Fabrication Of C_f/UHTC_p/SiC Composites Via Precursor Infiltration Pyrolysis Method And Properties Investigation

The development of hypersonic vehicles is in great need of high temperature resistant ceramic matrix composites with little ablation or even no ablation and enough mechanical properties and oxidation resistance. In this dissertation, 2D C_f/UHTC_p/SiC composites were designed and fabricated by PIP route from the viewpoint of availability and applicability. In the composites, carbon fibers were used as reinforcement to provide strength and toughness, ultra high temperature ceramics (UHTC) were used to provide high temperature and ablation resistance, SiC was used to provide oxidation resistance. The influence of PIP route and different UHTC and its content on the microstructures and properties of 2D C_f/UHTC_p/SiC composites were investigated. Also, the mechanical and anti-ablation properties of the composites were studied, and anti-ablation mechanism was discussed.The manufacturing of 2D C_f/UHTC_p/SiC composites via PIP included two steps: (1) the formation of the composites, namely fabrication of the body; (2) the densification of the composites, namely several infiltration/pyrolysis cycles were repeated to densify the body. Firstly, the formation and densification process of 2D C_f/UHTC_p/SiC composites were investigated. For 2D composites, brushing with slurry, stacking and curing of carbon fiber cloth was proper to fabricate the body of composites. The mass ratio of PCS/DVB was a key parameter to achieve a balance between content of UHTC and integrality of the body, and mPCS:mDVB=1:3 was proved to be desirable. Repeating cycles of vacuum infiltration with PCS/xylene and pyrolysis was found to be desirable for densification of the body.The influence of UHTC_p (ZrB₂, ZrC and TaC) content, molding pressure and heat treatment temperature on microstructure and properties of the composites was researched respectively. Incorporation of UHTC_p could remarkably increase the anti-ablation property of the composites. With UHTC_p contents increasing, the anti-ablation property of the composites was increased obviously. At the same time, the volume content of carbon fibers decreased, resulting in decrease in flexural strength of the composites. Increasing molding pressure could enhance mechanical and anti-ablation properties of the composites as a result of the increase of contents of UHTC_p and carbon fibers. The anti-ablation and mechanical properties could be increased further if curing was conducted with pressure.2D C_f/ZrB_2p/SiC composites, which prepared with a mass ratio (mPCS:mDVB:mZrB_2p) of 1:3:8.9, a molding-curing pressure of 7MPa, and a heat-treatment temperature of 1400℃, showed the best mechanical properties which were 252.0MPa in flexural strength and 35.5GPa in modulus. The volume content of ZrB₂, .carbon fibers and SiC in the composites were 25.5vol%, 22.6vol% and 32.5vol%, respectively. When ablated in oxyacetylene flame for 60 seconds, the surface temperature of the composites reached 2200℃. Under the circumstances, the mass loss rate and linear recession rate were 0.0260g/s and 0.0198mm/s, respectively. Increasing heat treatment temperature would improve anti-ablation property markedly, and decrease mechanical properties sharply at the same time. The composites fabricated with a heat treatment temperature of 1800℃exhibited very low mechanical properties which were 27.1MPa in flexural strength and 26.1GPa in modulus and improved anti-ablation with a linear recession rate of 0.0161mm/s and a mass loss rate of 0.0141g/s. Based the above results, it is proposed that much attention should be paid to how improve anti-ablation property not at the sacrifice of mechanical properties.2D C_f/ZrC_p/SiC composites, which prepared with a mass ratio (mPCS:mDVB:mZrC_p) of 1:3:23.3, a molding-curing pressure of 7MPa, and a heat-treatment temperature of 1600℃, showed the best mechanical properties which were 168.7MPa in flexural strength and 31.0GPa in modulus. The volume content of ZrC, carbon fibers and SiC in the composites were 33.3vol%, 20.3vol% and 26.5vol%, respectively. When ablated in oxyacetylene flame for 60 seconds, the surface temperature of the composites reached 2243℃. Under the circumstances, the linear recession rate and mass loss rate were 0.0037mm/s and 0.0073g/s, respectively.2D C_f/TaC_p/SiC composites, which prepared with a mass ratio (mPCS:mDVB:mTaC_p) of 1:3:34.0, a molding-curing pressure of 7MPa, and a heat-treatment temperature of 1600℃, showed the best mechanical properties which were 210.9MPa in flexural strength and 34.4GPa in modulus. The volume content of TaC, carbon fibers and SiC in the composites were 30.5vol%, 26.9vol% and 26.0vol%, respectively. The linear recession rate and mass loss rate were 0.0142mm/s and 0.0193g/s, respectively.The mechanical properties in room and high temperature and anti-oxidation property of the three composites were investigated. The mechanical properties of 2D C_f/UHTC_p/SiC composites were mainly decided by carbon fiber content. The tensile strengths of 2D C_f/ZrB_2p/SiC, 2D C_f/ZrC_p/SiC and 2D C_f/TaC_p/SiC were 78.5MPa, 67.4MPa and 118.9MPa, respectively. The shear strength and fracture toughness of the three 2D C_f/UHTC_p/SiC composites were～30.0MPa and～10MPa·m1/2 respectively. In order to improve the interlaminar bonding between carbon fiber cloth, z-pin through-thickness reinforcements was prepared. The resultant 2D C_f/ZrB_2p/SiC-Zpin composites showed 37.4MPa in shear strength which was increased by 20.6%. The improvement in shear strength was beneficial to the machinability and reliability of the composites. The flexural strength and flexural modulus of the resultant composites was almost the same as the unpinned sample. The compressive strengths in x, y, z directions of the three composites were all about 250.0MPa.The 2D C_f/UHTC_p/SiC composites exhibited better high temperature resistance compared with 2D C_f/SiC composites. The flexural strength of 2D C_f/ZrB_2p/SiC composites at 1800℃was 143.9MPa, 74.1% of original strength, and the flexural strength at 2000℃was only 61.8MPa. The flexural strength of 2D C_f/ZrC_p/SiC composites at 1800℃was 165.9MPa, 81.8% of original strength, and the flexural strength at 2000℃was 168.5MPa, 83.1% of original strength. The flexural strength of 2D C_f/TaC_p/SiC composites at 1800℃and 2000℃were 98.0MPa and 122.2MPa, respectively.Due to the formation of B₂O3 and Ta2O5 with self-healing ability, the 2D C_f/ZrB_2p/SiC and 2D C_f/TaC_p/SiC composites showed desirable oxidation resistance. After oxidized at 1200℃in static air, 2D C_f/ZrB_2p/SiC composites showed 184.3MPa in flexural strength, 69.2% of original strength, and 2D C_f/TaC_p/SiC composites showed 197.6MPa in flexural strength, 74.1% of original strength. However, 2D C_f/ZrC_p/SiC composites showed little strength after oxidation because the ZrO2 resulting from oxidation of ZrC had no self-healing ability.The anti-ablation properties of the three composites in two testing environments and different testing conditions were investigated. It was evident that UHTC_p played a key role in deciding anti-ablation property. The results indicated that the three composites exhibited anti-ablation property superior to 2D C_f/SiC composites due to the incorporation of UHTC_p. When ablated in oxyacetylene flame and surface temperature of composites reached 2200℃, 2D C_f/ZrB_2p/SiC composites showed the best anti-ablation property, 2D C_f/ZrC_p/SiC composites were placed in the middle, and 2D C_f/TaC_p/SiC composites showed the worst. The mass loss rate and linear recession rate of the three composites were 0.0062g/s and 0.0052mm/s, 0.0104g/s and 0.0111mm/s, and 0.0134g/s and 0.0187mm/s, respectively. In case of surface temperature of 2600℃, the mass loss rate and linear recession rate of the three composites were all increased markedly, and 2D C_f/ZrC_p/SiC composites showed the best anti-ablation property, 2D C_f/ZrB_2p/SiC composites were placed in the middle, 2D C_f/TaC_p/SiC composites showed the worst. When tested in plasma wind tunnel, the mass loss rate and linear recession rate of the three composites were further increased due to the more serious environment. Under the circumstance, 2D C_f/TaC_p/SiC composites showed the best anti-ablation property, 2D C_f/ZrC_p/SiC composites were placed in the middle, and 2D C_f/ZrB_2p/SiC composites showed the worst.Anti-ablation mechanisms of the three composites in different testing environments were discussed. In oxyacetylene flame, ablation of the composites was mainly ascribed to thermal chemical and thermal physical ablation, accompanied by airflow erosion and mechanical denudation to a certain extent. At the surface temperature of 2200℃, a layer of melted coating with high viscosity formed on the surface of 2D C_f/ZrB_2p/SiC composites, which could prevent the diffusion of oxygen and resist airflow erosion. Accordingly, the composites showed good anti-ablation property. Increasing surface temperature from 2200℃to 2600℃led to the decrease in viscosity of melted coating. In this case, the melted coating was prone to be blew away by airflow, resulting in the increase of mass loss rate and linear recession rate. As for 2D C_f/ZrC_p/SiC composites, viscous coating could not form until surface temperature of the composites reached 2600℃. Consequently, 2D C_f/ZrC_p/SiC composites showed better anti-ablation property at surface temperature of 2600℃than 2200℃. Viscous coating could not form on surface of 2D C_f/TaC_p/SiC composites at surface temperature of 2200℃and 2600℃because the melting point of Ta2O5 derived from oxidation of TaC is 1870℃. As a result, the composites exhibited the worst anti-ablation property.In plasma wind tunnel, thermal chemical ablation can be ignored because testing time is only 10 seconds. So, thermal physical ablation, airflow erosion and mechanical denudation play a significant role in determining anti-ablation property of the three composites. The melting points of ZrB₂, ZrC and TaC are 3040℃, 3530℃and 3880℃, respectively. Accordingly, when surface temperature reached 2800℃, 2D C_f/TaC_p/SiC composites showed the best anti-ablation property, 2D C_f/ZrC_p/SiC composites were placed in the middle and 2D C_f/ZrB_2p/SiC composites exhibited the worst, which were decided by anti-denudation ability of ZrB₂, ZrC and TaC at 2800℃. It was easier for stacked carbon fiber cloths to be blew away in plasma wind tunnel due to more serious airflow erosion and mechanical denudation than in oxyacetylene flame. As a result, the composites showed worse anti-ablation property in plasma wind tunnel.