چكيده به لاتين
Abstract:
In this study, a finite element-based numerical method was developed to simulate heat
transfer and stress distribution in conventional and functionally graded thermal barrier
coatings applied on the Hastelloy-x. Considering the geometry of the system, wodimensional energy and stress-displacement equations was solved under different conditions using ABAQUS commercial finite element package software. All physical, thermal and mechanical properties needed for simulation were considered to be temperature dependent and Vegard mixture model was used to determine the properties of composite coating. The mechanical model coupled with thermal model used to evaluate the behavior of elastic and plastic deformation. Comparisons of computed results with experiments reported in the literature and results obtained by nano-indentation stress measurement method showed good agreement. A combination of fuzzy linguistic based model and finite element method (FEM) has therefore been developed in the terminology of a combined or “hybrid model” in this study. The hybrid model was applied to predict residual stress during thermo mechanical process in functionally graded thermal barrier coating (FG-TBC). Additionally in this research, a micromechanical FE approach based on the real microstructure was utilized to simulate residual stress distribution and fracture mode in thermal barrier coating. In this method, actual microstructures of the TBC taken by a scanning electron microscope (SEM) were utilized as the representative volume elements (RVEs) in the computational model. In the present study, three different TBC systems including one duplex coating and two FGTBCs, having three and five layers, were prepared by atmospheric plasma spray (APS) process. The coatings were characterized using optical microscope, Scanning Electron Microscope (SEM) equipped with Energy Dispersive x-ray Spectrometry (EDS), and X-Ray Diffraction (XRD) analysis system. Moreover, thermal shock resistance of the coatings was determined.
The result of the simulation showed that temperature gradient along the duplex thermal
barrier coating was 102 °c. The thermal insulation was found to be diminished up to 30% in functionally graded YSZ/NiCrAlY coatings. The results of simulation showed that in the
samples without TGO after thermal shock the average value of the maximum stress is 29
MPa in duplex TBC ( at top coat /bond-coat interface), 15.3 MPa in three-layer FG-YBC
system (at the interface of 50% NiCrAlY - 50% YSZ / YSZ) and 8.1 MPa in five-layer FGYBC system (at the interface of 25% NiCrAlY- 75% YSZ / YSZ). It is noted that the stress distribution is more uniform in five-layer FG-TBC system that helps to increase the TBC performance and to extend the life time of thermal barrier system. The reason for this
phenomenon is that the coefficient of thermal expansion changed gradually through the fivelayer functionally graded coating. Using the results of parametric studies, intelligent fuzzy logic model was designed to predict the relationship between stress and structural properties of fuzzy interference system. This model showed an excellent potential of such finite element-artificial intelligence hybrid approach for analysis of high temperature imposing of these protective coatings. The achievements of this research also highlight that the implemented micromechanical model is able to simulate fracture mechanism and crack propagation of the high temperature TBC systems in a much realistic manner. Furthermore, it was found out that vertical and horizontal cracks nucleating at interface because of tensile stresses can be predicted for the two considered TBCs with and without TGO, which is in conformity with the experimental results.
Keywords: Functionally Graded Thermal barrier coating, Finite element method, Fuzzy logic, Micromechanics, Interface
