Influence of hydrothermal treatment on the microstructure and oxidation resistance of a Zn<sub>4</sub>B<sub>2</sub>O<sub>7</sub>·H<sub>2</sub>O (4ZnO·B<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O) coating for C/C composites
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Antioxidant modification for C/C composites by <i>in situ</i> hydrothermal synthesise at 140 °C of a 4ZnO·B<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O crystallite coating has been successfully achieved. The influence of hydrothermal time on the phase composition, microstructure of the as-prepared Zn<sub>4</sub>B<sub>2</sub>O<sub>7</sub>·H<sub>2</sub>O (4ZnO·B<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O), and its antioxidant modification for C/C composites were investigated. Samples were characterised by XRD, SEM, isothermal oxidation test and TG-DSC. Results show that, 4ZnO·B<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O crystalline coating is achieved on the surface of C/C composites after the hydrothermal treatment at 140 °C for time in the range of 2–12 h. A smooth and crack-free 4ZnO·B<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O layer can be obtained when the hydrothermal time reaches 8 h. Isothermal oxidation test demonstrates that the oxidation resistance of C/C composites is improved. The as-modified composites exhibit only 1.52 g·cm<sup>−2</sup> weight loss after oxidation at 600 °C for 15 h, while the non-modified one shows a 6.57 g·cm<sup>−2</sup> weight loss after only 10 h oxidation. For the uncoated C/C composite the oxidation rate is approximately linear with time (non-protective oxidation), thus at 15 h exposure one can estimate the mass loss to be 6.57 g·cm<sup>−2</sup> after 10 h for direct comparison with the coated samples.
