【作者】 张守明；

【导师】 陈朝辉； 王松；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2012， 博士

【摘要】 以高超声速飞行器、固体姿轨控动力系统和第三代远地点发动机为代表的新一代飞行器及关键部件对材料的耐高温性能提出了严峻的挑战，迫切需要开发具有良好力学性能和抗烧蚀性能的耐超高温材料。针对现有耐超高温材料的不足，本文以锆基耐超高温陶瓷复合材料为研究对象，开展低温熔渗反应工艺制备及其性能研究。首先系统分析了各种锆基耐超高温陶瓷和锆合金性能，设计了低温熔渗反应目标材料体系和浸渗剂种类。选择W-ZrC、ZrB₂-ZrC、ZrB₂材料分别代表金属-陶瓷、陶瓷-陶瓷和单相陶瓷三种材料体系作为目标材料；选择Zr₂Cu合金作为浸渗剂实现熔渗反应低温化。该浸渗剂熔点只有1000℃左右，Zr含量较高，异质元素Cu与目标产物不浸润、不反应，可随着熔渗反应过程中的体积膨胀而从锆基复合材料中挤出，残留的少量Cu在高温条件下还能起到发汗冷却的作用。根据目标材料体系，选择预制体原料，设计预制体结构，分析了熔渗反应可行性。选择WC、B4C和B分别为W-ZrC、ZrB₂-ZrC和ZrB₂基复合材料的熔渗预制体原料，计算出三种预制体的理论孔隙率分别为50.1%、58.3%和49.3%，热力学计算及实验验证了它们都可以与Zr₂Cu在低至1200℃反应生成目标产物。通过研究低温熔渗反应制备W-ZrC复合材料的预制体成型工艺及熔渗工艺，制备得到性能较好的W-ZrC复合材料。最佳工艺条件为：以4μmWC粉为原料，3wt%PCS作粘结剂，在20MPa压力下成型，1600℃烧成制备多孔刚性预制体，与过量Zr₂Cu合金在真空条件下加热到1400℃保温5h。制备得到的W-ZrC复合材料中W相含量为35.4vol%，ZrC相含量为57.7vol%，残余合金为6.9vol%。低温熔渗反应制备的W-ZrC复合材料具有良好的力学性能、抗烧蚀性能和抗热震性能。其断裂韧性为7.0MPa·m1/2，1800℃弯曲强度是室温强度的1.2倍，实现了金属W对ZrC陶瓷的增韧以及ZrC陶瓷对金属W高温力学性能的改进；相对于文献报道的Mo-ZrC、ZrC-SiC等材料，W-ZrC复合材料具有更好的抗热震性能，其热震临界温度为450℃；W-ZrC复合材料的抗烧蚀性能优异，氧乙炔焰烧蚀30s后，其线烧蚀率仅为0.0033mm·s-1，质量烧蚀率仅为0.0012g·s-1。通过研究WC与Zr₂Cu合金在不同情况下的反应产物结构，建立了WC与Zr₂Cu熔体间的熔渗反应机理模型。当Zr₂Cu熔体与WC接触后，受熔体影响，WC中碳原子往外扩散，逐步形成W2C相和W相，最终完全转化成W相。碳原子扩散进入熔体后，与锆原子反应生成ZrC相，弥散分布在熔体中。不饱和ZrC相中含W、Cu元素，C原子饱和后，Cu被挤出，W则在降温过程中脱熔析出。由于孔隙结构的影响，导致局部区域熔体过量或不足。在熔体过量情况下，锆原子扩散进入W晶粒内部形成W2Zr相和W-Zr固溶体。当熔体不足时，W原子扩散进入ZrC晶格形成（W,Zr）C固溶体。首次采用低温熔渗反应工艺制备ZrB₂-ZrC基复合材料，系统研究了原料粒度、粘结剂含量和熔渗温度对复合材料的影响。最佳工艺条件为：150μmB4C粉为原料，15wt%的PCS作粘结剂，20MPa压力成型，1600℃烧成，然后在1200℃真空条件下熔渗过量Zr₂Cu合金3h。制备得到的ZrB₂-ZrC基复合材料中ZrB₂含量为65.0vol%，ZrC含量为28.1vol%，残余合金含量为6.9vol%左右。低温熔渗反应制备的ZrB₂-ZrC基复合材料具有优异的耐高温性能和抗烧蚀性能。其常温弯曲强度为360.3MPa，弯曲模量为200.8GPa，断裂韧性为12.1MPa·m1/2，1600℃氩气保护下高温处理1h后，复合材料弯曲强度提高了30.4%，弯曲模量提高了为26.2%，断裂韧性提高了12.4%。在氧乙炔焰条件下考核300s后，复合材料的质量烧蚀率为-0.00063g·s-1左右，线烧蚀率为-0.0033mm·s-1左右，其烧蚀机理以氧化反应为主。深入研究了B4C与Zr₂Cu的反应机理，具体涉及到界面反应和溶解析出两种机制。当Zr₂Cu熔体接触到预制体中B4C颗粒时，发生表面反应，B原子具有更好的反应活性，首先生成ZrB₂，残余C然后反应生成ZrC相。随着熔体浸渍通过产物层，其活性由于锆原子的消耗而下降，富Cu液体残留在产物晶粒间隙处，为反应传质提供通道。同时，B原子也溶解在富Cu熔体中，Zr原子与这些溶解的B原子反应析出ZrB₂晶粒，这是溶解析出机制。首次采用低温熔渗反应工艺制备了ZrB₂基复合材料。具体工艺是以无定形硼粉为原料，15wt%PCS为粘结剂，100MPa压制成型，1600℃Ar保护下烧成，然后在1200℃真空条件下熔渗Zr₂Cu合金3h。熔渗制备的ZrB₂基复合材料主要由200nm左右的ZrB₂晶粒组成。ZrB₂基复合材料具有较好的力学性能及抗烧蚀性能。其弯曲强度为414.3MPa，模量为183.6GPa，断裂韧性为5.5MPa·m1/2。原料中添加SiC粉后，复合材料具有更好的抗烧蚀性能，其线烧蚀率为0.0013mm·s-1，质量烧蚀率为0.0028g·s-1，烧蚀后试样表面形成的氧化层有利于提高材料的抗烧蚀性能。最后对三种低温熔渗反应工艺及其制备的复合材料进行了总结对比，发现B4C具有较高的反应活性，制备的ZrB₂-ZrC基复合材料成本最低，并且该材料具有较低的密度、优异的断裂韧性和良好的抗烧蚀性能。更多还原

【Abstract】 The extreme operational conditions encountered for hypersonic and space vehiclesas well as rocket propulsion systems present a great challenge to the development ofultra high temperature materials. New and innovative structural materials capable ofprolonged operation at temperatures above2000℃are required for future spacesystems. In this paper, zirconium based ultra high temperature ceramics compositeswere proposed to be prepared by reactive melt infiltration at relative low temperature,furthermore, the properties of the composites were also investigated.Firstly, the W-ZrC, ZrB₂-ZrC, and ZrB₂composites were chosen as the resultingmaterials standing for metal-ceramic, ceramic-ceramic, and single-phase ceramicmaterials, respectively. Zr₂Cu was chosen as the infiltrant, because its melting point isonly1000℃, the zirconium content is74.2wt%, copper can be extruded out of theresulting composites easily, and the residual copper can be used as transpiration cooling.WC, B4C, and boron were chosen as the raw materials for the three preforms, theycould react with Zr₂Cu at1200℃from thermodynamics calculations and experiments.After reactions, solid volumes increased, to fill all pores and keep the shapes of thepreforms, the theoretical porosities were50.1%、58.3%and49.3%, respectively.Preparation of W-ZrC composites by the low temperature reactive melt infiltrationmethod was investigated. The best preform processing was that the raw WC particlesize was about4μm, the preform pressure was20MPa, the content of adhesive PCSwas3wt%. After the preforms were fabricated, they had better to infiltrate with Zr₂Cuat1400℃for5h in vacuum. The resulting W-ZrC composites had a content of5.4vol%W,57.7vol%ZrC, and6.9vol%residual alloy.The low temperature reactive melt infiltrated W-ZrC composites had excellentmechanical properties, ablation resistance, and thermal shock resistance. The fracturetoughness was7.2MPa·m1/2, the1800℃flexural strength was1.2times of the roomtemperature value. The thermal shock resistance was better than the reported Mo-ZrC,ZrC-SiC composites, the thermal shock critical temperature was450℃. When thecomposites were ablated by oxyacetylene flame for30s, the linear ablation rate was0.0033mm·s-1, the mass ablation rate was0.0012g·s-1.The reaction mechanism between WC and Zr₂Cu was investigated. It was foundthat when the Zr₂Cu melt came in to contact with WC, carbon atoms diffused out to form W2C and then W phase. The diffused carbon atoms in melt reacted with zirconiumatoms to form ZrC, dispersed in the melt. Because of the effect of pore structure in theprefoms, zirconium might be excessive or insufficient in some local place. When themelt was excessive, zirconium atoms diffused into the W grains to form W2Zr phase andW-Zr solid solution phase. While the melt was insufficient, tungsten atoms diffused intothe ZrC grains to form （W,Zr）C solid solution.Preparation of ZrB₂-ZrC based composites by reactive melt infiltration at relativelow temperature was firstly investigated. The composites were prepared with150μmB4C particle and15wt%PCS at1200oC had the best properties. They had a content of65.0vol%ZrB₂,28.1vol%ZrC, and6.9vol%residual alloy.The ZrB₂-ZrC based composites had outstanding high temperature resistance andablation resistance. The composites had a flexural strength of360.3MPa, a flexuralmodulus of200.8GPa, and a fracture toughness of12.1MPa·m1/2, after the compositeswere treated at1600oC in argon, the flexural strength, modulus, and toughnessincreased to30.4%,26.2%, and12.4%, respectively. After the composites were ablatedby oxyacetylene flame for300s, the mass ablation rate was-0.00063g·s-1, the linearablation rate was0.0033mm·s-1, and the main ablation mechanism wasthermal-chemical process.The reaction mechanism between Zr₂Cu and B4C involved surface reaction andsolution-precipitation was proposed. When the infltrating Zr₂Cu metal came intocontact with B4C particles in the preform, a surface reaction happened. Boron atom hada better affinity to react firstly to form ZrB₂, the residual carbon then reacted to formZrC layer. As the melt infiltrated though the product layer to the unreacted B4C surface,the activity of the infiltrant decreased with zirconium atoms consumed continually.Copper rich melt was left in the interstice of products and provided an easier route formass transfer, the melt also attacked B4C substrate and made boron dissolve in the melt,zirconium atoms might react with the dissolved boron in the melt to precipitate ZrB₂grains.ZrB₂based composites were also firstly prepared by reactive melt infiltration atrelative low temperature. The amorphous boron powder together with15wt%PCSwere pressured at100MPa and then were sintered at1600℃in argon to produceporous preforms. ZrB₂based composites could be prepared with the preforms andZr₂Cu at1200℃for3h. The composites were mainly composed of200nm ZrB₂grains. The mechanical properties and ablation resistance of ZrB₂based composites werealso investigated. The composites had a flexural strength of414.3MPa, a flexuralmodulus of183.6GPa, a fracture toughness of5.5MPa·m1/2. When the compositesprepared with SiC filler, the ablation resistance increased, they had a linear ablation rateof0.0013mm·s-1and a mass ablation rate of0.0028g·s-1. The oxidation coating on thesample was useful to resistance ablation more.The processing and the three composites were compared with each other at last. Itwas found that B4C was the most active, the cost of frabrication of ZrB₂-ZrC basedcomposites was the lowest, and the composites had a low density, a good fracturetoughness and ablation resistance.更多还原