Microstructure and ablation behavior of a novel La2Hf2O7/ZrC-SiC/SiC graded coating for ablative protection up to 2400 °C

A novel La₂Hf₂O₇/ZrC-SiC/SiC graded coating with CTE increasing gradually from the inner to outside layers was prepared by pack cementation and supersonic atmospheric plasma spraying. Due to the graded multi-layer structure, the coating exhibits good ablative property after different ablation times. As ablated under 3.2 MW/m² oxygen acetylene flame for 30 and 60 s, the ablation rates are increased from −11.3 μm/s, −0.53 mg/s to 6.79 μm/s, 1.12 mg/s. The ablation results demonstrate that La₂Hf₂O₇ outer layer maintains good thermal stability without decomposition or phase transformation at 2400 °C, and only partial melting and obvious growth of the La₂Hf₂O₇ grains are observed in ablation center. The scoured molten La₂Hf₂O₇ phase and the thinning of La₂Hf₂O₇ outer layer caused by the further sintering result in the increased ablation rates of the graded coating. The excellent thermal insulation and stability of La₂Hf₂O₇ outer layer provides effective protection for internal SiC-ZrC and SiC layers. The integrity three-layer structure and the well protected inner layers demonstrate the effective design of this novel graded coating for thermal protection of C/C composites at 2400 °C.