Preparation And High Temperature Anti-corrosion Properties Of Hafnium-based Composite Oxides

Owning to high melting point,low thermal conductivity,low oxygen permeability,good thermal and chemical stability,hafnium-based composite oxide ceramics are ideal candidates for new generation thermal barrier ceramics and oxygen-resistant materials.With the development of aerospace engines with high thrust-weight ratio and reentry vehicles,the development and performance evaluation of new generation high temperature materials are imperative.Herein,hafnium-based composite oxide ceramic powders were prepared by low-temperature solution combustion method.The corresponding ceramic and coating were prepared by high-temperature solid-state sintering and supersonic plasma spraying.The high-temperature properties of hafnium-based composite oxide ceramics were studied by high-temperature molten salt corrosion,reaction with Al₂O₃,thermal shock and ablation environment.The main research contents and results of this paper are as follows:Take Y₂Hf₂O₇ as the object,The effects of urea,glycine,glucose,citric acid and ethylenediaminetetraacetic acid（EDTA）as burners on the synthesis of hafnium-based composite oxide ceramics were studied.The results show that all kinds of precursors have formed coordination structures.Y₂Hf₂O₇ ceramic powders with good crystallinity were successfully prepared by combustion process and subsequent high temperature heat treatment of various precursors.It was found that the combustion reaction of urea precursors started at about 256°C,which is the lowest initial ignition temperature.Using urea as burning agent.The crystallinity of ceramic powders is better prepared at the ignition temperature of 550°C,and a higher ignition temperature has no better effect on the crystallinity of ceramics.La₂Hf₂O₇ and Sr Hf O₃ ceramic powders with good crystallinity were prepared by solution combustion using urea as combustion agent and starting temperature of 550°C.Y₂Hf₂O₇,La₂Hf₂O₇ and Sr Hf O₃ ceramics were prepared by pressureless solid-state sintering at 1600°C.X-ray Diffraction（XRD）and Raman measurements confirm that Y₂Hf₂O₇ has defective fluorite structure,La₂Hf₂O₇ has pyrochlore structure and Sr Hf O₃ has perovskite structure.The thermal physical properties of three kinds of ceramics were studied:The maximum thermal expansion coefficients of ceramics were 10.37,9.23 and 10.68×10^-6K^-1 from room temperature to 1300°C,respectively;The thermal conductivities ranged from room temperature to 1400°C are 2.78-1.34WM^-1·K^-1,2.92-1.44 WM^-1·K^-1 and 3.0-1.39WM^-1·K^-1,respectively;The typical thermal conductivities of 1200°C are 1.59,1.44 and 2.02WM^-1·K^-1,respectively,and the thermal conductivity of La₂Hf₂O₇is more stable at high temperature.La₂Hf₂O₇ and Sr Hf O₃ were selected to study the corrosion performance of hafnium-based composite oxide ceramics under high temperature molten salt.900,1050 and1200°C were chosen as corrosion temperature.Na₂SO₄,V₂O₅ and Na₂SO₄+V₂O₅（molar ratio is 1:1）were chosen as corrosives and the heat treatment atmosphere is air.The results show that there is almost no chemical reaction between Na₂SO₄ and La₂Hf₂O₇ ceramic during the tested temperature range.When La₂Hf₂O₇ ceramics are corroded in V₂O₅ and mixed molten salt,abundant La VO₄ octahedral particles appear on the surface,and the grain size of La VO₄ increases with the increasing temperature,while the grain size of Hf O₂ does not grow significantly.During corrosion of V₂O₅ molten salt,flocculent Sr₃V₂O₈ grains appeared on the surface of Sr Hf O₃ ceramics,and the accompanying Hf O₂ grains grew into rod-like grains with the increase of corrosion temperature;under mixed molten salt corrosion,the growth of Hf O₂grains was not obvious,but the flocculent Sr₃V₂O₈ grains grew rapidly into plate-like grains almost penetrating the whole corrosion layer.Na VO₃ with lower melting formed under the reaction between Na₂SO₄ and V₂O₅,which promotes the migration of Sr ions,thereby resulting in the rapid growth of Sr₃V₂O₈ grains and inhibiting the growth of Hf O₂.In corrosion environment,the concentration of Hf,La and Sr-rich layers makes the stress distribution uneven,and the stress concentration accelerates the degradation of ceramic specimens.Constructing La₂O₃-Al₂O₃-Hf O₂ complex phase ceramics to study the high temperature chemical stability of hafnium-based composite oxide ceramics.La₂O₃-Al₂O₃-Hf O₂ ceramics were prepared by solid-state method.XRD results showed that new phases of La Al O₃,Hf O₂and La Al₁₁O₁₈ are formed after the reaction of Al₂O₃with La₂Hf₂O₇ at high temperature.The reaction process was studied by TG-DSC,which exhibits that La Al O₃ phase formed near1149°C and La Al₁₁O₁₈ phase appeared near 1319°C.By means of Scanning Electron Microscope（SEM）and Energy Dispersive Spectroscopy（EDS）analysis,the introduction of Al₂O₃ promotes the La₂Hf₂O₇ grain fusion.La Al O₃ phase appears on the interface.With the increase of Al₂O₃ content,the interfacial phase hinders the further diffusion reaction,and La Al₁₁O₁₈ phase appears in the core region of Al₂O₃.The density of ceramics decreases with the increase of Al₂O₃ content.Contrasting the thermal conductivity and thermal expansion properties of La₂O₃-Al₂O₃-Hf O₂ ceramics,it was found that the thermal expansion coefficient increased gradually with the increase of alumina content.The thermal expansion coefficient ranged from 9.2 to 9.7×10^-6K^-1 at 1300°C,while the thermal conductivity of La₂O₃-Al₂O₃-Hf O₂ ceramics is non-line.When the composited ceramics with the ratio of La₂Hf₂O₇ and Al₂O₃ is 1:0.25 has the lowest the thermal conductivity and the thermal conductivities range from room temperature to 1400°C is about 1.76-1.12 W m^-1·K^-1.Y₂Hf₂O₇ ceramic coatings were prepared by supersonic atmospheric plasma spraying（SAPS）using its ceramic powders to study the performance of hafnium-based composite oxides in high temperature thermal shock and ablation protection.Firstly,Y₂Hf₂O₇ coatings were prepared on Si C coated C/C composites with distinct spraying power of 40,45,50 and55 k W.The effects of spraying power on the microstructure and mechanical properties of the coatings were studied.Combining XRD,Confocal Microscopy,SEM and scratch mechanics test,power has little effect on the phase composition of the coating,but the coating prepared at 50 k W has the best microstructure and mechanical properties,and its bonding force reach27.2 N.The thermal shock stability of air-cooled and water-cooled of Y₂Hf₂O₇ coatings during 1200°C and room temperature was studied.After 40 thermal shock cycles,the phase composition of Y₂Hf₂O₇ coatings remained unchanged,and the interface of composite coatings remained relatively intact.Under low heat flux(2.38 MW·m^-2)ablation environment,the mass and linear ablation rates were 0.16μm·s^-1 and 0.028 mg·s^-1after ablation for 40 s,respectively.Under the long-term ablative environment,the outer coating was eroded by the oxidation products of inner coating,and the volatilization of low-temperature glass phase and the thermal stress concentration eventually lead to the failure of the coating...