Prepared By Pressureless Sintering Behavior Of Tac Ceramic Oxide Ablation

The anti-oxidation and anti-ablation are key performances oftantalum carbide (TaC) ceramic-the ultra-high temperature materials. Inthis paper, TaC ceramic was prepared by the process of pressing andsintering under ordinary pressure; X-ray,SEM,EDS and Tg-DSCanalysis instrument were used to characterized the microstructure and theproperties of oxidation and ablation of TaC ceramic; Finally, theoxidation kinetic and the mechanism were also dicussed in this paper.The optimizing preparation method and process parameter wereobtained on the basis of the experimental investigation. TaC ceramicsintered from C and Ta powder at high temperature was porous becauseof fierce self-propagation reaction at high-temperature. Crack-free TaCceramic with relative density more than 91% could be prepared bypressing and active sintering at 2100℃, with raw powder of 97% TaCgreen powder and 3% C, Ta powder. When increased sinteringtemperature to 2300℃and the content of Ta,C additive to 6%, thedensity and the granularity of TaC ceramic increased.After oxidized at 1000～1600℃, the ceramic changed to Ta₂O₅; andthe microstructure of Ta₂O₅ was porous, with the size of the poreincreased with the oxidation temperature. In 1000～1400℃, the oxidationof TaC was controlled by oxidation reaction when weight gain of samplewas low than 0.3g/cm², with a line relationship between weight gain andthe oxidation temperature; and by oxygen diffusion through the porousoxide when weight gain exceeded to 0.3g/cm² with a parabolicrelationship. The activation energy of reaction was caculated,28.82kJ/mol between 1000℃and 1400℃,and 109.36 kJ/mol between1500℃and 1600℃. Correspondingly, the crystal structure of Ta₂O₅changed fromβ-Ta₂O₅ toα-Ta₂O₅ at 1500℃.Oxidation mechanism of TaC was analyzed through Tg-DSC andSEM techniques. The maxium weight gain of TaC powder was exceed thetheory calculation value, which indicated carbon was retained in powders.The reaction interface of TaC after oxided was observed, Because of Celement existed in TaC/Ta₂O₅ interface, the byproduct of oxidation at interface should be CO. There was a reaction area of CO and O₂ insample. The concentration distribution of CO, O₂ and diffusion modelhad been built on this basis. CO gas was the basic drive to form theporous oxide. The formation mechanism of the porous oxide was alsocombined with the effect of sintering and volume expand after theeconversion of TaC to Ta₂O₅.The ablation performance of TaC ceramic was studied byoxygen-acetylene flame ablation. During the ablation process, the mainablation mechanisms were the oxidation reaction at ultrahigh temperatureand the erosion of high speed airflow in the ablation center area. However,the main effect was the oxidation of ultrahigh temperature in the ablationfringe area, and themal oxidation and evaporation in the transition area.When ablation time was less than 30s, the ablation was mainly controlledby calefaction and themal oxidation process, with the thickness of oxideincreased with ablation time. When ablation time exceeded 30s theablation process was mainly effected by the erosion of high speed airflow,with a constant oxide thickness and weight loss with increasing time.