Effect of the BN content on the thermal shock resistance and properties of BN/SiO2 composites fabricated from mechanically alloyed SiBON powders

Abstract

Wave-transparent composites of BN/SiO₂ were prepared via hot pressure sintering at 1650 °C of mechanically alloyed amorphous SiBON powders. The mechanical, dielectric and thermal properties and thermal shock resistance of the composites were carefully investigated with different BN contents. With increasing BN content, the flexural strength and fracture toughness of the composites increased first and then decreased. The sample with the composition of SiO₂–5BN exhibited the highest flexural strength of 256.3 MPa and a fracture toughness of 3.15 MPa m^1/2. The relative density decreased with the increase of BN content, which would influence the thermal properties and the thermal shock resistance of the composites. The formation of borosilicate glass and crystallization of fused silica in the surface at the high temperature greatly improved thermal shock resistance of the samples. The SiO₂–6BN sample, possessing low relative density, exhibited the best thermal shock resistance, and the retained strength after a thermal shock of 1100 °C was 98.6% of the original strength. Besides excellent mechanical and thermal properties and thermal shock resistance, the as-sintered composites exhibited low dielectric constant (ε < 4.62) and loss tangent (tan δ < 0.002), meeting the required values for high temperature wave-transparent materials.