*2.2. Preparation of Composite Polymer*

PBN and PCS were dissolved in toluene separately, with appropriate ratios, and mixed at 60 ◦C in a rotary evaporator (Shanghaiyukangkejiaoyiqishebei Co. Ltd., Shanghai, China) for 4 h to form homogeneous hybrid precursor solutions. Then, the solutions were dried at 80 ◦C for 2 h to remove the toluene. After cooling to room temperature, yellow transparent bulk solids were obtained. Six composite polymers (P1, P2, P3, P4, P5, P6) with different PBN/PCS ratios were prepared. After heating up to 1000 ◦C in ammonia at a heating rate of 1 K/min and 1600 ◦C in N<sup>2</sup> at 2 K/min, the BN/Si3N<sup>4</sup> composite ceramics with different Si3N<sup>4</sup> mass contents were produced. The final BN and Si3N<sup>4</sup> mass contents were calculated according to the ceramic yield of each polymer, which was 33 wt% and 58 wt% for PBN and PCS, respectively. The mass contents of PBN and PCS of each hybrid precursor (named P0–P7) as well as the BN and Si3N<sup>4</sup> mass contents of the corresponding composite ceramics obtained at 1600 ◦C are listed in Table 1.


**Table 1.** Composition of hybrid PBN/PCS precursors and the BN/Si3N<sup>4</sup> ceramics derivatives (1600 ◦C).
