Effect of particle gradation on flexural strength and dielectric property of Si3N4w/Si3N4 composites

In order to obtain a nitride ceramic material with good flexural and dielectric properties, using silicon nitride whiskers (Si3N4w) as raw material, three kinds of Si3N4w spherical particle powders with different particle size distributions were prepared by spray drying process at first in this work, and the effect of the rotating speed of the atomizing disc on the particle size distribution of Si3N4w spherical particles was studied. Then, using the Si3N4w spherical particles obtained by spray drying as raw materials, three kinds of Si3N4w preforms with different particle gradations were prepared by dry pressing method, and the pore size distribution of the particle gradation Si3N4w preforms was studied. Si3N4 matrices were further prepared in three kinds of particle gradation Si3N4w preforms by chemical vapor infiltration (CVI) and precursor impregnation and pyrolysis (PIP) methods, and the phase and microstructure evolution of Si3N4w/Si3N4 composites during the preparation process were investigated. Finally, the effect of particle gradation on the microstructure, density, flexural and dielectric property of Si3N4w/Si3N4 composites was studied. The results show that the three kinds of particle gradation Si3N4w preforms all have bimodal pore structure characteristics, in which the pore sizes of small pores are all about 0.7 mu m, and the pore sizes of large pores are 45.2, 30.1 mu m and 21.3 mu m, respectively. Among the as-prepared three types of Si3N4w/Si3N4 composites with particle gradation, the flexural strength of the S13 composites reaches 81.59 MPa since it has the best particle gradation effect. In addition, the dielectric constant and dielectric loss of this sample are 5.08 and 0.018, respectively. Good flexural strength and dielectric properties indicate that the as-prepared Si3N4w/Si3N4 composites are expected to be used in the field of missile radomes.