Preparation And Properties Of Zr-based Nanoceramic Modified C/C-SiC Composites By Reactive Melt Infiltration

Spacecraft must withstand strong airflow and thermal load during fast flight in Ultra-high temperature environment with oxidation and ablation,so high requirements are made for the thermal protection materials.C/C-Si C composites,which combine the advantages of C/C composites and Si C ceramic,have good anti-ablation performances and have been used as the thermal protection materials.To improve the ablation property of C/C-Si C composites,Zr-based nanoceramic modified C/C-Si C composites were prepared by reactive melting infiltration.The effects of Zr-based nanoceramic content and composition on the density and open porosity,microstructure,ablation property and mechanical property were investigated.The reaction infiltration mechanism,ablation mechanism and fracture mechanism were analyzed.The main research contents and results are as follows:（1）The effect of Zr-based nanoceramic powder content and composition on the microstructure of Zr-based nanoceramic modified C/C-Si C composites was studied.When the content of nanoceramic powder was increased from 2 mol.%to 6 mol.%,the density of the prepared composite is not much different,but the porosity gradually increases from 4.0%to 10.7%.In addition,it can be found that a part of the distribution of the nanoceramic phase is relatively dispersed,in the state of fine particles,and the other part is inevitably agglomerated.When the content of the nanoceramic phase increases,the agglomeration will become more serious.The composition of the nanoceramic powder did not have a great influence on the infiltration effect and microstructure.During the infiltration process,the nano-ceramic powder fills the pores of porous C/C composites driven by the Si melt,and the gradient structure was formed due to the influence of gravity and capillary force.（2）The effects of Zr-based nanoceramic powder content and composition on the flexural properties of Zr-based nanoceramic modified C/C-Si C composites were studied.When the content of nanoceramic powder increased,the flexural strength of the prepared composites decreased from 200 MPa to 104 MPa.When the composition of the nanoceramic powder was changed but the content is unchanged,the flexural strength does not change much.The reason is that the increase of nanoceramic powder content increases the porosity inside the composites,resulting in the decrease of the flexural strength of the composites.（3）The effects of Zr-based nanoceramic powder content on the ablation property of Zr-based nanoceramic modified C/C-Si C composites was studied.With the increase of nanoceramic powder content,the mass ablation rate and linear ablation rate of the prepared composites gradually decreased from 0.0033 g/s and 0.0045 mm/s to 0.0020 g/s and 0.0039mm/s,and the degree of damage to the ablation surface was reduced.The increase of the appropriate nanoceramic content can generated more solid Zr O₂oxide layer during ablation,which can pinned the liquid Si O₂and reduced its loss,increasing the self-healing ability of the oxide layer.（4）The effect of Zr-based nanoceramic powder composition on the ablation property of Zr-based nanoceramic modified C/C-Si C composites was studied.When the content of nanoceramic powder remains unchanged and nano-Zr C ceramics was gradually replaced by nano-Zr B₂ceramics,the mass ablation rate and linear ablation rate of the prepared composites increased to 0.0040 g/s and 0.0050 mm/s.Zr B₂was oxidized to form B₂O₃with a lower melting point during the ablation process,and the volatilized B₂O₃caused the oxide layer on the surface of the composites to generate pores.The pinning effect of the non-dense oxide layer is bad,and the loss of liquid Si O₂is serious,resulting in poor self-healing effect of the oxide layer.