【作者】 王莹；

【导师】 朱圣龙； 曾潮流；

【作者基本信息】 中国科学技术大学 ， 腐蚀科学与防护， 2022， 博士

【摘要】 近年来,三元层状硼化物陶瓷Cr2AlB2由于具有纳米层状结构、导电导热性好、模量高、剪切变形阻抗较低、抗高温氧化性能好等优点正日益受到关注,在金属防护涂层、高温电极材料、燃气轮机结构件、吸波等领域具有广阔的应用前景。同时,Cr2AlB2也是制备二维层状材料CrBene的重要前驱体。此外,航空航天、电子技术等的迅速发展对铜提出了更高的要求,即在保证良好导电、导热等性能基础上,也需具有良好的力学性能、减摩耐磨等性能,这已成为电子电器、机械制造等工业领域的研究热点。引入增强相,研发高性能铜基复合材料是当前的热门课题。Cr2AlB2优异的性能使其有望用于铜基复合材料的增强相。制备高纯、结晶良好的Cr2AlB2粉体是急待解决的难点问题,也是开展相关性能研究的前提。目前Cr2AlB2粉体的制备存在工艺复杂、难以合成单相且所得粉体烧结严重等问题。因此探索Cr2AlB2粉体的制备方法对于深入认识其性能,并推动其工业应用无疑具有重要的意义。熔盐合成陶瓷因具有工艺简单、反应温度低、产物形貌可控等特点正日益受到关注。本文利用熔盐歧化反应,并结合熔盐自组装制备粒径均匀的Cr2AlB2粉体。进而采用热压烧结方法制备Cr2AlB2增强铜基复合材料,并研究其力学、摩擦性能。论文的主要研究内容和进展如下:(1)将2Cr/2Al/2B前驱体与等质量的(Na,K)Cl共晶盐混合、压片后,在1100℃保温0.5 h成功合成了较高纯度的Cr2AlB2。熔盐作为反应介质能够加速反应进程。Cr2AlB2粉体在900℃空气中氧化2 h表面能够形成具有保护性的Al2O3膜,故Cr2AlB2具有较优的抗高温氧化性能。(2)将Cr/1.05B与(Na,K)Cl共晶盐均匀混合,在850℃保温2 h能够合成高纯、细小(～1μm)而均匀的CrB粉体。其合成机制如下:首先,Cr粉在熔盐中溶解为Cr2+和Cr3+;随后,Cr2+在B粉表面歧化形成Cr原子;最后,Cr原子在B粉表面原位反应形成CrB。CrB和Al粉混合后在盐料比为15:1的熔盐介质中,于900℃保温2h可合成均匀且细小(～5 μm)的Cr2AlB2粉体。合成机制包括CrB和Al的互迁移、自组装以及Al向CrB的插层。系统自由能最小化是CrB颗粒和Al液滴在高温熔盐中快速装配的驱动力。(3)Cr2AlB2陶瓷粉体和Cu粉在850℃、40 MPa热压烧结1 h即可得到相对均匀、致密的陶瓷增强铜基复合材料。Cu中加入10 vol.%Cr2AlB2后,复合材料的硬度约提高至纯铜的2倍,抗拉强度为纯铜的168.98%,电导率为33.28%IACS。随Cr2AlB2含量升高,其屈服强度和抗拉强度进一步增大,而电导率和断裂延伸率降低。复合材料较高的硬度和强度得益于Cu(Al)固溶体和富Cr陶瓷的交叠结构与较强的界面键合。(4)Cu-20Cr2AlB2和Cu-25Cr2AlB2复合材料在2N和5N载荷下的磨损率低于纯铜的50%,摩擦过程中,复合材料的强界面键合和表面形成的均匀的机械混合层有助于提高耐磨性。Cu-20CrB和Cu-20Ni-P@Cr2AlB2复合材料在2 N载荷下的磨损率低于纯铜的50%,得益于其较高的硬度和磨痕表面形成的氧化物。更多还原

【Abstract】 Recently,ternary-layered boride Cr2AlB2 has attracted much attention because of its advantages including nano-layered microstructure,high thermal and electrical conductivities,high modulus,low shear deformation resistance,and high temperature oxidation resistance,etc.Therefore,it has a wide application prospect in protective coatings for metals,high-temperature electrode materials,structural parts of gas turbine,electromagnetic wave absorbing materials and other fields.At the same time,Cr2AlB2 is also an important precursor for the preparation of two-dimensional layered CrBene.With the rapid development of aerospace and electronic technique,higher requirements are raised for Cu including not only high electrical and thermal conductivities,but also good mechanical performances and wear resistance.This has become the research hotspot in the field of electronics,machinery manufacturing and other industrial fields.At present,it is a hot topic to introduce reinforcements to develop high performance Cu matrix composites.Cr2AlB2 has the potential as the reinforcement of Cu matrix composites because of its outstanding properties.The preparation of Cr2AlB2 powder with high purity and good crystallization is a problem to be solved urgently,and is also a prerequisite for further investigation of its properties.The current preparation processes of Cr2AlB2 are complicated,by which it is hard to synthesize single phase,with serious powder sintering.Hence,it is of great significance to explore a novel preparation method of Cr2AlB2 powder to deep understand its performance and promote its industrial application.The molten salt synthesis of ceramics has received increasing attention because of its simple process,low reaction temperature and controllable product morphology.In this work,Cr2AlB2 powder with uniform particle size was prepared by molten salt disproportionation reaction combined with the assembly of CrB and A1 in molten salt.Furthermore,Cu matrix composites reinforced with Cr2AlB2 were prepared by hot-pressing method and their mechanics and friction properties were investigated.The main research results are as follows:(1)Ternary-layered Cr2AlB2 powder with relative high-purity is successfully synthesized from 2Cr/2Al/2B precursors mixed and pressed with(Na,K)Cl molten salt in a mass ratio of 1:1 at 1100℃ within 0.5 h.Molten salt as a reaction medium can accelerate the reaction.A protective Al2O3 scale can be formed on the surface of Cr2AlB2 powder after oxidation in static air for 2 h at 900℃,showing the good high-temperature oxidation resistance of Cr2AlB2.(2)High-purity and homogeneous CrB powders with a grain size of around 1 μm are firstly synthesized from Cr/1.05B precursors in(Na,K)Cl molten salt at 850℃for 2 h.Cr can be dissolved into molten salt to form Cr2+ and Cr3+,followed by disproportionation of Cr2+ ions on the surface of B to form Cr atom.Finally,Cr atom reacts with B to form CrB.The as-achieved CrB powders and Al are used as precursors at a mass ratio of salt to precursors of 15:1 at 900℃ for 2 h to form homogeneous and fine Cr2AlB2 powders with a grain size of around 5 μm.The synthesis processes include mutual migration and assembly of CrB and Al in molten salt,and then intercalation of Al into CrB to generate Cr2AlB2.The fast assembly of CrB particles and Al droplets at high temperature is driven by the minimization of free energy in molten(Na,K)Cl.(3)Relative homogeneous and compact Cu matrix composites reinforced with Cr2AlB2 powders have been successfully prepared by hot-pressing at 850℃ and 40 MPa for 1h.The result shows that the hardness and the tensile strength of 10 vol.%Cr2AlB2 reinforced Cu matrix composite are around twice and 168.98%as much as pure Cu while its electrical conductivity decreases to 33.28%IACS.As the content of Cr2AlB2 increases,the yield strength and tensile strength increase whereas the electrical conductivity and fracture.elongation decline.The higher hardness and strength of composites are attributed to the alternative structure and good interfacial bonding of Cu(Al)solid solution and Cr-rich ceramics.(4)The wear rates of Cu-20Cr2AlB2 and Cu-25Cr2AlB2 composites under the load of 2 N and 5 N are less than half of pure Cu.The improvement of the wear resistance is relative to the strong interfacial bonding and uniform mechanical mixing layer formed on the surface of composites.The wear rates of Cu-20CrB and Cu-20Ni-P@Cr2AlB2 composites under the load of 2 N are less than half of that of pure Cu due to their higher hardness and oxides formed on the worn surface.更多还原