Novel thermal protection coating based on Zr0.75Ce0.25O2/phosphate duplex system for polyimide matrix composites fabricated via a combined sol–gel/sealing treatment process

• Novel Zr0.75Ce0.25O2/phosphate duplex coating was fabricated on polymer substrate.
• The microhardness of the surface was improved effectively by top duplex coating.
• The duplex coating exhibited good thermal shock and thermal ablation resistance.
• The duplex coating protected the substrate against thermal oxidation.
• Failure was attributed to the appearance of the vertical cracks and delamination.

Thermal protection coating based on Zr0.75Ce0.25O2/phosphate system was fabricated on polymer–matrix composites via a combined sol–gel/sealing treatment process. Phosphates sealed the cracks and enhanced the adhesion property via chemical bonding and binding. The Zr0.75Ce0.25O2/phosphate duplex coating exhibited good thermal shock resistance and improved thermal oxidation resistance of the substrate. Due to the protection of the duplex coating, the weight loss of the specimen reduced from (4.83 ± 0.12)% to (0.98 ± 0.08)% and the mass ablation rate decreased from 0.088 ± 0.002 mg cm−2 s−1 to 0.018 ± 0.002 mg cm−2 s−1 when testing at 810 °C. Coating failure was attributed to the formation of cracks and delamination.