Fabrication Of C/C-Zr-Hf-Si Composites Via Molten-salt-assisted Reactive Melt Infiltration

Traditional reactive melt infiltration（RMI）is one of the important methods for the industrial preparation of the ultra-high-temperature thermal structural components of hypersonic vehicles.However,the fabrication of the ultra-high-temperature ceramic-modified C/C composites（C/C-UHTCs）is hindered due to various factors,such as the inhibited suppression of the residual low-melting silicide,fiber damage at high infiltration temperatures,and low content of UHTC.To overcome these limitations of RMI,low-density C/C composites were used to prepare C/C–Zr–Hf–Si composites via the molten-salt-assisted RMI method.The effects of key processes,such as the molten-salt type and infiltration temperature,on the infiltration effect were studied.The mechanism of molten-salt-assisted RMI and the process of ceramic phase growth were investigated,and the effects of the microstructure on the ablative and mechanical properties of the materials were analyzed.On this basis,the surface defects of the modified matrix were sealed using a molten-salt-assisted thermal evaporation process to improve the overall ablation resistance of the material.The primary research findings are as follows.（1）Chloride-assisted RMI was found capable of decreasing the amount of low-melting silicide in the composites and increasing their density.The chloride salt was also able to optimize the formation of the fine-grained ceramic-inlaid Py C/Zr C–Si C interface,increase its carbon diffusion capacity.Additionally,CaCl₂,which has low viscosity and volatility,could be used to decrease the viscosity of the infiltration melt while increasing its liquid phase concentration and fluidity.Thus,the overall density of the material was increased substantially.（2）Fluorine-salt-assisted RMI was found capable of forming fine particles and continuous UHTC layers on the surface of Py C,including Zr C,Zr Hf C,Zr Ta C,and Ta Nb C.Further,the high melting point and inert nature of the Py C/UHTC interface protected the carbon fiber from reaction erosion resulting from the alloy melt.Additionally,the C/C–Zr Hf C–Si C composites with Py C/Zr Hf C interface protection could be prepared via a combination of RMI and fluorine-salt-assisted RMI.（3）Fluorine-salt-assisted RMI was able to overcome the carrying limitation of the Me–Si alloy,diffuse into the C/C matrix by ionizing the refractory metal in molten salt,and form UHTCs at low temperature.Additionally,the preparation temperature of MSI was approximately 400–600℃lower than that of RMI.The mechanism of molten-salt-assisted RMI is as follows:the metal ionizes and reacts with the fluorine ions and then diffuses into the C/C composite and forms UHTCs.The by-products of the reaction are dissolved fluorine gas and fluorine ions.（4）In the process of molten-salt-assisted RMI,a high concentration of molten salt helps form fine-grained ceramics,and the nucleation rate of ceramics is greater than their growth rate.As the temperature approached the boiling point of the molten salt,the equilibrium at the solid/liquid interface degenerated completely,and carbide nucleation ceased gradually.Simultaneously,the ceramic particles sintered,and the Ostwald ripening process reached maturity.（5）Compared to the C/C-UHTCs prepared via RMI,the C/C–ZrHfC–SiC composites with the Py C/Zr Hf C interface demonstrated decreases of43.8%and 33.3%in mass and linear ablation rate,respectively;furthermore,along the vertical and parallel fiber bundles,pressure planes increased by 21.5%and 35.4%,respectively,while the bending properties improved by approximately 50%.（6）CaCl₂ optimized the diffusion channel of the Zr–Si thermal evaporation masterbatch and improved the controllability of the Zr C–Si C sealing layer structure.Additionally,the surface Zr C–Si C hybrid layer demonstrated excellent oxidation resistance at low temperatures during ablation at 2800℃.The mass and linear ablation rate were 0.176±0.01mg/s and 0.21±0.02μm/s,respectively.