Effect of Preforming Process and Starting Fused SiO2 Particle Size on Microstructure and Mechanical Properties of Pressurelessly Sintered BNp/SiO2 Ceramic Composites.

Hexagonal BN particles (BN_p) reinforced fused SiO₂ matrix composite (BN_p/SiO₂) was regarded as a very important candidate material for potential applications on wave transparent thermal protection parts. In this study, pressurelessly sintered 15 vol% BN_p/SiO₂ composites were prepared by cold isostatic pressing (CIP) and gel-casting routes, respectively, using two kinds of fused silica powders with different particle sizes (5.82 and 3.24 μm in d₅₀, respectively) as starting SiO₂. It was demonstrated that the microstructure and mechanical properties of the composites were well dependent on the preforming techniques of green compacts. Gel-casting preforming route has an overwhelming superiority over CIP route on the thermal stability of fused SiO₂ matrix against α-cristobalite crystallization, sinterability, and thus the mechanical properties. Using the finer fused silica (3.24 μm in d₅₀) as the starting SiO₂, the 15 vol% BN_p/SiO₂ composite sintered at 1375°C get a density of 2.05 g/cm³, the silica matrix primarily keep amorphous phase with uniformly dispersed nanosized α-cristobalite crystallites (<5 nm in diameter). The bending strength, fracture toughness and Young's modulus all reach the highest values, 101.5 ± 4.3 MPa, 1.57 ± 0.04 MPa·m^1/2 and 61.3 ± 2.4 GPa, respectively. Microstructure characteristics, chemical bond information and mechanical properties of the as-prepared composites are correlated with their preforming routes, starting fused SiO₂ particle size, sintering temperature, etc. based on X-ray diffractometry, scanning electron microscope, transmission electron microscope, high resolution transmission electron microscope, and FT-IR analysis. [ABSTRACT FROM AUTHOR]