新型核壳结构WC-Cr2(C,N)-Co纳米复合粉的设计、制备及其合金强韧化机理研究

How to controllable fabricate the nanocrystalline hardmetals, of which the key challenge is to control the rapid grain growth during the fully densifying, have become an international problem in the field of high-end cutting tools. Therefore, in this project, the innovation is proposed that through design novel WC-Cr2(C,N)-Co core-shell structure (WC as the core, Cr2(C,N) as the inner rim, Co as the outer rim) nanocomposite powders, in order to improve the distribution of grain growth inhibitors, nitrogen and binder Co in hardmetals, which will enhance the inhibition performance on WC grain growth, optimize the microstructure and mechanical properties, and strengthen and toughen the nanocrystalline hardmetals. The main research in this work focus on: (1) controllable preparing the WC-Cr2(C,N)-Co core-shell structure nanocomposite powders, by using the hydrothermal synthesis, spray transformation method and the carbon thermal reduction nitride technology; (2) unfolding the formation mechanism of the WC-Cr2(C,N)-Co core-shell structure; (3) revealing the sintering densification behavior and metallurgy mechanism of the core-shell structure nanocomposites; (4) explaining the acting mechanism of the core-shell structure in inhibiting the WC grain growth and the strengthening-toughening hardmetals, combining with the calculations and simulations. This project will provide theoretical basis and technical support for the preparation of high-performance nanocrystalline hardmetals.

纳米硬质合金的可控制备是高端切削工具领域面临的国际难题，如何有效调控致密化过程中晶粒长大是制备纳米硬质合金的关键所在。为此，本项目提出了通过设计具有核壳结构(WC为核、Cr2(C,N)为内壳层、Co为外壳层)的WC-Cr2(C,N)-Co纳米复合粉，改善晶粒长大抑制剂、N元素和粘接相Co在合金组织中的分布，来抑制WC-Cr2(C,N)-Co合金中WC晶粒长大，优化合金的微观结构与性能，实现合金强韧化的新思路。并着重研究：(1)采用水热合成、喷雾转化技术、并结合碳热还原氮化工艺，可控制备核壳结构WC-Cr2(C,N)-Co纳米复合粉；(2)探讨WC-Cr2(C,N)-Co核壳结构的形成机理；(3)揭示该核壳结构复合体系的烧结致密化行为与冶金机制；(4)结合计算机模拟计算，阐明核壳结构抑制WC晶粒长大、强韧化合金的作用机理。本项研究将为高性能纳米硬质合金的可控制备提供有效的理论依据和技术支撑。

高性能硬质合金是先进制造业快速发展所必需的关键基础材料，如何有效调控致密化过程中晶粒长大、改善界面结合强度是实现其高性能化的关键所在。本项目在冶金热力学分析的基础上，系统研究了核壳结构WC-Cr2(C,N)-Co纳米复合粉末可控制备技术及其机理，分析了其致密化行为，进而阐明了其硬质合金强韧化机理，制备出了高性能硬质合金，发现：（1）通过反应热力学分析，论证了碳热还原氮化法制备Cr2(C,N)的可行性，且提高N2、降低CO分压，促进反应正向进行。WC-Cr2(C,N)-Co纳米复合粉末制备物相演变遵循：非晶态→(CoWO4、WO2、CrWO3、WO2.72)→(CoWO4、WO2、CrWO3、Co6W6C)→(W2C、Co3W3C、WC)→(WC、Co、Cr2(C,N))的规律。（2）当水热温度为180℃，水热时间为10h，葡萄糖含量为理论值140%，热处理温度1050℃，热处理时间为60min时，综合制备条件最优，制备的WC-Cr2(C,N)-Co纳米复合粉末粒径范围在25~75nm，颗粒分散均匀，粉末呈WC为内核、Cr2(C,N)为内壳层、Co为外壳层的结构。（3）反应温度、碳含量和保温时间对产物影响较大，是三个重要影响参数，适当提高反应温度，增加碳含量，延长保温时间，有助于WC-Cr2(C,N)-Co的形成。（4）氮可提高Cr在粘结相中的溶解度，并与Cr协同降低W在粘结相中的溶解量，有效限制WC的“溶解-析出”行为，显著抑制WC的晶粒生长，阻碍(W,Cr)Cx脆性层的形成，强化界面的同时强韧化粘结相，从而显著改善硬质合金的强度和硬度等力学性能，但对韧性略有降低。（5）随着WC-Cr2(C,N)-Co中Cr2(C,N)含量的增加，合金微观组织逐渐细小均匀，合金的密度不断降低，致密度先基本保持不变、后降低，抗弯强度先增加后降低，维氏硬度不断升高，断裂韧性逐渐降低。当Cr2(C,N)含量为1.00 wt.%时，硬质合金具有最优的综合力学性能。这为核壳结构纳米复合粉末及其合金的制备提供了理论基础，并为新结构高性能硬质合金的创新研制及应用提供了关键技术支撑。