| Ultra-high temperature ceramics(UHTCs)modified C/C composites(C/C-UHTCs)are widely used high temperature structural materials,which combine the excellent chemical stability of UHTCs and good mechanical properties of C/C composites.However,the current preparation method have some shortcomings,such as low densification efficiency and uneven UHTCs distribution.Chemical liquid vapor deposition(CLVD)is a process for C/C composites preparation,which has the advantages of high densification efficiency,simply operation and can control the microstructure of the composites.But there are few researches about the preparation of C/C-UHTCs composites by CLVD process,and the deposition mechanism of the composites and the effects of process parameters on the microstructure and properties of the composites are not clear.Thus,in this paper,CLVD process was used to prepare Zr C and Zr C-Si C modified C/C composites,and the Si C NW/Py C core-shell structure was constructed to toughen C/C-Zr C-Si C composites.The preparation process,deposition mechanism,microstructure and properties of the composites were studied.The main research contents and results are as follows:The C/C-Zr C composites were prepared by CLVD process.The deposition behavior and mechanism of the composites were studied,and the effects of deposition temperature on the microstructure and properties were explored.The results presented during the deposition,the deposition front was formed in the radial center of the preform,and the front moved to the undensified area,which made the composite dense gradually;when the deposition temperature was 800-900 oC,the composites had the dense microstructure,high content of Zr C and uniform distribution of ceramics,and the matrix/fiber interface bonding was good;as the deposition temperature increased to 1000-1100 oC,the porosity of the composites increased,and the Zr C content decreased and its distribution was segregated;when the deposition temperature was 900 oC,the flexural strength of the composites was at the highest value of 160.26 MPa,and after the ablation for 60 s under the heat flux of4.18 MW/m2,the linear and mass ablation rates were the lowest,3.10×10-3mm/s and2.57×10-3g/s respectively.The gradient distributed Zr C-Si C modified C/C composites were produced.The effects of precursor addition rate on the microstructure of the composites were studied.The influence of gradient distribution structure on the ablation resistance of the composites was investigated,and the formation and evolution mechanism of the oxide layer on the ablation surface was analyzed.The results showed when the precursor addition rate were 30 and 50 g/h,the Zr C content decreased from the center to the edge of the composites,while the Si C content increased gradually,presenting a gradient distribution trend;as the addition rate increased to 70 g/h,the ceramics content at the edge was high,while at the center the ceramics content was low,showing the different distribution characteristics;after the ablation for 90 s at heat flux of 2.38 MW/m2,the linear and mass ablation rates of the gradient modified composites prepared at 50 g/h were 0.52×10-3mm/s and 0.39×10-3g/s,which were66.7%and 60.9%lower than that of the uniformly modified composites;after the ablation for 90 s at heat flux of 4.18 MW/m2,the linear and mass ablation rates of the gradient modified composites were 1.59×10-3mm/s and 1.57×10-3g/s,which were67.2%and 55.9%lower than that of the uniformly modified composites;the ablation resistance of the gradient modified composites was better than that of the uniformly modified composites in two ablation environments;the gradient distribution of Zr C and Si C leaded to the formation of various oxide coating on the ablation surface;The ablation center surface was covered by Zr O2 coating,the transition region near the ablation center was covered by Zr O2 enriched Zr O2-Si O2 coating,the transition region near the ablation edge was covered by Si O2 enriched Zr O2-Si O2 coating,and the ablation edge was covered by Si O2 caoting with Si O2 nanowires.The C/C-Zr C-Si C composites were prepared by CLVD combined with PIP process.The densification characteristics of two processes were studied,and the influences of the deposition time on the microstructure,flexural and ablation resistance of the composites were discussed.The ablation mechanism of the composites was analyzed.The results showed CLVD process could densify the non-woven layers preferential,and PIP process could densify the fiber webs preferential;when the deposition time in CLVD process was 8 h,the composites have high density and uniform ceramics distribution,resulting in an excellent flexural property and good ablation resistance,while the flexural and ablation resistance was reduced by shortening or prolonging the deposition time;the joint processes could improve the ceramics distribution of the non-woven layes and fiber webs,which promoted the continous oxide coating during the ablation and enhanced the ablation resistance.The C/C-Zr C-Si C composites toughened by Si C NW/Py C core-shell structure were fabricated.The effects of Py C deposition time on the microstructure,flexural and anti-ablation properties were explored.The toughening mechanism of core-shell structure and ablation mechanism of the composites were analyzed.The results indicated when the heat-treatment temperature was 1700 oC,the distribution of Si C NW was uniform and the nanowires grew by vapor-solid(V-S)mechanism;with the increasing Py C deposition time(3-10 h),the diameter of the core-shell structure increased(1-5?m)and the density of the composites was improved,while long deposition made the core-shell structure inapparent;when the Py C deposition time was 6 h,the composites displayed the pseudo-plasitc fracture and had the highest flexural strength and fracture toughness,212.09 MPa and 11.24 MPa·m1/2;during the ablation,the core-shell structure could fix Zr O2,which promoted the formation of dense oxide coating and provide the protection for the composites. |
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