纳米孪晶金刚石/立方氮化硼复合超硬块体材料的制备及性能研究

Based on the principles of the nanocomposite enhanced hardness and elastic modulus, combining the high hardness and toughness of nt-diamond and the high chemical and thermal stabilities of nt-cBN, in the range of pressure for 8-25GPa and temperature for 800-2200℃, nt-cBN/nt-diamond superhard nanocrystalline composite materials with an extremely high Vickers hardness (>300GPa) and fracture toughness (>30 MPa•m1/2) as well as good stability will be prepared by the high temperature and high pressure (T25) synthesis technique. The phase constitution, characterization of chemical bonding and herero-interface structure of nt-cBN/nt-diamond namocomposite will be ascertained. The influence of synthesis pressure and temperature on phase constitution, nanotwinned and heterostructure interface’s behaviours will be investigated. We will also accurately measure the properties of nt-cBN/nt-diamond nanocomposite such as hardness, fracture toughness (elastic limit), thermal stability and chemical stability for transition metals. The relationship between properties and microstructure as well as herero-interface structure of nt-cBN/nt-diamond namocomposite will be shown. Basing on the research results in this project, we should be able to give a new effective method to synthesize nanocomposite ultrahard materials with excellent properties.

采用高压高温合成技术(T25)，依据纳米复合强化原理，将纳米孪晶金刚石(nt-diamond)块体高的硬度和断裂韧性与纳米孪晶立方氮化硼(nt-cBN)块体高的热稳定和化学稳定性相结合，制备具有极高硬度(大于300GPa)和断裂韧性(大于30 MPa·m1/2)及良好稳定性的nt-cBN / nt-diamond复合超硬材料；在8-25GPa和800-2200℃的压力温度范围内，研究合成工艺参数对nt-cBN/nt-diamond复合材料相组成和孪晶形态及界面行为的影响规律，确定其合理的相分布和异质界面的结构及化学键特征；精确测定nt-cBN/nt-diamond纳米复合块体的硬度、断裂韧性(弹性极限)和热稳定性以及与过度金属间的化学稳定性，建立nt-cBN / nt-diamond纳米复合材料的性能与微观组织和界面结构的关系，发展制备综合性能极其优异的纳米复合超硬材料新技术。

采用洋葱结构氮化硼(o-BN)、片状hBN和石墨(o-graphite)为原料，在12~25GPa和1600~2200℃的条件下，分别合成了纳米孪晶金刚石/纳米孪晶立方氮化硼（nt-diamond/nt-cBN）复合材料、nt-diamond/M相（金刚石多型结构）自生复合材料、nt-diamond/nt-cBN层状复合块体材料、nt-diamond/cBN仿生结构复合材料等多种样品。nt-diamond/M相自生复合块状样品中含有大量的M-diamond相，由此构成了一种纳米孪晶金刚石与M相的自生复合材料；初步的力学测定发现其硬度为200~400GPa，断裂韧性为17~40 MPa·m1/2；并且出现硬度和断裂韧性同时明显提高的趋势，这与我们先前预期的结果相一致。纳米孪晶自生复合材料的部分内容已撰写一篇文章后投递到Nature杂志，正在审稿中。应用第一原理计算的方法，对金刚石与立方氮化硼复合结构进行了理论预测研究，获得了多种一维和二维导电性超硬sp2-sp3杂化BC2N和BC6N新结构。利用氮化硼和碳所固有的较强离散性，采用片状六方氮化硼和洋葱结构碳为原料，基于择优分散的原则，我们进行了仿生结构超硬材料的探索性研究，获得了相分布较为合理的多个样品，并进行了初步的微结构分析。在国产420缸径六面顶压机上完成大尺寸超高压合成装置的改造，并已获得直径大于5mm、高度大于2mm纳米孪晶金刚石样品，该样品包含相当数量的金刚石多型结构，导致其具有较好的导电性。