Investigation Of C/C Composites Modified By Zr-Ti Alloy Using Reactive Melt Infiltration

In order to improve the oxygen and ablation resistance of the carbon/carbon (C/C) composites and meet the requirements of the thermal protection for the components of new high-speed aerocraft, the C/C composites were modified by the Zr-Ti-C ceramics with good ablation resistance using the reactive melt infiltration in this work. The factors influencing the infiltration performance, microstructure of the components and the interfaces, physical property and the heating damage mechanisms of the modified composites were thoroughly investigated. According to the work above together with the modification methods of substrate and the coating, the C/C-(Zr-Ti-C-B/SiC) composites were manufactured, and their property and microstructure were thoroughly studied. The main research contents and results are as follows:1. Based on the infiltration dynamic, the essential characters controlling the infiltration rules have been revealed. The mechanisms of infiltration were studied. The sealing of the pores adjacent bundles could be delayed by the acceleration of content of Zr in melts. The final density of the modified composites increased with the increase of the volume and the size of the pores adjacent bundles in the pre-modification composites.2. The microstructure, phase and evolution mechanism of ceramics were investigated. In the ceramics, the atoms of Zr and Ti showed a graded distribution, due to the precipitation mechanisms of the carbide in melts. More Zr atoms prior to the Ti atoms were moved to the eutectic carbide and then separated out, indicating that parts of Ti atoms stayed in the carbides separated out later from the melts, which resulted in the grated distribution of atoms. Moreover, the growth of the carbide in the melts was dominated by the laminar boundary between the carbide layer and the melts.3. The effects of ceramic content, structure of preform and density of composites on the ablation property were studied. In addition, two ablation mechanism models had been proposed. The modified composites with high content of Zr in ceramics exhibited outstanding ablation resistance at2500℃and2000℃. The mass and linear ablation rates after ablation at2500℃for180s were0.001mm/s and1.60mg/s, respectively. The excellent property could be attributed to the protective layer consisted of amorphous ZrTiO2pined by the ZrO2crystals on the composites during the ablation test. This layer owned sealing ability for the defects and the anti-scouring ability of the high-speed gas.4. The mechanical property of the modified composites would to some extent decline and the fracture behaviors of the composites would be varied after the experience of RMI process. The reaction of volume reduction between Zr-Ti and C, as well as the mismatch of CTE of the components in composites would produce high thermal stress and residual thermal stress. Thermal stress could be released through the damage of the fibers, pyrocarbon, ceramics and their interfaces, as well as the elastic strain of the components above. The release behavior of stress in the composites with fine-weave pierced preform (FWPP) mainly included the damage of the interfaces. However, the release behavior of stress in the composites with chopped web needled preform(CWNP) mainly included the damage of the fibers. The two damage behaviors above were found in the composites with needle-integrated preform (NIP).5. Thermal conductivity (TC) of the modified composites was investigated. The TC of modified composites with NIP was7.5mm2·s-1, and that of composites with CWNP was6.7mm·s-1at room temperature, which was lower than that of C/C-SiC and C/C composites. In addition, the TC decreased with the increase of temperature. The TCs of composites with NIP and CWNP at800℃were2.2mm·s-1and3.1mm2·s-1, respectively.6. The integrated modification of the substrate and coatings was achieved by the pack cementation together with the substrate modification, and the C/C-(Zr-Ti-C-B/SiC) composites were successfully prepared. The superficial ceramics were composed of ZrC-ZrB with atoms coherent interface and the solid solution of Ti. The substrates were composed of Zr-Ti-C-B ceramics and the carbon substrates. The ceramics distribution in the coating and substrate showed continuous structure.