| Due to their unique physical and chemical properties, iridium(Ir) and rhenium(Re) have been considered as the most promising candidates for thermal structural components used at high temperatures(>2000°C) in aerospace and military fields. In this dissertation, for the purpose of developing Ir/Re and Ir/Re/C/C combustion chambers used in the third-generation liquid-propellant rocket engines, Ir/Re coating was prepared by chemical vapor deposition(CVD) and electrodeposition in molten salts(ED). The effects of processing parameters of CVD and ED on morphology, microstructure and uniformity of Re and Ir coatings were studied. The interface, interdiffusion, high-temperature oxidation and ablation of the Ir/Re coating were investigated. As a result, the failure mechanism of the Ir/Re coating at high temperatures was analyzed. Further more, a preliminary investigation on modifications of Ir coating was carried out to further improve the oxidation resistance of Ir/Re coating.Considering the complexity of configurations of combustion chambers, the effects of CVD processing parameters on the deposition rate, yield and surface morphology of the Re coating deposited on a mandrel of the combustion chamber were studied. It is found that the yield of the Re coating increases with increasing deposition temperature, while decreases with increasing flow rate of chlorine and total pressure. The deposition rate of the Re coating in a certain region of the mandrel increases with increasing temperature and concentration of reactant, while decreases with increasing thickness of the boundary layer in the corresponding region. The surface roughness of Re coating increases with the increase of the deposition temperature and the decrease of the flow rate of chlorine and the total pressure. The distributions of the temperature, concentration of reactant and gas velocity in different regions of the mandrel can be optimized by changing the shape of coil, the configuration of deposition chamber and the total pressure. As a result, a uniform Re coating was produced on the mandrel of the combustion chamber. The as-deposited Re coatings consist of two sub-layers, i.e., an inner nucleation layer of equiaxed grains and an outer growth layer comprising <002> oriented columnar grains. The CVD Re coating attains 99.4% of theoretical density and has a purity of 99.966%, whose tensile strength and elongation at room temperature are 772 MPa and 11.9%, respectively.The composition and structure of molten salts, as well as the stability of Ir ion in molten salts were investigated to optimize the composition of the molten salts for ED. In the selected molten salts, the effects of cathodic current density, temperature and atmosphere on morphology and microstructure of Ir coating were studied. The results show that the melting point of the molten salt mixture of Na Cl-KCl-Cs Cl-Ir Cl3 decreases with increasing Cs Cl content. As the content of Cs Cl increases to 60 wt.%,the melting point of molten salts drops to 479°C. The Cs Cl component reacts with Ir Cl3 to form Cs2 Ir Cl6 and Cs3 Ir Cl6 during the melting process of molten salts, which increases the solubility and stability of Ir Cl3 in molten salts. In air atmosphere, with increasing of current density from 5 to 50 m A/cm2, the surface roughness of the Ir coating increases, while the grain size decreases; the macroscopic compactness of Ir coating increases, while the microscopic compactness decreases. As the temperature is increasing from 520 to 640°C, the surface roughness of the Ir coating decreases, while the grain size increases; the macroscopic compactness of Ir coating decreases, while the microscopic compactness increases. In Ar atmosphere, the Ir coating electrodeposited under the same deposition condition has a higher degree of preferred orientation of <111> and a better microscopic compactness. The bond strength between ED Ir coating and Re substrate is over 16 MPa. The as-prepared ED Ir coating attains 98% of theoretical density and has a purity over 99.976%, whose tensile strength and elongation at room temperature are 258 MPa and 2.3%, respectively.The diffusion behavior of Re into Ir was studied in the temperature range of 1800 to 2200°C using the semi-infinite diffusion model. The diffusion coefficients of Re into Ir at 1800, 2000 and 2200°C are 2.8×10-11, 5.6×10-11 and 9.7×10-11 cm2/s, respectively. The diffusion activation energy is 132.5 k J/mol. The oxidation behavior of the Ir/Re was studied at 2000°C. The results show that the failure of the Ir/Re in high-temperature oxidizing environment is caused by both the direct oxidation of Ir and the fast diffusion of Re into Ir coating along the grain boundaries. After oxidation test, a large amount of micropores form along the grain boundaries of Ir coating, which are due to the migration and aggregation of the voids in the coating, and the preferential oxidation of those regions with high Re content caused by the fast diffusion of Re along the grain boundaries of Ir coating.The adhesion and high-temperature ablation behavior of the Ir/Re coating on the C/C composite material were investigated. The results show that, after applying a Re coating on the C/C composites, the contact angle of molten salts on the C/C substrate decreases from 128.5° to 43.4°, and the bond strength between Ir coating and C/C substrate increases from 3.2 MPa to 7.9 MPa. However, after high temperature annealing treatment, the bond strength is weakened, due to the infiltration of molten salts through microcracks in the Re coating during the ED process. During entire high-temperature ablation process, the Ir/Re coating on the C/C composites keeps intact without any signs of ablation damage. However, macroscopic degradations of the coating like cracking and bubbling start to appear during the cooling stage after ablation. The Ir coating gets smoother and a bigger grain size after ablation, with lots of micropores distributed along the grain boundaries and the preferred orientation changing from <311> to <220>.Based on the failure mechanism of the Ir/Re coating at high temperatures, two aspects of modifications on structure and composition of Ir coating were taken into account to improve the oxidation resistance of Ir coating. Laminar Ir coatings were obtained by pulse current electrodeposition. It is found that the columnar growth trend of Ir coating is broken by using the pulse current electrodeposition method. The as-deposited laminar Ir coating is quite smooth(Ra1.01±0.09μm) with extremely high degree of preferred orientation of <111>, and its laminar structure is well developed with clear boundaries and uniform thickness of sub-layers. The Ir coating was aluminized by pack cementation. By adjusting the temperature, both single-layer Ir Al(600°C) and double-layer structure with an Ir Al inner layer and an Ir Al2.7(3) outer layer(700~800°C) can be obtained on Ir coating. The oxidation resistance of the aluminized Ir coating is improved notably compared with that of the pure Ir coating. After oxidation tests at 1850~1900°C for 90 min and 100 min, the weight losses of aluminized Ir coating and pure Ir coating were 0.372 mg/cm2 and 81.1 mg/cm2, respectively. The as-oxidized aluminized Ir coating exhibits multilayer structure of Ir, Ir+Al2O3 and Al2O3 from inside out. |
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