Study On Design,Preparation And Properties Of High Entropy Ceramics And Their Application In Thermal Protective Coatings

With the development of aerospace exploration,aircraft will face more complex and changeable service environment.The new generation of thermal protection materials will develop in the direction of lightweight,wide temperature range,repeatable,etc.The traditional thermal protection materials cannot give consideration to the comprehensive properties of lightweight,oxidation resistance and toughening.In recent years,the integrated thermal protection materials have attracted people’s attention due to their low density,low thermal conductivity and good oxidation and ablation resistance,which is in line with the development direction of the new generation of aircraft.In this paper,the composition system and content of integrated thermal protection materials are designed according to the urgent needs of thermal protection materials.The porous carbon/carbon composite is used as the thermal insulation layer,(Zr_0.25Hf_0.25Ta_0.25Nb_0.25)C is used as as main phase of anti-ablation layer and(Cr_0.2Nb_0.2Ta_0.2Mo_0.2W_0.2)Si₂ is used as auxiliary phase of anti-ablation layer to prepare the integrated thermal protection materials.The stoichiometric deviation is introduced into the high-entropy carbide system to promote the formation ability of single-phase solid solution.Based on the traditional disilicide infrared emitters,the multi-principal components are introduced to improve the infrared emissivity of the disilicide system.The method of preparing porous carbon/carbon composites by hydrothermal reaction is proposed,and the required compressive and thermal insulation properties of thermal insulation materials are studied.The integrated thermal protection materials are prepared,and the key properties of their thermal protection application,such as compression resistance,thermal conductivity,oxidation and ablation resistance,are studied.Based on the principle of thermal expansion matching,the components of the anti-ablation layer are selected.The high-entropy carbides are selected as the main phase.At the same time,the high-entropy disilicides are selected as the auxiliary phase to improve the infrared emissivity of the anti-ablation layer.The first principle calculation is used to select the most easily synthesized(Zr_0.25Hf_0.25Ta_0.25Nb_0.25)C system.The density of thermal insulation layer is determined according to the relationship between thermal conductivity,strength and porosity.The law of thermal conductivity and compressive strength with porosity in porous carbon/carbon composites is revealed.The density of porous carbon/carbon composites is designed to be 0.2 g/cm³.According to the principle of minimum thermal residual stress,the content of each component in the integrated thermal protection composite is designed.The volume fraction of high-entropy carbide/high-entropy disilicide is determined to be 1.59%,and the volume fraction of silicon carbide is 20%.The carbon stoichiometry deviation is introduced into the high-entropy carbide ceramics.By weakening the strong chemical bonds and increasing the atomic diffusion rate,the carbon stoichiometry deviation can increase the formation ability of single-phase solid solution and promote the sintering of high-entropy carbide.The carbon stoichiometry deviation can also control the mechanical properties,electrical conductivity,thermal conductivity and oxidation properties of high-entropy carbide.SPS-ZHTNC_0.8has the best comprehensive properties,with a bending strength of 419 MPa,a fracture toughness of 4.73 MPa·m^1/2,and a high temperature thermal conductivity of 20.34W/（m·K）.High-entropy disilicide ceramics are prepared by introducing multi principal components into binary system disilicide.(Cr_0.2Nb_0.2Ta_0.2Mo_0.2W_0.2)Si₂（HE-2）has the best performance due to its largest lattice distortion.Compared with traditional infrared emitters（Mo Si₂ and Ta Si₂）,the emissivity of the prepared high-entropy transition metal disilicide is substantially improved,and the total infrared emissivity of HE-2 is 0.42,which is about twice that of Ta Si₂.In addition,HE-2 has low thermal conductivity and excellent oxidation resistance.Glucose is used to prepare porous carbon/carbon composites by high efficiency deposition of hydrothermal carbon on the surface of carbon fibers at low temperature.The deposition law of hydrothermal carbon is revealed.The density of porous carbon/carbon composites is adjusted based on the number of reactions,and the compression strength and thermal conductivity of the composites are adjusted on demand.When the density of composites increases from 0.166 g/cm³ to 0.252 g/cm³,the yield strength of the composites increases from 0.2 MPa to 0.7 MPa,the thermal conductivity at room temperature increases from 0.159 W/（m·K）to 0.231 W/（m·K）,and the thermal conductivity at 1000℃increases from 0.481 W/（m·K）to 0.587 W/（m·K）.The integrated thermal protection materials are prepared based on the designed component and content.The mechanical properties,thermal properties and ablation performance of integrated thermal protection materials are examined based on thermal protection applications.The insulation materials present excellent compression strength.The compression strength of the porous material is 14.5 MPa,which is nearly an order of magnitude higher than that of the composites with similar density.When the temperature increases from room temperature to 1000℃,the thermal conductivity of the integrated thermal protection materials gradually increases from 0.38 W/（m·K）to 1.06 W/（m·K）.The integrated thermal protection materials have excellent ablation resistance.After ablation at 1800℃,linear ablation rate is-2.3×10^-4 mm/s,the mass ablation rate is 8.4×10^-5 g/s.The integrated thermal protection materials have high infrared emissivity before and after ablation.The total spectral infrared emissivity of the composites before ablation,after ablation at 1600℃and after ablation at 1800℃are 0.79,0.82 and 0.82 respectively.