| Due to the desirable combination of high melting temperature, high thermal conductivity, good thermal shock resistance and oxidation resistance, ZrB2-based ultra-high-temperature ceramics have become the most promising candidates for use in thermal protection systems and propulsion systems for hypersonic flight vehicles and rockets. However, due to the low quality raw powders and deficient high-temperature properties evaluation, the practical applications of ZrB2-based ceramics in extremely ultra-high temperature environments are limited. Above these issues will be carried out in this thesis. Firstly, high quality ZrB2powders were synthesized by boro/carbothermal reduction and new borothermal reduction methods, and the effects of carbon and boron impurities from powders on densification, microstructure and mechanical properties of ZrB2-based ceramics were discussed. Then ZrB2-based ceramics were prepared by hot pressing using commercial or self-synthesized ZrB2powders, and high-temperature properties including thermal stability, creep behavior and oxidation resistance were systematically investigated. Detailed research contents and results were summarized as follows:(1) Using ZrO2, B4C and graphite as raw materials, ZrB2powders were synthesized by boro/carbothermal reduction in vacuum at1650℃. According to thermodynamic calculations and XRD results, the reaction processes of the boro/carbothermal reduction were analyzed. The particle size of synthesized ZrB2powders was0.88μm. The oxygen content and carbon content were0.70wt%and0.39wt%, respectively. Compared with commercial powders, self-synthesized ZrB2powders have relatively high sinterability due to small particle size and low oxygen content. Then ZrB2-SiC ceramics were prepared by hot pressing using as-synthesized ZrB2and commercial SiC powders. The influence of carbon impurities coming from ZrB2powders and graphite-rich sintering environment on phase composition, microstructure, and mechanical properties of ZrB2-SiC ceramics was studied. Results showed the carbon impurities from ZrB2powders removed the inherent oxygen impurities, refined the microstructure and increased the hardness, but degraded the toughness. The carbon impurities from sintering environment induced the formation of secondary ZrC phase, maintained the coarse microstructure, and degraded the toughness. This study provided two suggestions on preparation of ZrB2-based ceramics:(a) A refined microstructure can be obtained by controlling the carbon impurities content from the starting ZrB2powders;(b) Densification should be carried out at lower temperature to avoid the carbon uptake from sintering environment.(2) Submicrometer ZrB2powders with low oxygen content were synthesized by a new borothermal reduction route in vacuum using ZrO2and boron as raw materials. By exploring the conventional borothermal process, it was found that the coarsening of ZrB2powders mainly depended on the presence of boron oxides. Based on this discovery, a new borothermal reduction route was developed, namely, two-step reduction plus intermediate water-washing. This new route included the borothermal reduction of ZrO2with boron at1000℃for2h, water-washing process, and the removal of residual oxygen impurities at1550℃for1h. Using conventional process, the particle size and oxygen content of ZrB2powders were2.3μm and0.68wt%, respectively. Based on the new route, the particle size and oxygen content were0.6μm and0.40wt%, respectively. Then, ZrB2ceramics were prepared by pressureless sintering using as-synthesized ZrB2powders with boron impurities, and the influence of boron on densification and microstructure was studied. The1.2wt%boron increased relative density from54.3%to96.1%after sintering at2000℃, whereas it also led to an exaggerated grain growth from1-3to20-80μm. The faster grain growth led to the formation of intragranular pores, and prevented the ZrB2ceramics from reaching full density. With the presence of3wt%boron impurities, a fully dense ZrB2ceramics with relative density of99.7%and grain size of5-10μm was obtained after sintering at2000℃. The improvement of densification was attributed to the synergistic effects of boron by removing the oxide impurities and pinning the grain-boundary.(3) Thermal stabilities of three kinds of ZrB2-based ceramics (ZrB2-SiC, ZrB2-SiC-Yb203and ZrB2-MoSi2) were evaluated at2000℃in argon by high temperature heat treatment method. Firstly, the effect of heat treatment on microstructure and mechanical properties of ZrB2-SiC containing10vol%,20vol%and30vol%SiC was investigated. The SiC with higher volume fraction strongly inhibited ZrB2grain growth during both sintering and heat-treatment processes, and improved hardness and fracture toughness. The grain growth kinetics analysis showed that the ZrB2matrix-coarsening rate was controlled by grain-boundary pinning of the second-phase SiC particles, and the ZrB2grain growth rate in theZrB2-30vol%SiC was25times lower than that for ZrB2-10vol%SiC. Preliminary investigation showed that the hardness and fracture toughness of ZrB2-SiC were not influenced by the heat treatment. Secondly, the effect of heat treatment on densification, phase composition, microstructure, and mechanical properties of ZrB2-20vol%SiC-3vol%Yb2O3was investigated. The heat treatment led to the depletion of Yb-containing phases by decomposition and evaporation, the formation of some pores, and the change of fracture mode from mixed inter/transgranular to fully transgranular. Compared with ZrB2-SiC ceramics, the Yb2O3-doped ZrB2-SiC ceramics had more significant grain growth under same heat treatment condition. Due to the formation of pores, significant grain coarsening and the change of fracture mode, the hardness, fracture toughness and flexural strength of ZrB2-SiC-Yb2O3were severely degraded. Finally, the effect of heat treatment on microstructure of ZrB2-20vol%MoSi2was investigated. After heat treatment, the materials changed from uniform structure to multiphase layered structure. From surface to center, the layered structure consisted of (a) a surface Mo layer,(b) a subsurface Si layer,(c) a dense ZrB2-MoSi2layer and (d) a porous layer of ZrB2-MoSi2from which MoSi2had been partially depleted. Based on the present results, it was found that the ZrB2-SiC ceramics showed relative good high-temperature stability among three ZrB2-based ceramics.(4) Four-point flexure creep behavior of ZrB2and ZrB2-SiC ceramics was studied in argon atmosphere at high temperature by home-made apparatus. Firstly, the creep behavior of hot-pressed ZrB2ceramics with WC sintering aid was characterized at1900℃under a static load of25MPa as a function of time (0-60min). The WC induced the formation of the W-containing phase containing W, Co, Zr, C and O by EDS analysis. After60min bending deformation, large strain (about8%) was achieved without fracture, suggesting excellent high temperature deformation property. On the tensile side, many cavitations were observed, and the W-containing phase inhibited the coalescence of cavities by its deformation. Then, the creep behavior of hot-pressed ZrB2-30vol%SiC ceramics was characterized under a static load of19MPa as a function of temperature (1500-1600℃) and time (0-100h). During the steady-state stage (16-100h), the creep rate at1600℃was3.7times higher than that at1500℃. Some cavitations were nucleated at the triple junctions. These cavitations were always associated with SiC particles due to the fact that SiC particles acted as the hard phase. The cavitations in both size and number showed no obvious difference from26to100h at1500℃, whereas that showed a significant increase from26to100h at1600℃. Present study suggested that ZrB2-SiC exhibited good creep resistance at1500℃in argon atmosphere.(5) Static oxidation behavior at1500℃in air for ZrB2-10vol%SiC (ZB10S) and ZrB2-30vol%SiC (ZB30S) ceramics, and the effect of oxidation on the room temperature flexural strength were investigated. After oxidation, the oxide scale was composed of an outer glassy layer and an inner extended SiC-depleted layer. ZrO2inclusions were found in the glassy layers of ZB10S and almost no ZrO2inclusions were observed in ZBS30, due to the difference of the B2O3content in glassy layer. The changes in weight gain, glass layer thickness, and extended SiC-depleted layer thickness with oxidation were measured. It was found that the extended SiC-depleted layer was the most suitable to evaluate the oxidation resistance. Based on thickness of the extended SiC-depleted layer, the oxidation of the ZB10S followed parabolic kinetics, while the oxidation of the ZB30S exhibited cubic kinetics, indicating that the oxidation resistance of ZB30S was better than that of ZB10S. The improved oxidation resistance in ZB30S was attributed to the higher viscosity and dense of the glassy layer. Because of the healing of surface flaws by the outer glassy layer, the flexural strength increased significantly by110%for ZB10S and by130%for ZB30S after oxidation for0.5h. |
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