Microstructure, high-temperature mechanical and dielectric properties of novel Si3N4 f/SiNO wave-transparent composites

Continuous Si3N4 fiber reinforced SiNO matrix composites (Si3N4 (f)/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si3N4 (f)/SiNO composites were investigated in detail. The as-fabricated Si3N4 (f)/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si3N4 (f)/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 degrees C for the surface reactions between the SiNO matrix and Si3N4 fibers. After heat-treated at 1550 degrees C, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si3N4 fibers at this temperature. Si3N4 (f)/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7-18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10(-3). The dielectric constants and dielectric loss of Si3N4 (f)/SiNO composites are relatively stable from RT to 800 degrees C. The as-fabricated Si3N4 (f)/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.