Preparation And Properties Of Modified C/C Composite By Si-Zr Alloyed Melt Infiltration

C/C composite has a number of merits such as low density and high specific strength. However, the long fabrication period with high cost and feasibility of oxidation and ablation at high temperatures limit its widespread applications in aviation, aerospace and metallurgy areas. Introduction of ultra high temperature ceramics into C/C composite is an effective method to improve its oxidation and ablation resistance. Large amount of ultra high temperature ceramics can be introduced into C/C composite by reactive melt infiltration process（RMI）, which can not only improve the oxidation and ablation resistance of C/C composite, but also effectively decrease the preparation period and cost. This modification process has extensively potential application.Generally, metal melts（silicon or zirconium et al） infiltrates into a porous C/C preform under the driving force of capillary and then reacts with the pyrolytic carbon in the preform, resulting a dense high temperature ceramic matrix during RMI. Two main process, infiltration and reaction process are included in the RMI. The infiltration of pure silicon and zirconium must be performed at very high temperatures and the process is quite difficult to be controlled. Moreover, certain amount of residual metal is usually found in the resulting composite, which is deleterious for the composite applications at high temperatures. In the present work, the eutectic Si-Zr10 and Zr-Si8.8 alloy based on Si-Zr binary system were chosen as the infiltrators during RMI. Compared with pure silicon and zirconium, these alloys have relatively low melting points, which means the RMI temperatures can be accordingly decreased. The residual alloys in the RMI composites can also be transformed to high temperature zirconium silicides. The properties of the modified C/C composite can be further improved.In this paper, the RMI process was firstly systematically investigated. The reaction kinetics and infiltration dynamics during RMI were established. The reaction mechanism and infiltration-controlled factors were analyzed. Then, C/C-SiC and C/C-ZrC composites were respectively fabricated by RMI with Si-Zr10 and Zr-Si8.8 alloys. The mechanical strength, oxidation resistance and ablation resistance of the C/C-SiC and C/C-ZrC composites were also evaluated.The reaction of zirconium with carbon was more favorable than the reaction of silicon with carbon according to thermodynamics calculation. Both the reactions of carbon with zirconium and silicon were exothermic. A continuous carbide layer was respectively formed after the reaction of carbon with zirconium and silicon. Both the reaction kinetics equations were parabolas. The activation energy of the above reactions based on Arrhenius equation was 177.1 k J/mol and 313.2 k J/mol. The growth speeds of the carbide interfacial layers were calculated based on the diffusion of carbon. The calculated results were in good agreement of the experimental data. The reaction of Si-Zr10 alloy with carbon included the following stages: dissolution of carbon, formation of the discontinuous ZrC layer, formation of the continuous SiC layer and its growth, precipitation of the discontinuous SiC layer during cooling stage and the solidification of the residual alloyed melts. The reaction of Zr-Si8.8 alloy with carbon included the following stages: dissolution of carbon, formation of the continuous ZrC layer and its growth, precipitation of ZrC grains during cooling stage and the solidification of the residual alloyed melts.The pore structure of the porous C/C preform was characterized using various techniques. The properties of the Si-Zr10 and Zr-Si8.8 alloyed melts, such as density, surface tension, viscosity and wetting angles on the C/C preform, were studied according to experiments and calculation. The influence of alloy element on the surface tension and viscosity were evaluated and the kinetics of wetting on the C/C preform was analyzed. Both the infiltration dynamics controlled by the viscosity resistance of alloyed melts and interfacial reaction were respectively investigated using experiments and modeling calculation consponding to the disagreement of the infiltration dynamics controlled by the viscosity resistance of alloyed melts and interfacial reaction. The results demonstrated that both the infiltration dynamics controlled by the viscosity resistance of alloyed melts and interfacial reaction made sense and they worked in different stages of infiltration. At the beginning, the infiltration was controlled by the interfacial reaction. In the middle and ending of stages, the infiltration speed controlled by the interfacial reaction became larger than that by the viscosity resistance, and the infiltration of the alloy melts was controlled by the viscosity resistance of alloyed melts.C/C-SiC composite has been prepared by Si-Zr10 alloyed melt infiltration. The microstructure and mechanical strength of the C/C-SiC composite were greatly influenced by the RMI parameters and C/C preform density. With the increase of infiltration temperature and time period, more and more ZrC phase was formed in the C/C-SiC composites and the flexural strength of the resulting composite initially increased and then decreased. The densities of the C/C-SiC composite decreased with the increase of C/C preform densities while the flexural strength initially increased and then decreased. The C/C-SiC composite had good oxidation resistance compared with the C/C preform and high density C/C composite. The improved oxidation resistance of C/C composite can be attributed to the formation of Zr O₂, Si O₂ and Zr Si O4 phases after high temperature oxidation. The ablation tests by oxyacetylene flame indicated that the linear ablation rates of the C/C-SiC composite decreased with the increase of ablation time and distance from the ablated surface to the flame. The linear ablation rates of the C/C-SiC composite increased with the increase of C/C preform densities. Multilayered ablation products composed of melting Zr O₂ layer, coarse layer and glassy ablation layer were formed from the ablated surface to the surface of the composite matrix.C/C-ZrC composite has been prepared by Zr-Si8.8 alloyed melt infiltration. The effects of RMI parameters and C/C preform density on the microstructure and mechanical strength of the C/C-ZrC composite were investigated. With the increase of infiltration time periods, the flexural strength of the resulting composites decreased while the mechanical strength of the composites initially increased and then decreased with the increase of the infiltration temperatures. The densities of the C/C-ZrC composites decreased with the increase of C/C preform densities while the flexural strength initially increased and then decreased. The C/C-ZrC composite had relatively poor oxidation resistance. Loose Zr O₂ layer was formed on the oxidized surface, which could not seal the cracks and pores on the composite surface and the composites had a large weight loss. The oxidation resistance of the composite needed further improvement. Linear ablation rates of the C/C-ZrC composite decreased with the increase of ablation time and distance from the ablated surface to the flame and increased with the increase of C/C preform densities. The ablation process was greatly influenced by the temperature distribution and temperature variation during ablation. The ablation mechanism was different in different stages. Multilayered ablation products composed of melting Zr O₂ layer, coarse layer and loose ablation layer were formed from the ablated surface to the surface of the composite matrix.