics have extremely high hardness and bending strength. The bending strength could be as high as 1090 MPa, but it decreases sharply with increasing porosity. With the development of light weight and miniaturization of radomes, the performance of the base materials (silicon nitride ceramics) has higher requirements, so the preparation of low-density and high-strength porous silicon nitride ceramics is a challenge.

In this work, fiber α-Si₃N₄ was used as the main raw material and 5 wt.% nano-sized Y₂O₃ was used as the sintering aid. The Si₃N₄ powder was mixed with Y₂O₃ of different particle sizes through ball milling for 12 h. The mixtures were finally magnetically stirred for 2 h. The resulting slurries were evaporated in a rotary evaporator at 70–80 ℃ for 2 h and then dried in an oven at 80 ℃ for 12 h. The dried powder was sieved through a 150-mesh sieve (aperture 100 μm) and die pressed into samples with dimension of 5 mm×5 mm×50 mm. The green bodies were sintered in a furnace (Highmulti-5000, Fujidempa Co., Ltd., Osaka, Japan) at 1700 ℃ for 2 h, at N₂ pressure of 0.5 MPa, at heating rate of 5 ℃·min^-1. X-ray powder diffractometer (XRD) was used to determine phase composition of the samples. Microstructure was characterized by using scanning electron microscopy (SEM, Phenom pro X, accelerated voltage 10 k V, backscattered electron imaging mode). The samples were machined to bending test bars with a size of 3 mm×4 mm×50 mm. The tensile surface was ground by using an 800-grit diamond wheel and the edges were beveled to reduce the effect of edge cracks. Three-point bending testing was conducted to determine flexural strength on a testing machine (Instron1195, Instron Co., London, UK) with a span of 16 mm at a crosshead speed of 0.5 mm·min⁻¹. Image J software was used to randomly select at least 500 grains for particle size distribution and at least 100 pores for pore size distribution statistics in SEM images of sample fractures.

The addition of Y₂O₃ with different particle sizes as sintering additives had weak effect on the green density, linear shrinkage and bulk density of porous silicon nitride, with porosity of 54%–56%. The silicon nitride had completed the phase transformation from α to β after sintering at 1700 ℃ for 2 h. The average grain size decreased and the glass phase distribution became more uniform as the particle size of the sintering aid decreased. The decrease in pore size of the porous silicon nitride ceramics was observed with increasing grain size, which was attributed to the effective improvement in the bending strength from 82.3 MPa to 135.4 MPa. Dielectric constant of the porous silicon nitride ceramics changed slightly, ranging from 2.85 to 3.08, while the dielectric loss was less than 2×10^-3. The dielectric properties of the materials met the needs of low dielectric wave-transmitting materials.

Porous silicon nitride ceramics with high strength and porosity were successfully prepared by using pressureless sintering. The Y₂O₃ with smaller particle size was more uniformly distributed in the matrix, resulted in uniform distribution of the liquid phase after sintering. With the decrease of Y₂O₃ particle size, the grain size decreased significantly and grain distribution was more uniform, which was attributed to the high nucleation rate of β-Si₃N₄ in the phase transformation. The pore size decreased with the decrease of grain size, while grain refinement and pore size reduction were the main factors for improving mechanical properties. As the particle size of Y₂O₃ decreased, the bending strength increased from 82.3 MPa to 135.4 MPa. Dielectric properties depend mainly on the porosity of the materials.