Microstructure evolution and ablation mechanisms of Csf/ZrB2-SiC composites at different heat fluxes under air plasma flame

In this study, short carbon fibre (Csf) reinforced ZrB2-SiC composites were fabricated via direct ink writing followed by precursor infiltration and pyrolysis. The ablation behaviour of the composites was investigated at heat fluxes of 4-6 MW/m2 under an air plasma flame, which revealed excellent ablation resistance with negative linear ablation rates of - 0.1 to - 0.5 mu m/s. The results showed that ZrO2-SiO2 oxide layer was formed on the ablation surface after ablation, which provided effective protection to the internal materials. With an increase in heat flux, ZrO2 precipitated from the SiO2 melt first in the form of nano-tetragonal ZrO2 (t-ZrO2), owing to the vaporisation of SiO2. During cooling, the nano t-ZrO2 particles were retained, and some of them reacted with SiO2 to form ZrSiO4, thus improving the thermal stability of the SiO2-ZrO2 oxide layer. In contrast, t-ZrO2 agglomerated and grew owing to capillary force and finally transformed into monoclinic ZrO2 (m-ZrO2). Moreover, a higher heat flux led to the size of precipitation ZrO2 increases, meanwhile increasing the content of tZrO2 and ZrSiO4 in the oxide layer.