基于火花放电颗粒种植的叶尖耐磨涂层形成机理与调控机制的研究

With the unceasing promotion of aeroengine properties, the research of abrasive coatings on blade tips in high temperature stage is increasingly concerned. One of the dominated coating structures is that the particulates are held in a metal matrix with some parts of them protruding from the matrix surface. In this way, the particulates can cut into the abradable coating on the casing to improve the efficiency and power output of the engine. Current processes of coating preparation, such as electrodeposition, brazing, and laser cladding are usually used to produce abrasive coatings on blade tips. However, because of the weak mechanical anchorage between particulates and bond matrix, performance of the electrodeposited coating can be limited, for instance, by particulate dislodgement and even particulate pullout from the surrounding bond matrix during service. The brazed abrasive coatings on blade tips can produce higher bonding strength, but the addition of melting point depressant, such as Si, B, C and P, may destroy the structure of the base metal or form hard brittle phases by interdiffusion. Laser cladding may also produce cracks or recrystallization when working on superalloy. As a result, a new process of producing abrasive coatings on blade tips based on electrospark discharge is proposed in this project, and is called “particulate planting based on electrospark discharge”. This process can realize metallurgical bonding between particulate, bond matrix and base metal, with little harmful effect to the microstructure and properties of the base metal. Due to the introduction of particulates in the discharge gap between the electrode and the workpiece, the discharge process is distinctly different from that of the traditional electrospark discharge. Therefore, in this project, combination of process experiment and numerical simulation is employed to investigate the formation mechanism of abrasive coating produced by particulate planting process. The mass and heat transfer numerical model will be build on the basis of data collected in the process experiments. By calculating the energy distribution coefficients of electrode and workpiece, observing the coating structure, analyzing the temperature and flow fields, measuring the high temperature wear resistant property, we will investigate the effect of particulate properties on the energy distribution and mass transfer during particulate planting, and illuminate the relationship and regulation mechanism between energy distribution, mass transfer, coating structure and wear resistance. The results will provide a new idea and establish corresponding theoretical and experimental foundation for the preparation of abrasive coating on blade tip in high temperature stage.

随着航空发动机性能不断提升，高温段叶片叶尖耐磨涂层的研究逐渐引起重视。传统制备工艺如电沉积、钎焊、激光熔覆等在制备高温段叶片叶尖耐磨涂层时常存在结合力不足或对母材影响较大等缺点。本项目借鉴电火花沉积工艺快速且对母材影响极小的特点，提出火花放电颗粒种植工艺，旨在不影响母材组织与性能的前提下，实现颗粒-基体-母材冶金结合的叶尖耐磨涂层。因在电极与工件放电间隙中引入了颗粒，使得其放电过程与传统电火花沉积存在明显差异，故本项目拟采用工艺实验与数值模拟相结合的方法，在工艺实验的基础上建立火花放电颗粒种植工艺传热传质数值模型，通过计算电极与工件的能量分配系数、观察涂层组织结构、分析温度场与流场、检测涂层高温耐磨性能，研究颗粒属性对种植过程的能量分配与质量转移机理的影响规律，阐明能量分配与质量转移、涂层结构、涂层耐磨性能的关系及调控机制，为制备高温段叶片叶尖耐磨涂层提供新思路并奠定相应的理论与实验基础。

随着航空发动机性能不断提升，高温段叶片叶尖耐磨涂层的研究逐渐引起重视。该涂层的主流结构之一是将耐磨颗粒埋入涂层基体保留顶部露出，以切削可磨耗涂层形成密封气路。传统制备工艺在制备高温段叶片叶尖耐磨涂层时常存在结合力不足或对母材影响较大等缺点，已难以满足需求。本项目提出一种新的叶尖耐磨涂层制备工艺：火花放电颗粒种植工艺，利用火花放电时间短能量高的特点，实现在不影响单晶高温合金母材组织和性能的前提下，凸出颗粒/涂层基体/单晶母材的冶金结合。由于在电极与工件放电间隙中引入了颗粒，使得其放电过程与传统电火花沉积存在明显差异，本项目采用工艺试验与数值模拟相结合的方法，计算电极与工件的能量分配系数、观察涂层组织结构、分析温度场、检测力学性能，研究颗粒对种植过程的能量分配与质量转移机理的影响规律，阐明能量分配与质量转移、涂层结构和性能的关系及调控机制。获得以下有意义的结果：1）建立了一套过程数据采集系统和数值模型，得出上放电点分配能量为13.03%~28.79%，下放电点为3.67%~9.25%，且随放电过程的脉冲频率和脉冲宽度变化。因此，通过调整脉冲频率和脉冲宽度的大小可以调控种植过程的能量分配。2）火花放电颗粒种植过程的上放电点为典型的火花放电，而下放电点更接近于电阻瞬时加热；种植过程可分解为三个组成部分，即火花放电过程、颗粒破碎过程、电毛细上升过程。3）火花放电过程的电脉冲促进了陶瓷颗粒与金属熔体的界面反应，形成了共格的界面反应层，使颗粒与基体的结合力显著提高，平均剪切应力最高可达到325.37 MPa，并且随脉冲频率的增加和脉冲宽度的减小而减小。相关研究成果已经在Materials Science and Engineering A、Surface & Coatings Technology、Journal of Alloys and Compounds等期刊上发表SCI收录论文8篇，并获得江西省科技进步二等奖1项（项目负责人为第一完成人），参加国内学术会议4次，申请专利11项，其中已授权8项，培养硕士生4人。本项目研究为制备高性能单晶叶片叶尖耐磨涂层提供新思路并奠定相应的理论与实验基础。