封严涂层多相异质结构非均质超声背散射响应机制研究

Abradable seal coating is one of the key issues in the coating protection of aircraft engine. It consists with voids and phases including metallic and nonmetallic, the scale of which ranges from tens to hundreds of microns. The compactness and multi-phase uniformity determine the properties of wear and erosion resistance. This project aims at the morphology characteristics and distribution of the nonmetallic phase and the voids in thermal spraying coating. Based on the random medium theory, the elastic disturbance characteristics of nonmetallic phase and voids distributed on the metal matrix will be described and a model of inhomogeneous structure with random multi-phase media will be established. In virtue of the time domain finite difference simulation with a staggered variable step size meshing method, multiple scattering characteristics of high volume fraction random multi-phase will be discussed. The ultrasonic wave propagation behavior and law of the ultrasonic backscattering under different volume fraction, shape, size and distribution of voids will be analyzed. Thus the relationship between the acoustic properties including the velocity, attenuation coefficient and backscattering coefficient, and the coating parameters related to the structural integrity, such as the porosity, dispersion and average size of the nonmetallic phase and the voids will be established. The ultrasonic backscattering response mechanism to the inhomogeneous structure with random multi-phase will be further revealed. The proposed ultrasonic nondestructive characterization method on the compactness and the multi-phase uniformity would provide a theoretical and technical foundation for the integrity of the seal coating, and also the solution of the inverse problem of high performance manufacturing.

可磨耗封严涂层是航空发动机涂层防护的核心问题之一。封严涂层由数十至百微米金属、非金属相和微孔组成，涂层致密性和多相均匀性决定可磨耗和抗冲蚀性能。本项目针对热喷涂封严涂层非金属相和孔隙形貌特征及分布，依据随机介质理论描述金属基体镶嵌的非金属相及分布孔隙的弹性扰动特性，建立封严涂层异质结构随机多相介质模型。借助交错变步长网格划分结合时域有限差分方法数值模拟，研究高体积分数多相随机散射体的多重散射特点，分析非金属相和孔隙不同体积分数、形态、尺寸及分布条件下的背散射超声波传播行为及变化规律，建立声速、衰减系数、超声背散射系数等超声参量与涂层孔隙率、非金属相和孔隙分散度、平均尺寸等涂层完整性参数之间的关系，揭示封严涂层多相异质结构非均质超声背散射响应机制。所提出的封严涂层致密性和多相均匀性超声定量表征方法，能够为保证封严涂层完整性奠定理论和技术基础，并为高性能制造反问题的解决创造条件。

针对孔隙率0.5%-7.0%的铝硅聚苯酯封严涂层，基于SEM观测和显微CT三维成像分析获得的铝硅聚苯酯封严涂层统计学特征，结合随机介质理论与统计学方法，提出了组成相形貌不规则、分布随机的封严涂层随机多相介质建模方法，所建模型能够较好刻画涂层中聚苯酯和孔隙的不规则形貌及随机分布状态。借助自相关长度对涂层组织均匀性进行量化描述，定义了分布均匀性长度用于定量描述涂层组织均匀性。采取仿真和实验相结合的方法初步阐明了封严涂层组成相类型、含量等单因素条件下及多因素耦合对声衰减特性的影响，依据超声波散射分区近似理论和广义自洽理论，建立了多相非均质封严涂层的声衰减机制分析模型。基于超声衰减机制确定了多尺度超声衰减系数作为输入特征参量，构建了粒子群优化-支持向量回归机器学习预测模型，实现了小样本条件下封严涂层孔隙率、孔隙和聚苯酯分布均匀性的超声定量表征。研究成果为航空发动机可磨耗封严涂层质量控制提供了有效途径。