(Ta,Hf)C超高温陶瓷的水基凝胶浇注成型及常压放电等离子烧结研究

Ultra-high temperature ceramics (UHTCs) are ideal materials for use in key hot components, such as nose cones and sharp leading edges of hypersonic aircrafts. Development of near-net-shape forming in combination with high-efficiency sintering technologies to fabricate UHTC components with complex shapes and high precision has now become a seriously urgent issue to tackle. The purpose of this project is to prepare novel (Ta,Hf)C UHTCs through the combination of aqueous gelcasting with pressureless spark plasma sintering technique. Considering the problems of high densities of TaC and HfC powders and their poor stability during aqueous dispersion, inner together with outer coatings are proposed in the present project for the preparation of aqueous TaC-HfC slurries with high solids loading and low viscosity. The stabilizing mechanisms of the highly dispersed TaC-HfC slurries will be revealed. Gelcasting will be conducted subsequently to obtain highly dense and homogeneous green bodies. Finally, pressureless spark plasma sintering technique will be adopted to achieve rapid densification in (Ta,Hf)C ceramics. The thermo-physical properties, and mechanical properties at both room and high temperatures will be studied. The sintering kinetics will be illustrated, and the effects of solid solutioning, electric and plasma fields on the densification progress will be investigated. The project has great theoretical value and scientific significance on the near-net-shape and high-precision forming, as well as high-efficiency sintering technologies of novel (Ta,Hf)C UHTCs.

超高温陶瓷是高超声速飞行器的鼻锥、前缘等关键热端部件的理想材料。复杂形状、高精度超高温陶瓷部件的近净尺寸成型和高效烧结则是亟待解决的难题。本项目旨在通过联用水基凝胶浇注成型与常压放电等离子烧结来制备新型(Ta,Hf)C超高温固溶体陶瓷。针对TaC、HfC密度大及其在水溶液中悬浮稳定性差的问题，通过对粉体表面进行内外两层包覆改性来实现高固相含量、低粘度水基浆料的制备，并揭示TaC-HfC高分散浆料的微观稳定机制，以此为基础通过凝胶浇注成型制备高致密度、高均匀性的TaC-HfC陶瓷素坯。最后，采用常压放电等离子烧结的手段实现(Ta,Hf)C陶瓷的快速致密化并研究其热物理性能、常温及高温力学性能。阐明该体系的烧结动力学机理，研究高温固溶过程、电场和等离子体对致密化进程的具体影响方式。该项目研究对于高性能(Ta,Hf)C超高温陶瓷的近净尺寸、高精密成型以及高效烧结具有重要的理论价值和科学意义。

立方相碳化钽铪(Ta,Hf)C由碳化钽和碳化铪（TaC和HfC）在全化学计量比范围内固溶得到，其固溶体系中Ta0.8Hf0.2C熔点接近4000°C，是目前高温热稳定性最佳的超高温陶瓷材料，有望被用作高超声速飞行器耐热烧蚀结构件。作为耐热烧蚀结构件，(Ta,Hf)C基超高温陶瓷应具有高致密、高均匀的微结构、良好的力学性能。但TaC、HfC均具有极强的共价键特性且自扩散系数低，使得它们难以原位固溶并致密化。.本研究围绕着(Ta,Hf)C基超高温陶瓷的固溶行为、陶瓷烧结、微结构调控、性能评价及提升的主线，从理论和实验两方面着手，系统地研究了(Ta,Hf)C基超高温陶瓷“组成－结构－性能”之间的构效关系，建立高强韧、高温材料与防热结构一体化设计原理与方法。主要研究进展如下：.（1）通过无压烧结工艺，分别采用直接外加(Ex-situ)和原位引入(In-situ)的方式，制备了Ta0.8Hf0.2C-SiC复相陶瓷.复相陶瓷的致密度随亚微米级Ex-situ SiC的增加而提高。加入10vol%SiC的样品致密度可达99.6%；主相Ta0.8Hf0.2C晶粒被抑制长大：利用HRTEM观察到SiC与固溶相间存在扩散能垒较高的过渡层，可得SiC颗粒的钉扎作用影响TaC和HfC间固溶与致密化的物质输运的推论；.利用HfSi2与碳黑反应原位引入了高长径比SiC，制备了性能连续可调的Ta0.8Hf0.2C-SiC复相陶瓷，并考察了综合性能。.（2）通过优选烧结助剂（裂解碳、Cr3C2），制备出相对密度高达97%的(Ta,Hf)C超高温陶瓷.深入探究了在常压固相烧结进程中，TaC和HfC的高温原位固溶行为其致密机理。由酚醛树脂得到的裂解碳在高温烧结后石墨化，富碳环境下TaC和HfC的固溶传质过程由晶界扩散主导。TaC和HfC的原位固溶过程持续温度区间为[1600oC, 2200oC]，固溶过程降低了晶界扩散激活能，促进了Ta0.8Hf0.2C的致密化；.利用Cr3C2在TaC/HfC中固溶度不同，在1800oC，使用2mol%Cr3C2添加成功制备出致密度高达97%的(Ta,Hf)C固溶相陶瓷，烧成样品的平均晶粒尺寸约4μm。验证了2000oC保温阶段利于获得固溶完全的(Ta,Hf)C陶瓷，并探究了Cr3C2作为瞬时液相烧结的致密化机理，显著降低了(Ta,Hf)C陶瓷的烧结致密化温度。