Preparation Process And Mechanical Properties Of C_f/Si₃N₄Used For Absorbing Material

Abstract:Silicon nitride ceramics have excellent performance such as good mechanical properties in high and room temperature, good thermal stability, low dielectric loss (the dielectric constant is8-10) and high erosion resistance. As a kind of excellent microwave transmission materials, they are widely used in the rocket, spacecraft, missiles, recoverable satellites and other fields. In this paper, silicon nitride was prepared by gel-casting as wave-transmitting material and short carbon fibers were added to prepare Cf/Si3N4composites as absorbing agent and toughening phase. The effects of addition short carbon fibers on mechanical properties of carbon fiber/silicon nitride composites were studied. The bending strength and fracture toughness of materials were measured by three point bending beam and SENB (single edge notched beam). XRD, SEM and EDS were used to study the composition, microstructure and material toughening mechanism of material. The main research contents and conclusions were as follows:(1) The effects of the pretreatment of the raw powder, the content of dispersant (10wt%tetramethylammonium hydroxide solution) and AM (acrylamide), milling time and solid loading (solid volume ratio of raw material powder) on the viscosity of suspension were studied. A silicon nitride suspension with solid loading (55vol%) and low viscosity was successfully prepared. The research result shows that the key factors influence the properties of sintered bodies mainly include solid loading, the content of AM and sintering additives, sintering temperature and sintering methods. The bending strength of sintered bodies are as high as635MPa when the solid loading is45%and the AM and sintering additives (Al2O3-Y2O3) content is15%, the green bodies are sintered at1800℃using buried powder sintering. Silicon nitride mainly exists in the form of long cylindrical β-Si3N4 (2) Cf/Si3N4composites were prepared by gel-casting with Si3N4as matrix and short carbon fibers as absorbing agent and toughening phase. The effect of amount of carbon fibers on properties of composites was discussed. The results show that, with the increase of carbon fiber content, the bending strength of the composite decrease but fracture toughness increase. Bending strength of the composite is325MPa and fracture toughness is8.91MPa·m1/2with4vol%carbon fibers content, which is nearly2times more than Si3N4matrix. The composite can be used as functional materials under certain conditions. The main toughening mechanisms are fiber pull out and interface debonding.(3) The basic research on densification of non-woven web of carbon fibers is studied. The interface between carbon fiber and Si3N4matrix is improved by fiber surface C/SiC coatings and modification of Si3N4matrix by ZrO2. Low pressure of dipping is the main reason to lead low strength of sintered body.