钢基表面微/纳米NbC梯度层的原位制备及强韧化机理研究

Carbide and steel surface composites have both the advantages of high hardness and high modulus of ceramics, high toughness and high plasticity of the steel matrix, thus being widely used in metallurgy, mining, machinery and other fields. However, the toughness of steel surface is significantly reduced due to the further increase in the volume fraction of high hardness carbides, which raises the issue of the low bonding strength between the carbide and the matrix micro-interface, and the composite layer/the matrix macroscopic interface. In this paper, micro/nano NbC gradient layer was prepared on the steel surface by the niobium plate/steel in-situ reaction method. The goal was to obtain the in-situ self-generated enhanced phase, to improve the bonding strength between NbC and the steel-based micro-interface, and to optimize the combination of the composite layer/the macroscopic interface by using the gradient characteristics of the structure. The in-situ formation mechanism of the surface micro/nano NbC gradient layer was elucidated by studying the thermodynamic driving forces, kinetic parameters and evolution of the structure. The relationship among the enhancement phase, the matrix orientation and coherency level was investigated. In addition, the bonding strength between the gradient layer and the substrate under continuous loading was studied. Moreover, surface fracture toughness and deformation of the plastic deformation performance were characterized. Above findings create conditions to further improve the ceramic/metal composite with good strength and toughness based on micro/nano composite and gradient structure coupling. The successful implementation of the project provides a novel idea and method for the research of ceramic/metal surface composites, which possesses great academic value.

碳化物／钢表面复合材料兼具陶瓷高硬度、高模量及钢基体高韧性、高塑性优势，在冶金、矿山和机械等领域应用广泛。然而，随着钢基表面碳化物体积分数的进一步提高，表面层韧性大幅下降，且存在碳化物与基体微观相界面、复合层与钢基宏观界面结合强度较低等问题。本项目采用铌板/钢-原位反应法在钢基表面制备微/纳米NbC梯度层，汲取增强相原位自生优势提高NbC与基体微观相界面结合强度，利用梯度结构提高复合层与钢基宏观界面结合强度。通过研究体系热力学驱动力、动力学参量以及组织演化规律，阐明表面微/纳米NbC梯度层原位形成机制。表征增强相与基体位向关系和共格程度，结合连续加载下梯度层与基体的结合强度、表面断裂韧性及变形区域塑变性能，揭示微/纳米复合和梯度结构耦合的增强增韧机制，为进一步提高陶瓷/金属复合材料良好强韧匹配提供理论支撑。项目的成功实施将为陶瓷/钢表面复合材料的研究提供新的思路与方法，具有一定的学术价值。

碳化物／钢表面复合材料兼具陶瓷高硬度、高模量及钢基体的高韧性、高塑性优势。但是，随着碳化物颗粒体积分数提高，复合材料强度提高的同时韧性却大幅下降。因此，如何改善陶瓷/金属复合材料强韧性倒置问题成为复合材料研究领域的一个关键难题。本项目采用高纯Nb板/轴承钢-原位反应法制备钢基表面微/纳米NbC梯度层，可耦合微/纳米NbC陶瓷与梯度结构两种增韧方式以改善陶瓷/钢复合时的强韧性倒置问题。主要研究包括有：(1) Fe-C-Nb-Cr体系热力学计算表明增强相NbC能够实验条件下生成，系列实验及生长动力学分析推导可得NbC增强层生长厚度与保温时间、反应温度的函数关系，便于对NbC增强层在不同工艺条件下的原位制备进行预测。(2) 根据不同区域NbC增强相含量将其分为高体积分数微/纳米NbC复合区[A]和次表层为微米NbC-Fe梯度区[B]，[A]中的增强相NbC体积分数大于90%，其中NbC陶瓷颗粒细小且排列紧密，物相为NbC相和少量α-Fe。 [B]中NbC体积分数为90-60%，主要物相为NbC、Fe3C、α-Fe和Cr7C3等。微/纳米NbC梯度层的形成机制主要是C原子的扩散填充所导致的晶格结构转变以及金属Nb原子与C原子的晶格重组。(3) 沿表面向基体内部，微/纳米NbC梯度层的纳米硬度(H)和弹性模量(E)分别呈梯度减小：19.70-13.75 GPa和443.17-300.66 GPa。增强层的H和E明显高于轴承钢基体。(4)压痕法结合裂纹类型判断，并选择经验公式计算可得NbC增强层的断裂韧性为7.68~11.55 MPa•m1/2。在划痕测试中，微/纳米NbC复合区为微小局部的剥落，而NbC梯度层仅出现了一些微裂纹和颗粒堆积，并未出现严重的磨损失效行为，对钢基体保护作用良好。综上，组织结构和力学性能梯度变化的表面微/纳米NbC梯度层，可通过偏转、桥联和绕过等机制延长裂纹扩展路径改善韧性，能够实现强韧性的良好匹配；并能有效解决传统叠层增强时界面应力集中所导致的力学突变问题。项目的成功实施为碳化铌增强钢基复合材料的研究提供新的思路与方法，具有一定的学术价值。