| With the rapid development of hypersonic vehicle,its flight speed and duration are constantly upgraded,which arises extremely drastic aerodynamic heating.Therefore,much harsher requirements are placed on thermal protection system to ensure the safety of the aerocraft.At present,the service temperature of traditional oxide rigid insulation tiles is below 1300℃,which cannot meet the present requirement of thermal protection system.Therefore,it is of great significance to develop new ultra-high temperature(>2000℃)heat protection/insulation material systems.Ultra-high temperature ceramics(UHTCs)are excellent high temperature-resistance materials because of their ultra-high melting points.But the high density and high intrinsic thermal conductivity limit their application in ultra-high temperature insulation field.The aim of this thesis is to develop new lightweight ultra-high temperature insulation materials,work on design and synthesis of porous high-entropy ultra-high temperature insulation materials and investigating their oxidation resistance,as well as developing high infrared emissivity coatings on their surface.Ultra-high porosity high-entropy carbide(HEC)(Zr1/5Hf1/5Nb1/5Ta1/5Ti1/5)C was fabricated through foam-gelcasting-freeze drying technology and in-situ pressureless reaction sintering method,by using ZrC,HfC,NbC,TaC and TiC powders as initial raw materials,and SiC powder as additive.The as-prepared porous HEC samples possess outstanding overall performance,including controllable ultra-high porosity(86.4%95.9%),low density(1.27 g/cm3-0.38 g/cm3),high compression strength(11.77 MPa0.70 MPa),low thermal conductivity(0.239 W/(m·K)-0.164 W/(m·K)),and excellent high temperature stability,as well as good oxidation resistance of HEC powders.Compared with compact bulk HEC prepared by the same raw materials and additive,the porous HEC has clean grain boundaries and extremely low thermal conductivity(thermal conductivity of dense bulk HEC:13.12 W/(m·K),thermal conductivity of porous HEC with the porosity of 95.9%:0.164 W/(m·K)).The oxidation behavior of porous high-entropy carbides with porosity of～94%at 1500℃ and oxygen partial pressure of 100 Pa was systematically studied.After oxidation,the surface color of the porous sample changed from silver gray to black gray,and the oxidized layer thickened(261 μm-1263 μm)with the increase of oxidation time(10 min-60 min).After oxidizing for 40 min,cracks appeared between the oxidized layer and internal matrix.The mass change of oxidized samples was linearly correlated with the oxidation time,which is obviously different from the oxidation behavior of the dense bulk.XRD results show that the phase compositions of black gray external surface are mainly solid solution carbide and some oxides;while the interior of oxidized samples is still virgin single-phase HEC.It indicates that the oxidation of porous HEC under low oxygen partial pressure occurs merely in the surface layer of the sample.Then the oxidation mechanism of porous HEC under low oxygen partial pressure is discussed through the particularity of porous structure,the multiple components of HEC and thermodynamic calculation.High melting point ZrO2 and Y2O3 were covered on the skeleton of HEC by vacuum impregnation using oxide sol,and the oxidation resistance of samples after oxide modification was investigated.Compared with the oxidation resistance of ZrO2/HEC and Y2O3/HEC samples under 1500℃ and oxygen partial pressure of 100 Pa,the oxidation resistance was not improved for ZrO2 modified porous HEC sample.For the porous HEC sample modified with Y2O3,the thickness of oxidized layer decreases obviously and the time of cracking between the oxidized layer and internal matrix is delayed,which indicates that modification with Y2O3 has significantly improvement on oxidation resistance.The Y2O3 coated on the skeleton of porous HEC does not react with the matrix,and avoids the direct contact between most of HEC grains and oxygen.Meanwhile,the Y2O3 cladding with low oxygen diffusion coefficient slowed down the diffusion of oxygen into the HEC grains,thus enhancing the oxidation resistance of the sample.To further improve the oxidation resistance of porous HEC,the porous dual-phase high-entropy(DPHE)boron-carbide ceramic was prepared by foam-gelcasting-freeze drying technology,by introducing ultra-high temperature boride components with better oxidation resistance into HEC matrix.Compared with porous HEC,the porous DPHE sample has slightly lower strength and slightly higher thermal conductivity,but it still has excellent comprehensive performance.It includes high porosity of 90.1%96.4%,low density of 0.87 g/cm3-0.31 g/cm3,high strength of 4.17 MPa-0.45 MPa and low thermal conductivity of 0.281 W/(m·K)-0.202 W/(m·K).After oxidation at 1500℃ and oxygen partial pressure of 100 Pa for DPHE samples with the porosity of～95%,the oxide thickness of porous DPHE sample is obviously smaller than that of porous HEC,and there is no separation between the oxidized layer and matrix.Under the low oxygen partial pressure,the preferential oxidation of HEB particles in the surface layer region slows down the oxidation process of DPHE samples,and thus the oxidation resistance is significantly improved.Four high infrared emissivity coatings were developed on porous HEC surface by slurry brushing method,by using MoSi2 as the main radiation agent,LaB6 and SiB6 with high emissivity as dopants.All as-prepared coatings are uniform and complete,and there is a transition zone and no obvious interface between the coating and the porous HEC matrix.Four coatings all exhibit high interfacial bonding strength(2.33 MPa-4.21 MPa)and high emissivity(≥0.856).And the coating doped with LaB6 has the highest emissivity(room temperature:0.934 and 300℃:0.946).In addition,the emissivity of each coating at 300℃ is higher than room-temperature emissivity,indicating that four coatings all have excellent high-temperature infrared radiation performances. |
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