The Analysis Of Thermal Stress And The Crack Propagation Within Real TGO Interface In Thermal Barrier Coatings By Finite Element Modeling

Thermal barrier coatings(TBCs) have been widely utilized as a thermal insulator in the gas flow fluid of engines, thereby increasing the operating temperature of turbine blade, improving the thermal efficiency of the engines, and prolonging the life of the turbine components. A temperature drop of 200 K can be achieved through thermal insulation in TBCs due to their low thermal conductivity. Typical TBCs have an extremely complex multilayered structure, which usually consist of three layers: a top coating(TC), a metallic bond coating(BC) and a substrate. Inevitably, a fourth layer, named thermally grown oxide(TGO), is formed between the TC and BC because of the diffusion and reaction of oxygen and aluminum during the long thermal cycle, which has generally been recognized as the first key failure factor. The huge differences in the physical, thermal and mechanical properties developed in TBCs may generate a concentration of tresses, which further results in coating failure. It is well-known that the major failure mode is the spallation and delamination of the TC, which is mainly the result of the crack initiation and propagation at the interface of the TC and TGO. This is severely limited in the application of TBCs; therefore, it is essential to study the stress variation and assess the failure mechanism of TBCs as a matter of urgency. In this paper, the finite model of TBCs with real TGO morphology will be presented to investigate both the distribution of thermal stress and the failure mechanism of TC/TGO interface, which based on the computational micromechanics method. The main contents in this thesis are listed as follows:Firstly, the scanning electron microscopy(SEM) image of TBCs with different TGO morphology is used to process gray value, segment threshold value, and extract the TGO morphology. Furthermore, the real TGO interface is subjected to vector quantization and imported into ABAQUS. Subsequently, the material property, boundary condition and mesh grid are applied to the geometrical model of TBCs within real TGO morphology. The detailed processing procedure and an example are described in this thesis.Secondly, the two-dimensional finite element models of TBCs with different real TGO morphology(earlier stage, middle stage and later stage) have been developed to investigate the thermal stress distribution. The effect of real temperature variation, at different position of turbine blade(lead edge, trailing edge, suction side and pressure side), on thermal stress evolution was simulated. The simulated results indicate that the tensile stress locates at valley region, the compressive stress locates at peak region, and that the maximum shear stress lies in middle region at TC layer. However, the normal stress distribution of BC layer is contrary to that of TC layer. It can be also found that the location of maximum stress is change with the variation of TGO morphology, due to the stress concentration result from the irregularity of TGO morphology. The stress value distinctly increases with an increase in the relative roughness LA0 at first, and then flattens out, which is consistent with the analytical result.Thirdly, the two-dimensional finite element models of TBCs with different real TGO morphology(earlier stage, middle stage and later stage) have been developed to investigate the failure mechanism of TC/TGO interface. The simulated results indicate that the horizontal crack tends to propagate through the TC layer toward the interface, while the interface crack initiates at the point of the middle region before simultaneously propagating to both directions during the cooling stage. It can be also found that Mode I delamination plays a key role in the process of propagating the crack along the interface of the valley region, and mode II delamination plays the dominant role in the other process of the crack propagation.In summary, two failure mechanisms of the TC/TGO interface were observed based on the analysis of the crack propagation;(i) the spallation and delamination of the TC layer as a result of the development of the interface crack, in which there is no micro-horizontal crack;(ii) the spallation and delamination of the TC layer as a result of the interaction of the development of both the horizontal crack and the interface crack, in which there is micro- horizontal crack. It is very beneficial to build the failure mechanism of TBCs and develop the life-time estimating model in the process of service.