Residual Stress Measurements Of Thermal Barrier Coating Systems By Hole-drilling Method With Digital Image Correlation Technique

With the rapid development of aerospace industry, we have an increasing demand for the engine performance, which it required of high thrust weight ratio and large driving force, in order to get the high performance of the engine, spraying thermal barrier coatings is an important method on the surface of the blade. However, residual stress is generated during the preparation of the coating because of the serving conditions and the mismatch of material parameters. A larger residual stress of coating will directly impact on service performance. It is of great scientific significance to study the residual stress of thermal barrier coatings in preparation and improve coating service life and reliability. Based on this, this paper developed residual stress field drilling constitutive model for a multilayer coating system, independently developed a set of hole-drilling method test system with digital image correlation, the residual stress of thermal barrier coatings were comprehensive studied, At the same time, the residual stress obtained by drilling method was contrasted with that acquired by X-ray diffraction and Raman spectroscopy method, with which the feasibility of constitutive model and experimental test process of drilling method was confirmed. The main conclusion are summarized as follows:Firstly, we accomplished a constitutive model of the residual stress field for Multilayer structure coating system. Considering the influence of stress transfer in layers which has different materials parameters, it is assumed that the residual stress in the micro layer is uniform, so the relationship between the residual stress of each layer and the drilling depth was established, based on the principle of hole-drilling method about single block material, The expression of the residual stress in each layer of the multi-layer structure is derived through matrix and fractional integral method. The residual stress of each layer is obtained along with three directions of radial strain with digital image correlation method, therefore we can receive the residual stress of each layer. The model can be used as the theoretical basis of the residual stress testing experiments in the following chapters.Secondly, by the independent design and assembly of the hole-drilling test system, we tested average residual stress in different thickness of the plasma spraying before and after thermal treatment. The average residual stress measurement thermal barrier with different thickness were tested. The results show that: for different samples, the average residual stress of the coating decreased from 230.43 MPa to 144.73 MPa along with the increase of the thickness of the ceramic layer. For the same sample, the residual stress increased from 200.49 MPa to 226.88 MPa, then reduced to 188.34 MPa; The average residual stress of plasma sprayed thermal barrier coating material were also tested. The results show that: with the increase of heat treatment time, the residual stress of the coating increased from 182.73 MPa to 198.6 MPa and reduced to 88.14 MPa. At the same time, the residual stress of original samples within the coating surface of 10 μm is measured by XRD and Raman spectroscopy,which is consistent with the drilling method.Thirdly, by the independent design and assembly of the drilling test system, we tested average residual stress of electron beam physical vapor deposition thermal barrier coatings with different heat treatment. Firstly, the average residual stress of the thermal barrier coating before and after thermal t treatment had been measured. It is concluded that, average residual stress on the top coating increased firstly and then decreased along with the depth, the residual stress on top coating changes from 97.2 MPa to 175.6 MPa and reduces to 89.4 MPa; as the continue of heat treatment, the average residual stress on the top coating increased firstly and then decreased, the residual stress on top coating varies from 130.6 to 196.8 MPa and then reduced to 97.2 MPa. In addition, taking a scanning for the thermal barrier coating samples disposed by electron beam physical vapor deposition and founded that the microstructure consists of many separate columnar crystal composition and each of the columnar crystals combined firmly with the underlying. The results provided theoretical support for the properties and preparation of the materials.