【作者】 徐连勇；

【导师】 荆洪阳；

【作者基本信息】 天津大学 ， 材料加工工程， 2007， 博士

【摘要】 热喷涂层的可靠性关系到构件的安全运行,对涂层特性的了解和把握是其工业应用的关键条件。本文从探讨涂层/基体系统完整性评定的方法出发,对LX88A涂层的力学性能、涂层的断裂行为基本特性、涂层与基体间界面的断裂参量以及基于局部法评定涂层/基体的界面断裂行为等几个方面进行了研究。本文通过三点弯曲试验测量了LX88A涂层的弹性模量(E),结果表明涂层的孔隙率对涂层E具有很大的影响,E的测量值随孔隙率的减小而增大。随着涂层厚度的增加,整个涂层的平均孔隙率变小,使得E随之增大。另外,发现相同厚度涂层的显微硬度和孔隙率均具有较大的分散性,并且涂层E随显微硬度的增大而减小,因此,相同厚度涂层的E具有较大的分散性。通过对三点弯曲试样中LX88A涂层断裂行为的观察,发现涂层中某位置所受力矩达到一定值时,该处会产生垂直于界面的裂纹,故可将力矩是否达到临界值作为该位置产生裂纹的条件。有限元分析结果也证明了上述结论,对于三个不同位置的裂纹,当裂纹产生时该位置处界面附近的应力水平基本接近。本文将断裂力学实验和有限元分析相结合获得界面断裂时裂纹尖端的应力场,并根据界面复应力强度因子(K)的定义计算了对LX88A涂层与Q345钢界面裂纹的K。结果表明,在三点弯曲试样中拉伸载荷和剪切载荷共同主导界面裂纹尖端附近区域,甚至剪切效果大于拉伸效果。研究发现只有部分试样的界面裂纹尖端附近存在K主导区,即K有效。另外,由于七个试样发生界面断裂时的J积分值分散很大,所以不适合使用单一断裂参量J积分来评价界面断裂行为。本文推导了将局部法用于界面断裂分析的理论基础。提出了两种断裂控制区的定义,一种将界面作为断裂控制区,一种将靠近界面的涂层薄层和基体薄层作为断裂控制区。对应两种定义提出了两种统计学分析的微观力学模型,对应两种模型讨论了界面裂纹的断裂准则,并根据最薄弱环节理论将局部法用于界面断裂行为分析。另外,制定了界面裂纹威布尔参数的估计方法和步骤,并编制了估计界面威布尔参数的程序。本文将局部法用于分析界面断裂行为对试样几何形式的依赖性。发现不同几何形式的试样发生界面断裂时,在相同断裂概率下其威布尔应力基本相同,并基于局部法通过预制裂纹试样的试验结果成功预测了两种缺口试样断裂数据的分布,说明局部法可以用来描述界面断裂行为,并用于界面完整性评定。更多还原

【Abstract】 The reliability of thermal spraying coatings was related to safe operation of the components, and the knowledge of coatings’characteristics was critical for the widespread application of coatings. For the purpose of seeking a method to evaluate the integrity of coating/substrate system, the mechanical property of coatings, fracture behavior characteristics of coatings, interface fracture parameters and evaluation of interface fracture behavior based on the local approach had been studied in this paper.The 3-point bend test had been used to measure the Young’s modulus (E) of the LX88A coating produced by HAVS. The test results showed the coating’s porosity had important effect on E. E increased with the increase of porosity of coatings. It was found that the average porosity of whole coating decreased with the increase of coating thickness, so E increased. In addition, the microhardness and porosity of coatings with same thickness had notable dispersity, and E decreased with the increase of microhardness. Therefore, E of coatings with same thickness have large dispersity.The fracture behavior of LX88A coating in 3-point bend specimen had been observed. It was found that the crack normal to the interface in the coating occurred where a fixed moment of force was reached. So whether the moment reached to the critical value can be used as the condition of crack occurrence. The FEA results proved the previous conclusion. The stress levels at the different locations where the cracks occurred near the interface almost were same.The fracture mechanics experiment and FEA had been used to compute the complex stress intensity factor (K) of the interface between LX88A coating and Q345 steel in the paper. The result showed that phase angle (ψ) of K for the 3-point bend specimen was more than 45°. It indicated that both the tensile load and shear load near the crack tip were dominant, even the shear load had higher effect than the tensile load. The research found the K-dominant zone did not existed near the crack tip for total specimens, but for some specimens. In addition, the JC values of the total specimens when the interface fracture occurred were very dispersive. Therefore, JC can not be used as the single fracture parameter to evaluate the interface fracture behavior.The theory of the local approach applied to analysis the interface fracture had been deduced in the paper. In the theory, two different methods were proposed to define the fracture process zone. In one method, the interface was fracture process zone. In another method, the fracture process zone consisted of a thin layer of coating and a thin layer of substrate adjacent to the interface. Subsequently, two micromechanics models for statistics analysis were built corresponding to the two definitions, and the different fracture criterions for interface crack were proposed corresponding to the two models. Lastly, the local approach was developed to analyze the interface crack based on the weakest link theory. In addition, the estimation procedure of the interfacial Weibull parameter was determined.The local approach was used to analyze the geometry dependence of coating specimens for interface brittle fracture initiation. It was found that the Weibull stress (σW) for all specimen geometries almost were identical under the same fracture probability when the interface fracture initiation occurred for different specimen geometries. Moreover, the interface fracture behavior of two types of specimens with notch had been predicted from the test results of pre-crack specimens based on the local approach for interface brittle fracture, and the predicted distribution of the critical load for the notched specimens gave a good agreement with the test results. In a word, the geometry dependence of the interface brittle fracture toughness data can be reduced through the local approach’s application. It showed that the local approach not only can be used to describe the interface fracture behavior, but also can be used in the integrity evaluation for interface between different materials.更多还原