| TaxHf1-xC ceramics as a solid solution of Ta C and Hf C,have been proposed to have the highest melting point among the known materials as well as higher hardness,better resistance to oxidation and ablation compared with the individual Ta C and Hf C binary ceramics,which is expected to become a new desirable materials for structures to operate at high temperature in the future after the hot ultra-high temperature ceramics,such as Zr B2?Hf B2.However,it is extremely difficult to obtain highly dense,single phase TaxHf1-xC solid solution ceramics,due to the nature of ultra-high melting point and low self-diffusion coefficients.Because of the limitation of material preparation,the basic performance of TaxHf1-xC solid solution ceramics,especially the oxidation and ablation behavior research has not been reported.Therefore,in this work,highly dense and single phase TaxHf1-xC solid solution ceramics have been controllable prepared by a joint process of solvothermal-carbothermal-hot pressing,firstly.Then,six composites of Ta C,Ta0.8Hf0.2C,Ta0.5Hf0.5C,Ta0.25Hf0.75C,Ta0.2Hf0.8C and Hf C,which covering the whole system,were selected as the representative research samples to study the microstructures,basic properties and oxidation ablation behavior of the materials.Finally,the correlations between the materials components and the basic properties,oxidation ablation behavior of the TaxHf1-xC solid solution ceramics have been systematically constructed,which provided data support for the engineering application of TaxHf1-xC solid solution ceramics.Detail research contents are as follows:The controllable preparation of the whole system TaxHf1-xC solid solution ceramics was achieved through joint process of solvothermal,carbothermal and hot pressing.The correlation between the material components and mechanical properties,thermophysical properties was clarified.TaxHf1-xC powders were synthesized successfully through a joint process of solvothermal and carbothermal under the temperature of 1400-2100°C.The content of oxygen impurity in the obtained powders was less than<1 wt.%,and the particle size was about 50nm.Subsequently,using the TaxHf1-xC powders as the raw materials,the sintering of TaxHf1-xC solid solution ceramics were achieved by hot pressing under the condition of 2100°C-70MPa-60min.The relative densities of the sintered samples were all higher than 95%and the ideal single-phase solid solution structures of the sintered TaxHf1-xC ceramics were demonstrated by both XRD and TEM analysis.The mechanical properties of TaxHf1-xC solid solution were systematically characterized,and the results showed that the micro-hardness of TaxHf1-xC solid solution ceramics was between 30-40 GPa,which was more than 50%higher than that of the same samples reported in other papers.Moreover,the results of thermophysical properties indicated that the thermal conductivity of TaxHf1-xC solid solution ceramics decreases with the increase of Hf C content,while the coefficient of thermal expansion increases with the increase of Hf C content.The thermal conductivity was between20-50W·m-1·K-1,and the coefficient of thermal expansion was between 6.79-7.23×10-6/K.The oxidation behavior of TaxHf1-xC solid solution ceramics was systematically studied,and the correlation between oxidation behavior and material composition was established.Results showed that the oxidation rate of TaxHf1-xC solid solution ceramics was significantly lower than that of individual Ta C and Hf C ceramics.After isothermal oxidation at 1500°C for 30min,Ta C were completely oxidized,and the oxidation conversion of Hf C was as high as 0.4,while that of Ta0.5Hf0.5C and Ta0.25Hf0.75C were less than 0.2.And also,the non-isothermal oxidation was carried out at 5°C/min in the temperature range from room temperature to 1400°C.The Ta C and Ta0.2Hf0.8C were completely oxidized at temperature up to 1100°C and 1200°C,respectively,while the oxidation conversion of Ta0.5Hf0.5C and Ta0.25Hf0.75C were both only about 0.3,even the oxidation temperature was increased to 1400°C.The oxidation behavior of TaxHf1-xC solid solution ceramics was analyzed and explained by kinetic calculation.The oxidation rate constant and oxidation activation energy of TaxHf1-xC solid solution ceramics were obtained.With the increase of content of Hf C,the oxidation rate constant of TaxHf1-xC showed a trend of first decreasing and then increasing.The oxidation rate constant of Ta0.25Hf0.75C sample was calculated as2.08-5.27×10-3·s-1,only about one third of 9.15-13.91×10-3·s-1 of Hf C,which was an order of magnitude lower than 38.54-48.03×10-3·s-1 of Ta C.The variation trend of oxidation activation energy of TaxHf1-xC solid solution ceramics with Hf C content was opposite to that of oxidation rate constant,in which the oxidation activation energy of Ta0.25Hf0.75C was the highest,reaching 120 k J/mol.The oxidation behavior of TaxHf1-xC solid solution ceramics was further explained by the phase and microstructure analysis of the reaction products as well as the calculation of the oxygen diffusion coefficient.The phase composition of the oxide layers of TaxHf1-xC solid solution ceramics showed the transformation rule of Ta2O5?Ta2O5+Hf6Ta2O17?Hf6Ta2O17?Hf6Ta2O17+Hf O2?Hf O2 with the increase of Hf C content.And among these,single-phase Hf6Ta2O17 oxide layer had an obvious double-layer structure,the upper layer was relatively loose while the transition layer was continuous and dense.In contrast,the Ta2O5-Hf6Ta2O17 oxide layer had obvious eutectic structure characteristics.The diffusion rate of O2 in different oxidation layers was significantly different.Within the temperature range of 1400-1600°C,the oxygen diffusion coefficient of O2 in Hf6Ta2O17 was only 0.14-0.31×10-6/cm2·min-1,while in Ta2O5 and Hf O2,the oxygen diffusion coeffient was 1.56-4.51×10-6/cm2·min-1 and0.53-1.82×10-6/cm2·min-1,respectively.The ablation resistance of TaxHf1-xC solid solution ceramics was significantly better than that of individual Ta C and Hf C ceramics.Under the condition of low heat flux density of 4.14MW/m2,there would be obvious ablation pits due to the melting of the ablation products in the Ta rich samples of TaxHf1-xC solid solution ceramics.In this case,the linear ablation rate of Ta0.5Hf0.5C was about 0.19?m/s,which was two orders of magnitude lower than that of Ta C(10.76?m/s).By contrast,the Hf rich samples formed an unmelted oxide layer,and the ablation rate of Ta0.25Hf0.75C was-0.33?m/s,which was about 1/5 for Hf C ceramics.Taking Ta0.5Hf0.5C and Ta0.25Hf0.75C as the representatives,the ablation behaviors of Ta rich and Hf rich samples were further compared.The ablation pits was obvious on the two samples surface,while the Ta0.5Hf0.5C showed better ablation resistance.The phase and microstructure of the ablation products were studied,and the volatility diagram of TaxHf1-xC solid solution ceramics in ultra-high temperature environment was constructed.The phase compositions of TaxHf1-xC solid solution ceramics ablation products were basically the same as that of high temperature oxidation products.The reason why Ta0.5Hf0.5C exhibited the best ablation performance is that the dense outer layer of Ta2O5+Hf6Ta2O17 eutectic can effectively prevent the O2diffusion and the porous inner layer can effectively insulate heat.Although the ablative layer of Ta rich samples also had a relatively dense upper layer and porous inner layer,a large number of Ta2O5(1850°C)with low melting point on the upper layer was easy to lose and form holes,and the ablative continues to the interior of the materials.In Hf rich system,due to the lack of Ta2O5,only a losse and porous monolayer structure was formed,which resulted in serious ablation. |
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