Research On Fracture Behavior Of Interface Between Coating And Substrate

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.