Phase-angle effects on damage mechanisms of thermal barrier coatings under thermomechanical fatigue (original) (raw)
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Defect evolution in thermal barrier coating systems under multi-axial thermomechanical loading
Surface and Coatings Technology, 2005
In service, gas turbine components with thermal barrier coatings experience high cyclic mechanical and thermal loading. Important, but not yet considered sufficiently, are the multi-axial stresses arising from thermal gradients. In this study, multi-axial stresses were simulated in laboratory experiments using a specially designed test rig. Cyclic thermomechanical fatigue experiments with radial thermal gradients (TGMF) were performed on tubular specimens consisting of a directionally densified superalloy substrate, a NiCoCrAlY bond coat, and a ceramic thermal barrier coating (TBC). The test setup enables surface temperatures of 1000-C, temperature differences over the whole wall thickness of the specimen of about 170-C, and high heating and cooling rates. The resultant defects have specific features consisting of cracks parallel to the bond coat/TBC interface. They are located within the bond coat close to this interface. Weakening thus this interface, the defects enhance TBC spallation. Finite element analyses, calculating stress distributions during the quasi-stationary condition of the TGMF test cycle at high temperature and the transient stress distributions during cooling, were used to discuss the evolution of these specific defects.
Thermo-Mechanical Buckling Failure of Thermal Barrier Coatings with Arbitrary Delamination Location
A one-dimensional interface delamination model is introduced to analyze the thermo-mechanical buckling characteristic of Thermal Barrier Coatings (TBCs) system with arbitrary across-the-width delamination. Equilibrium equations, stability equations and characteristic equation governing buckling under external thermal and mechanical loadings are derived on the basis of the first order shear deformation theory and the State Space Method (SSM). Different types of thermal loading and temperature gradient across the thickness are considered, and the thermo-mechanical buckling loadings are accurately obtained. Finally the effects of Thermal Barrier Coatings (TBCs) system aspect ratio, relative thickness, delamination location and temperature gradient on thermo-mechanical buckling failure of TBCs are all discussed.
Thermal fatigue failure induced by delamination in thermal barrier coating
International Journal of Fatigue, 2002
The paper presents the experimental and theoretical investigation on the thermal fatigue failure induced by delamination in thermal barrier coating system. Laser heating method was used to simulate the operating state of TBC (thermal barrier coating) system. The non-destructive evaluation such as acoustic emission (AE) detect was used to study the evolution of TBC system damage. Micro-observation and AE detect both revealed that fatigue crack was in two forms: surface crack and interface delamination. It was found that interface delamination took place in the period of cooling or heating. Heating or cooling rate and temperature gradient had an important effect on interface delamination cracking propagation. A theoretical model on interface delamination cracking in TBC system at operating state is proposed. In the model, a membrane stress P and a bending moment M are designated the thermal loads of the thermal stress and temperature gradient in TBC system. In this case, the coupled effect of plastic deformation, creep of ceramic coating as well as thermal growth oxidation (TGO) and temperature gradient in TBC system was considered in the model. The thermal stress intensity factors (TSIFs) in non-FGM (functional gradient material) thermal barrier coating system is analytical obtained. The numerical results of TSIFs reveal some same results as obtained in experimental test. The model is based on fracture mechanics theory about heterogeneous materials and it gives a rigorous explanation of delaminations in TBC system loaded by thermal fatigue. Both theoretical analysis and experimental observation reveal an important fact: delaminations are fatigue cracks which grow during thermal shocks due to compressive stresses in the loading, this loads the delaminations cracks in mixed I and II mode.
Thermal barrier coatings (TBCs) are used in gas turbines to provide insulation against high temperature and to provide oxidation and corrosion resistance for the superalloys on which they are deposited. TBCs are deposited on hot parts in the combustor and on the turbine blades, and must consequently be compatible with the various superalloys used there. The influence of substrate material on the durability of TBCs has therefore been studied. Air plasma sprayed TBCs have been deposited on Hastelloy X and Haynes 230, which are alloys used in the combustor. The TBC systems have been thermally cycled until failure and their fracture surfaces have been studied. The thermally grown oxides and the substrate/coating interdiffusion have also been analysed by energy dispersive spectroscopy. The fatigue life, fracture mechanism and the oxide composition and kinetics were similar for the two TBC systems; however, one of the TBC systems is thought to have failed prematurely.
Influence on Thermal Barrier Coating Delamination Behaviour of Edge Geometry
Fracture of Nano and Engineering Materials and Structures
Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150µm and 350µm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB 2 B OB 3 B partially stabilised ZrOB 2 B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.
Finite Element Analysis of Thermal Fatigue in Thermal Barrier Coatings (TBC)
A Finite element model of plasma sprayed TBC's was developed to estimate the stress induced by thermal cycling experiments. A heat transfer analysis was performed to evaluate the temperature distribution on the specimen during the cooling under an impinging air jet; temperature measurements performed with an infrared pyrometer on the cooled samples show good agreement with the evaluated data. These results were then integrated in a structural mechanic model as thermal load. The COMSOL Multiphysics ® Thermal-Structural interaction model allowed to determine the dependence of the stress on the temperature fields.
Fracture Mechanical Approach and Models for Failure Analysis of Thermal Barrier Coatings
tecsis.ca
Two simplistic models using fracture mechanics considerations are used to advance understanding of the failure conditions in TBC system. One model assumes isostrain behavior prior to the onset of crack initiation and is based on elastic energy balance approach. The other model is used for crack propagation behavior. The analysis for crack initiation suggests that the crack tip driving force, K I can reach a high value that is comparable to the fracture resistance of the coating material at and near the TBC/TGO interface even for a small nominal applied stress. The level of stress required to yield the crack driving force (K I) values above the fracture resistance of TBC materials are found to be in the range of 0.05 to 0.5 GPa. This appears to be an order of magnitude lower than the reported tensile stress values of 1 to 2 GPa. High crack driving force resulting from low stress and small size defects (less than 10 microns) facilitates early crack initiation in TBC system.
Mechanisms controlling the durability of thermal barrier coatings
Progress in Materials Science, 2001
The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to ®nal failure by large scale buckling and spalling. Because of diering manufacturing approaches and operating scenarios, several speci®c mechanisms are involved. These mechanisms have begun to be understood. This article reviews this understanding and presents relationships between the durability, the governing material properties and the salient morphological features. The failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface. This ampli®cation induces an energy release rate at cracks emanating from the imperfections that eventually buckle and spall the TBC.