| Thermal protection materials using in hypersonic vehicle, need to meet the requirement of shape maintaining structure, load bearing and reusability, are subjected to high thermal loads owing to the lifting body structure and extreme aerothermal environment. Zirconium diboride based ultra-high temperature ceramics(UHTCs) are being considered as the promising candidate materials for some key parts of hypersonic vehicle. However, very little attention has been paid to the oxidation behavior of UHTCs at simulated service environment and the influence of surface oxidation on performance. In the present thesis, the oxidation behavior at simulated service environment and the microstructural evolution of oxide scale have been studied. The oxidation kinetic model of Zr B2-Si C ceramic was built, and the oxidation mechanism was also analyzed. Furthermore, the effects of surface oxidation on flexural strength, thermal shock resistance, surface grinding performance, radiative properties and temperature response were investigated in detail.The oxidation behavior and oxidation resistance of Zr B2-Si C ceramic were strongly dependent on both pressure and temperature. The total pressure was responsible for the evaporation of amorphous glass, and the oxygen partial pressure influenced the variation of oxidation reactions and the amount of oxidant. As the pressure decreased, a transition from passive to active oxidation can be found, and the thickness of oxide scale increased first and then gradually decreased.The thickness of oxide scale for Zr B2-Si C ceramic increased significantly after oxidation in atomic oxygen. Oxygen atoms had high reaction activity and promoted the oxidation of ceramic. However, the oxidation products, surface morphology and microstructure of oxide scale in atomic oxygen were not fundamentally different from those in molecular oxygen.The oxidation model of Zr B2-Si C ceramic was dependent on both temperature and oxygen partial pressure. The critical oxygen partial pressure of passive/active transition increased with temperature. The oxidation of additive Si C had a significant influence on the oxidation behavior and oxidation resistance of Zr B2-Si C ceramic. The oxidation of Si C depended on the oxygen partial pressure in reaction region. At the interface between substrate and oxide scale, oxidant was preferentially reacts with Si C grains. Owing to low oxygen partial pressure, the active oxidation of Si C resulted in the gradual shrinking of grains, and leaving large amounts of pores in oxide scale.The thickness of oxide scale of Zr B2-Si C ceramic as a function of oxidation time followed parabolic kinetics. Nevertheless, the influence of oxidation temperature was more significant than that of time. After oxidation at 1700°C in air, many pits and bumps were dispersed on the surface of scale, and the surface was rough. The thickness of oxide scale increased significantly accompanied by the existence of depleted layer. There were large cracks in the scale of specimen oxidized at 1900°C, and the reliability of oxide scale degenerated significantly when the specimen was oxidized at 2000°C.The oxidation kinetic model of Zr B2-Si C ceramic was built, and the effects of initial value, time steps and porosity on thicknesses of oxide scales have been analyzed. The calculated thicknesses of oxide scales with oxidation time and temperature were consistent with the experimental results. Additionally, the vapor pressures of oxidation products, glass viscosity, diffusion coefficient, permeability coefficient, porosity of oxide scale and velocity of the ambient fluid affected the thickness significantly. The calculation error could be partially attributed to the escape of large amounts of bubbles from surface glass. The bubbles containing gaseous products could accelerate the volatilization of glass and transmission of oxidation products, which enhanced the oxidation of ceramic.The oxidation products, especially surface glass, were beneficial for the flexural strength of Zr B2-Si C ceramic. However, the strengths of specimens coated by resin or B2O3 glass were not fundamentally different from original strength. The flexural strengths of specimens oxidized at 900°C were greater than those oxidized at 1300°C, indicating that thick oxide scale was detrimental to the strength owing to the presence of flaws in scale. The flexural strength of ceramic after oxidation was not sensitive to oxidation time within a certain range. In addition, due to the coefficient of thermal expansion mismatch, compressive stress on top surface of specimen could improve the mechanical properties. For Zr B2-Si C-G ceramic, the variation of flexural strengths of specimens after oxidation was approximately consistent with that of Zr B2-Si C ceramic, except that the strengths of oxidized Zr B2-Si C-G specimens after immersion were lower than original strength. Because of the poor stability for B2O3 glass, Zr B2-Si C-G ceramic should be oxidized above 1200°C.The surface oxidation was beneficial to improve the thermal shock resistance of Zr B2-Si C ceramic. The critical temperature difference of Zr B2-Si C ceramic oxidized in air was greater than other specimens, whereas the specimen oxidized at 1500°C under 500 Pa had biggest residual strength retention rate. The critical temperature difference of Zr B2-Si C-G ceramic after surface oxidation at 1300°C increased with oxidation time.The emissivity of Zr B2-Si C ceramic oxidized in air was above 0.74 due to the existence of glass on the surface. In contrast, the emissivity of specimen oxidized at 1500°C under 1000 Pa was small in the wavelength range of 2.5~25 μm. The change of emissivity for original specimen from 300°C to 900°C could be attributed to the surface oxidation. The emissivity of original specimen increased significantly when the specimen was oxidized at 700°C. As the temperature increased further to 900°C, the emissivity of original specimen is roughly equal to that of specimen oxidized in air. The emissivity of all of specimens increased with temperature.Under the same ablation condition, Zr B2-Si C original specimen and the specimen oxidized in air exhibited low temperature and good ablation resistance, however, the surface temperature of specimen oxidized at 1500°C under 100 Pa increased above 2300°C. This different temperature response of Zr B2-Si C ceramic could be attributed to the combination of oxidation, heat conduction and radiation associated with porous Zr O2 exposing on surface. When Zr B2-Si C original specimen immerged into atomic oxygen, a temperature jump could be observed. At the same time, the surface temperature of pre-oxidized specimen maintained steady. The main reason of temperature jump in atomic oxygen could associate with oxidation reaction, which released large amounts of energy on the surface. |
PDF Full Text Request