SYNTHESIS AND PROCESSING OF SILICON BORON CARBONITRIDE/BORON NITRIDE NANOTUBE COMPOSITES WITH HIGH THERMAL SHOCK RESISTANCE

Ceramics matrix composites (CMC) have both thermal resistivity and high customizability, but the manufacturing typically requires high temperatures and a long duration of pyrolysis or sintering with a low yield. Efforts to accelerate manufacturing, especially in the case of emerging polymer- derived ceramics, can result in void and crack formation or even catastrophic failure of the ceramic product. High stiffness and excellent chemical stability make BNNT ideal material for reinforcement in ceramics. This research findings reveal that polymer-derived Silicon Boron Carbonitride (SiBCN) ceramics were reinforced by boron nitride nanotube (BNNT) networks effectively, enabling them to withstand substantial volume changes during pyrolysis. SiBCN was synthesized from liquid polyborosilazane precursor followed by curing and pyrolysis, which exhibit structural singularities that allow their microstructure to remain amorphous. From the EDS analysis, the element of boron was detected from this black glassy ceramic which may have the composition of Si26.7B13.4C16.7N11,4O31.9. This ceramic showed high thermal stability and oxidation resistance under 1350 °C for 30 min without any weight loss. After adding BNNT into SiBCN, pyrolysis can also proceed with a much faster temperature ramping rate for both heating and cooling cycles, enabling much faster manufacturing throughput than conventional pyrolysis for dense-structure ceramics. This reinforcement results in the production of high-quality ceramics characterized by extremely low porosity and enhanced mechanical and thermal properties, encompassing improvements in the density and thermal shock resistance. By increasing the BNNT weight concentration to 25%, ceramic thin films were obtained with a density of 2.77 g cm–3, while the density of SiBCN was 1.55 g cm–3. No degradation of mechanical properties was observed after tens of thermal shock cycles with a sudden temperature drop of about 1100 °C at a rate of about 2190 °C s–1, making such CMC materials a promising candidate for applications like high- temperature Combustor manufacturing.