常压烧结制备TaC陶瓷及其氧化烧蚀行为的研究

【作者】 曾玉林；

【导师】 熊翔； 李国栋；

【作者基本信息】 中南大学 ， 材料学， 2007， 硕士

【摘要】 氧化烧蚀性能是超高温材料研究领域的重点。本文采用模压烧结法制备了TaC陶瓷材料，研究了TaC试样的高温氧化及超高温烧蚀行为，结合X-ray、SEM、Tg-DSC等分析手段对组织结构特点、氧化动力学及氧化烧蚀机理进行了研究。通过实验确定了TaC陶瓷材料的制备方法和工艺。以单质C、Ta粉末为原料压制成型常压烧结TaC时，由于发生自蔓延反应，产物疏松多孔，不能制备致密的TaC陶瓷。以少量（3％）单质Ta、C粉末为烧结助剂活化烧结TaC陶瓷，能够在2100℃烧结制备相对密度达91％的无裂纹TaC材料；提高单质粉末添加量到6％，或将烧结温度提高到2300℃，TaC的密度均有一定的提高，同时TaC晶粒的尺寸增加。TaC陶瓷在1000℃～1600℃范围氧化产物都为多孔结构的Ta₂O₅，并且孔隙尺寸随温度的升高而增大；1000℃～1400^C单位面积的氧化增重小于0.3g／cm²，TaC的氧化为反应控制阶段，增重随氧化时间的延长而增加；增重大于0.3g／cm²，氧化为扩散控制阶段，增重随氧化时间按抛物线关系增长；1500℃～1600℃时整个氧化过程都为反应控制过程，氧化增重随时间正比例增加。计算了各温度TaC陶瓷氧化的反应活化能：1000℃～1400^C活化能为28.82kJ／mol，而1500℃～1600℃高温区为109.36kJ／mol。XRD分析发现，在1500℃产物Ta₂O₅的晶型发生了转变。采用Tg-DSC、SEM等分析了TaC的氧化机制。TaC粉末在Tg氧化过程中的增重最大值超过了理论增重，出现了碳的滞留。TaC块体试样高温氧化后观察到反应界面处的存在无定形状物质，能谱分析为C。在有残留C条件下反应界面处的气体产物只能为CO，因此氧化层中存在向外扩散的CO与向内扩散的O₂发生反应的反应区，并以此初步建立了气体分布与扩散模型。排出副产物CO导致氧化后试样形成疏松多孔结构，结合烧结原理，推测了多孔氧化物的形成机制。采用氧-炔焰烧蚀方法考察了TaC陶瓷的烧蚀行为。在烧蚀过程中TaC烧蚀中心区主要为高温氧化和高速气流的机械冲刷作用，边缘区烧蚀主要为高温氧化作用，过渡区主要为氧化和热蒸发作用；烧蚀开始阶段（少于30s）为加热及热氧化过程，氧化层随烧蚀时间的增加增厚，试样烧蚀后增重；烧蚀30s后，气流冲刷作用才起主导作用，氧化层厚度稳定，试样表现为不断失重。更多还原

【Abstract】 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.更多还原