Ablation Behavior Of Carbon/Carbon Composites Doped By ZrC-based Ceramics

Carbon/carbon（C/C）composites have been widely used in the thermal protection systems and propulsion systems of advanced aircraft due to their rare combination of low density,low coefficient of thermal expansion and high specific strength.C/C composites are exposed to the oxidative environment above 2000℃and subjected to the high temperature oxidation with scouring of high-speed airflow in the hypersonic flight environment,resulting in ablation.Therefore,it is an important strategic significance to improve the anti-ablation of C/C composites.The improved anti-ablation performance of C/C composites can be achieved by adding ultrahigh temperature ceramics into C/C composites.In the present study,the 2D needling C/C composites doped by ZrC-based ceramics were prepared by the major processes of precursor infiltration and pyrolysis（PIP）with reactive melt infiltration（RMI）.The single and cyclic ablation testing of the as-prepared composites were performed on an oxyacetylene torch with different heat fluxes and the designed component with wedge shape were tested for a relatively long time.The ablation behavior and anti-ablation performance of the composites was analyzed and the ablation mechanism was explored by the characterization of the phases,compositions,microstructure pre-and post testing using XRD,SEM,EDS and surface temperature.C/C-ZrC-SiC and C/C-ZrC composites were prepared by PIP combined with CVI.Flexural properties and ablation behavior of the composites were investigated comparatively.The results indicated that the flexural strength of C/C-ZrC-SiC composites was lower than that of C/C-ZrC composites due to the weak bonding strength between pyrolytic carbon layer and matrix induced by SiC.After ablation under the torch with heat flux of 2.38 MW/m² for60s,C/C-ZrC composites showed a better ablation resistance than that of C/C-ZrC-SiC.The porous Zr O₂ was formed on the surface of C/C-ZrC-SiC composites and the formed Zr O₂layer with a dense and holonomic microstructure on the surface of C/C-ZrC composites.Exposing to the torch with the heat flux of 4.18 MW/m²,the ablation pits occured on both composites.C/C-ZrC-SiC composites showed a higher mass ablation rate than that of C/C-ZrC composites due to evaporation of a large amount of gaseous product,but its linear ablation rate was lower than that of C/C-ZrC composites.The ablation behavior and anti-ablation property of C/C-ZrC-SiC composites prepared by RMI were investigated under the torch with heat fluxes of 2.38 and 4.18 MW/m².The results showed that the reaction-bonded ZrC-SiC was distributed in the fiber web and SiC was surrounded by ZrC.At the heat flux of 2.38 MW/m²,the formed"island"-like Zr O₂ and Si O₂layer on the ablated surface of C/C-ZrC-SiC composites as the effective barriers were capable of resisting the ablation heat and slowing down the ingress of oxygen into the inner material,the mass and linear ablation rates of C/C-ZrC-SiC composites was 76.8%and 88.4%lower than C/C composites,and 66.9%and 58.3%lower than C/C-SiC composites,respectively.At the heat flux of 4.18 MW/m²,the porous Zr O₂ layer formed on the center region of surface C/C-ZrC-SiC composites was peeled due to the high temperature and the intense mechanical scouring of the torch,the mass ablation rate of C/C-ZrC-SiC composites was 60.8%and 44.8%lower than that of C/C and C/C-SiC composites,respectively.The linear ablation rate of C/C-ZrC-SiC composites was 7.3%higher than that of C/C,but 13.5%lower than that of C/C-SiC composites.Microstructure characterization of C/C-ZrC-SiC composites fabricated by PIP combined with RMI revealed that the reaction-bonded ZrC-SiC formed by RMI was distributed in the fiber web and showed a high compactness.Uiform distribution occured between SiC and ZrC.After ablation at the heat flux of 4.18 MW/m² for 60s,the anti-ablation performance of C/C-ZrC-SiC composites by PIP combined RMI was superior to the composites by single RMI.Microstructure of the ablated surface revealed that the grain size of Zr O₂ had changed from larger to small due to temperature gradient from the center to the brim region.The formed Zr O₂ with a framework structure exhibited a high configurational stability.No evidence of the Zr O₂ spallation was observed after undergoing the mechanical scouring of the high temperature and high speed torch.Cyclic ablation behavior of C/C-ZrC-SiC-ZrB₂ composites prepared by a combined process of RMI,infiltration of the B₄C/phenolic resin slurry and CVI was investigated following 30s×4 and 60s×2 cycles under the torch with heat fluxes of 2.38 and 4.18 MW/m²,respectively.At the two heat fluxes,the surface temperature of cyclic ablation of 30s×4 was lower than that of 60s×2.At the second ablation in cyclic ablation of 30s×4,the surface emissivity was changed and the surface temperature was lowered due to the incresed spallation of Zr O₂.The mass loss and thickness loss of the composites in cyclic ablation of30s×4 was higher than that of the composites in 60s×2 because of the peeled amount of oxides was increased by the increased numbers of cyclic ablation.Microstructure characterization of the ablated surface indicated that the center and transition region were consisted of a layered structure of Zr O₂outer layer with Zr O₂-Si O₂ sub layer.The consumption of Si O₂ was decreased due to the lower temperature in the sub layer,which made the grain size of Zr O₂ in the sub layer smaller than that of Zr O₂ in the outer layer.Cyclic ablation behavior of 30s×4 and single ablation behavior of 120s were carried out on C/C-ZrC-SiC and C/C-ZrC composites prepared by RMI using the torch with heat flux of2.38 MW/m².The results showed that the surface temperature of cyclic ablation of the two composites was lower than that of the single ablation.With the cyclic ablation number increased,the surface temperature of the composites increased and the peeling amount of oxides increased.In the case of cyclic ablation,Si O₂ formed on the C/C-ZrC-SiC composites was favorable to generate a good adhesion between Zr O₂,Si O₂ and matrix material.Simultaneously,Zr O₂ embeded in the Si O₂ layer had a higher performance in resistance to scouring of the torch than that of the single Zr O₂.After cyclic ablation,the amount of oxides on the surface of C/C-ZrC-SiC composites was significantly higher than that of C/C-ZrC composites,and C/C-ZrC-SiC composites showed a lower linear ablation rate than that of C/C-ZrC composites.The ablation behavior and anti-ablation performance of wedge shape components of C/C-ZrC-SiC-ZrB₂ and C/C-ZrC-SiC composites fabricated by RMI were investigated using the torch with heat flux of 2.38 MW/m² for 120s.The results showed that C/C-ZrC-SiC-ZrB₂and C/C-ZrC-SiC components well maintained the complete shape despite having underwent a large temperature gradient at the initial ablation stage as well as ultrahigh temperature associated with intense mechanical scouring of the torch at the steady ablation stage.The anti-ablation performance of both composites was superior to that of C/C composites.There was a structure evolution from the region close to the tip of the C/C-ZrC-SiC-ZrB₂ and C/C-ZrC-SiC components to the tail region on the surface due to the effects of temperature gradient.Microstructure characterization revealed a porous Zr O₂ layer forming in the region near the tip,a layered structure consisting of the Zr O₂ outer layer and the Zr O₂-Si O₂ sub layer in the middle region and Si O₂ layer in the tail region.Particularly in the middle region,the grain size of Zr O₂ in the sub layer was smaller than that of Zr O₂ in the outer layer.