High-temperature properties and associated structure evolution of continuous SiNO fiber-reinforced BN composites for wave transparency

Continuous SiNO fiber-reinforced boron nitride (SiNOf/BN) composites for high-temperature wave transparency have been fabricated by a precursor infiltration pyrolysis (PIP) process using borazine as the BN precursor. The evolution of the properties of the composites at elevated temperatures have been measured in terms of strength, dielectric constant, and loss tangent, and the associated structure evolution has been investigated by XRD, SEM, HRTEM, and FTIR. A flexural strength of 138 MPa was maintained up to 1200 °C. Deterioration of the mechanical properties at elevated temperature was due to crystallization of the amorphous SiNO fibers into Si2N2O and internal pore formation, coupled with interface reactions. The composites display a low dielectric constant of 3.38 and a low loss tangent of 0.0017, which increases slightly with temperature due in part to the crystallization into Si2N2O and high-temperature polarization. The high-temperature mechanical properties and good dielectric properties of these composites make them useful for wave transparency.