Synthesis And Oxidation Resistance Of Multicomponent Single Phase Carbide And Boron Containing Carbide Ceramics

Due to the ultra-high melting point,high hardness,high stability and good high-temperature strength,carbide ultra-high temperature ceramic materials are potential candidates candidate materials for ultra-high temperature thermal protection system.However,poor oxidation resistance of traditional carbides at high temperatures restricts their applications.In order to improve the oxidation resistance of carbides and broaden the application temperature range of carbide ultra high temperature ceramics,so as to meet the needs of thermal protection materials,a series of novel multicomponent single phase carbide and boron containing carbide ceramics were synthesized by designing and regulating the metallic and non-metallic components in the carbides.The effects of metallic components（Zr,Ti,Hf and Ta）,nonmetallic components（C and B）and carbon vacancy on the oxidation behavior of multicomponent single phase carbide and boron containing carbide ceramics,and the mechanisms of improving the oxidation resistance were clarified.The main innovative research results were as follows:（1）The oxidation behavior of non-stoichiometric（Zr,Ti）C_x（x=0.7～1.0）powders was investiagted.The mechanisms of enhancing oxidation resistance of C and Ti components were clarified.The onset oxidation temperature of(Zr_0.8Ti_0.2)C_0.8 was 672±2℃,which was higher than those of(Zr_0.8Ti_0.2)C_1.0（573±3℃）and Zr C_0.8（599±5℃）,showing better oxidation resistance.The decrease of C content reduced the cracks and pores in the oxide scale and hindered the diffusion of oxygen into the carbide matrix.In addition,the existence of carbon vacancy decreased the free energy of Zr C_x with the increase of temperature,which improved the crystal structural stability of Zr C_x.However,too many carbon vacancies increased the vacancy channels,leading to the reduction of oxidation resistance.The addition of Ti resulted in formation of t-（Zr,Ti）O₂ solid solution in the oxide scale,which further reduced the formation of cracks and improved the oxidation resistance.Meanwhile,the carbon diffusion in(Zr_0.8Ti_0.2)C_0.8 was improved by the substitution of Ti for Zr,and a carbon layer with a thickness of 9±1 nm was formed on the surface of the matrix after oxidation at 500℃,which hindered the diffusion of oxygen.（2）The effect of metallic atom composition on the oxidation behavior of non-stoichiometric（Zr,Hf,Ti）C_x quaternary system carbide powders was systematically studied.It was found that the oxidation process of carbides was controlled by oxygen diffusion or phase-boundary reaction,which was affected by the different metallic atom compositions.Meanwhile,due to a high Hf content among the metallic atomic composition in(Zr_0.3Hf_0.5Ti_0.2)C_0.8,it was beneficial to the formation of amorphous Zr-Hf-Ti-C-O oxycarbide layer,inhibiting oxygen diffusion and improving the oxidation resistance of carbides in the oxygen diffusion controlled stage.When the content of each metallic component was equal in(Zr_1/3Hf_1/3Ti_1/3)C_0.8,the growth rate of t-（Zr,Hf,Ti）O₂ was reduced and the phase stability of oxide was improved.The oxidation activation energy of oxidation in the phase-boundary reaction controlled stage was increased.（3）A novel solid phase diffusion process of boron atom method was invented.The element powder was firstly reacted at high temperature and then B₂O₃ was used as boron source for solid phase diffusion.Based on the optimization of sintering process,a series of high relative density multicomponent single phase boron containing ceramics were synthesized.In addition,it was confirmed that B improved the oxidation resistance of（Zr,Ti）C_xB_y quaternary ceramics at 1600℃in air.The oxidation rate constant（k_p）decreased from 67.0±5.9 mg²/（cm⁴·min）of(Zr_0.75Ti_0.25)C_0.82 to 46.9±2.7 mg²/（cm⁴·min）of(Zr_0.75Ti_0.25)C_0.82B_0.06.（4）The effect of metallic component on oxidation resistance at high temperature of（Hf,Ta）C_xB_y quaternary ceramics was discussed.The mechanism of high temperature oxidation resistance of quaternary ceramics with a high Hf content was revealed.The weight gain of(Hf_0.75Ta_0.25)C_0.88B_0.09 was only 28.7%after oxidation at 1600℃for 60min in air,and the k_p was 6.5±0.5 mg²/（cm⁴·min）.However,(Hf_0.2Ta_0.8)C_0.88B_0.07 with a low Hf content was completely oxidized under the same conditions for 20 min.The dense and high thermal stability oxide layer was consisted of Hf₆Ta₂O₁₇ as skeleton and a small amount of Ta₂O₅ and B₂O₃ filling pores,and thus improved the oxidation resistance of ceramics.（5）The relationship between the microstructure evolution and high temperature oxidation resistance of（Zr,Hf,Ti）C_xB_y quintuple ceramics was studied.The(Zr_0.5Hf_0.25Ti_0.25)C_0.88B_0.04 ceramic was initially oxidized to form Ti-rich（Zr,Hf,Ti）C_xB_yO_1-x-y,（Zr,Hf）O₂ and amorphous carbon,followed by the formation of（Zr,Hf,Ti）O₂,Ti O₂,B₂O₃ and graphite carbon.The interlayer composed of these oxidation products exhibited a strong resistance to oxygen diffusion.The oxygen content was decreased by 41.3%when it diffused through the interlayer.(Zr_0.5Hf_0.25Ti_0.25)C_0.88B_0.04 was completely oxidized at 1600℃for 180min in air and k_p was 11.4±1.0 mg²/（cm⁴·min）,whereas(Zr_0.5Hf_0.25Ti_0.25)C_0.88 was completely oxidized after 20 min under the same conditions.