梯度纳米贝氏体化涂层剪切带增强增韧调控及其摩擦学行为研究

Laser cladding is one of the most important technologies for surface strengthening and remanufacturing, and to balance the strength and toughness of the coating has be attracted much attentions. In our previous investigation, we found that the nano-crystalline bainitic modified layer with balanced strength and toughness can be fabricated on the alloy steel by laser remelting during the bainite phase transition temperature range. However, the microstructure control mechanism of the surface coating and its effect on the tribological behavior are still need to be explained. Based on the thermal stress effect and the bainitic transformation theory, a new laser-fast cooling and subsequent isothermal transformation process has been developed to prepare the shear bands structure such as stacking fault, twin and ε-martensite on the surface of alloy steel coatings in this project. The shear bands type and distribution, application of gradient residual stress on the formation and regulation of the shear bands, and the surface properties and tribological behavior of the gradient nano-bainitic coating will be investigated. Moreover, the enhanced regulation of strength and toughness by shear bands in nano-bainitic coating will be investigated, the tribology mechanism of gradient nanostructured coatings will be illustrated, and the relationship between the microstructure and the friction and wear characteristics will be established. The investigated results can provide a theoretical basis and technical measures for the application of the bainitic coatings with satisfactory strength and toughness.

激光熔覆技术是零件表面强化与再制造的重要技术之一。解决激光熔覆涂层强度与韧性之间的制约关系备受关注。前期研究我们发现，在贝氏体相变温度区间对合金钢进行激光重熔可以形成强度和韧性匹配均衡的纳米贝氏体化改性层。然而，该表面涂层微观结构调控机制及其对摩擦学行为的影响有待阐释。本项目基于热应力效应和贝氏体相变理论，利用激光快冷及后续等温转变组合新工艺，在合金钢表面制备含有堆垛层错束、孪晶和ε马氏体等剪切带结构的激光熔覆层。研究梯度纳米贝氏体化涂层剪切带类型与分布、涂层梯度残余应力对剪切带形成与调控和梯度纳米贝氏体化涂层的表面性能与摩擦学行为，全面揭示贝氏体化涂层剪切带调控机理，阐释梯度纳米结构涂层的摩擦磨损机理，建立显微结构与摩擦磨损特性的关联行为，通过诱发形成剪切带实现贝氏体化涂层强度与韧性的调控，进而为提升高强韧贝氏体化涂层长寿命、高可靠应用提供理论基础和技术措施。

铁基涂层强度和韧性往往难以调和，如何同时提高涂层的强度与韧性备受关注。基于热应力效应和贝氏体相变理论，本课题提出利用激光快冷及后续等温转变组合新工艺制备强韧兼具铁基涂层的新方法。课题重点研究了三方面的工作：（1）梯度纳米贝氏体化涂层剪切带类型与分布表征；（2）涂层梯度残余应力对剪切带形成与调控的作用；（3）梯度纳米贝氏体化涂层的表面性能与摩擦学行为。课题的主要结论如下：控温激光表面处理可以诱发位错形成，发现了贝氏体涂层中大量存在的纳米孪晶、ε-马氏体和堆垛层错等塑性相与位错滑移有关。采用电解腐蚀技术对涂层进行逐层腐蚀剥离，测定了涂层不同深度残余应力、微观结构与力学性能，发现随深度增加，涂层的残余应力由拉应力逐渐减小，并过渡为压应力。与此同时，涂层中残余奥氏体含量逐渐增加，而ε-马氏体含量逐渐降低，涂层硬度亦随深度增加逐渐降低。建立了堆垛层错束、纳米孪晶、ε-马氏体和 α‘-马氏体形成与堆垛层错堆叠方式的关系，阐释了残余应力驱动位错滑移形成堆垛层错的同时能够缓解残余应力。此外，位错密度增加和铁素体固溶碳含量增加是涂层微观结构中贝氏体—铁素体硬度提升的关键。摩擦学试验表明控温激光表面处理能够有效的提高涂层试样表面耐磨性。在摩擦磨损过程中，贝氏体涂层表面残余奥氏体体积分数逐渐减小。项目研究结果对于重新认识表面涂层制备过程中产生的残余应力，思考如何通过在线控温熔化-凝固全流程工艺影响涂层相变，从而获得高性能涂层材料提供了新的方法，具有一定的工程应用价值。