Pyrolysis of boron-silicon hybrid phenolic resin and ablation performance of carbon/phenolic composite materials

Fiber-reinforced phenolic composites, as an ablative thermal protection material, is widely used in the aerospace applications to enhance the thermal oxidation resistance and ablation performance, which is the key to improving the quality and reliability of aerospace vehicles. For this reason, in this study, it used the diphenyldimethoxysilane as the silicon source and phenylboronic acid as the boron source. By using a simple solvent-free method, the boron and silicon elements were introduced into the thermoplastic phenolic resin (NR), and a boron-silicon hybrid thermoplastic phenolic resin (BSiNR) with an excellent thermal-oxidative stability was designed and synthesized. Carbon fiber (CF) was selected as the reinforcement phase to fabricate a carbon fiber-reinforced hybrid phenolic resin matrix composite (CF/BSiNR) while its mechanical performance, anti-ablation performance and carbon layer after ablation were researched. Compared with NR, BSiNR exhibits an initial thermal decomposition temperature (T_5%) of 391.4℃ and a char yield at 1000℃ (R_1000℃) of 71.5% under the nitrogen atmosphere. Through the thermogravimetry-infrared spectroscopy, XRD, and SEM analyses of the pyrolysis products, it is revealed that the introduction of silicon and boron elements progressively enhances the graphitization degree of the pyrolytic carbon while gradually forming an amorphous ceramic structure, thereby reducing the thermal and oxygen exchange efficiency. CF/BSiNR demonstrates the outstanding ablation performance, with a linear ablation rate and mass ablation rate as low as 0.005 mm/s and 0.0354 g/s, representing reductions of 89.79% and 50.42%, respectively. It proves that this new type of resin with excellent high-temperature oxidation stability and anti-ablation performance is expected to be applied in the thermal protection systems.