PSI - Issue 13

Elena Fedorova et al. / Procedia Structural Integrity 13 (2018) 741–745 E.Fedorova et al./ Structural Integrity Procedia 00 (2018) 000 – 000

742

2

TBC system is typically composed of three layers: the Ni-based bond coat (BC), the thermally-grown oxide (TGO) and the yttria-stabilized zirconia top coat (TC). The TGO layer consists predominantly of  -Al 2 O 3 that forms under service conditions due to oxygen diffusion through the TC and oxidation of aluminum present in the BC. The mechanisms controlling the durability and the damage evolution in TBC has been extensively studied over the past 20 years by many authors and clearly described by Evans A.G. et al. (2001), Pindera M.-J. et al.(2005), Hille T.S. et al. (2011). The residual stresses developed during thermal cycling are of primary importance, as their magnitude and distribution govern the durability of TBC systems. As analytical methods are limited to certain simplified problems, finite element simulation is often used to predict the stress distribution and evaluate failure mechanisms in TBC. Finite element modelling of various TBC systems has been performed for a long time (M. Baker and Ph. Seiler, (2017). Now, it is well-acknowledged that the mismatch between the thermal expansion coefficients of components and the interface shape bring about stresses at the interfaces when a TBC system is cooled down from a stress-free state (assuming here that the TGO growth stresses are negligible). Due to the interfacial asperities, stresses normal to the interface can develop. Stresses are largest (in absolute value) at the maximum convex (peak) or minimum concave (valley) regions of the interface profile. When TGO is thin or absent, the stresses are tensile in the peak regions and compressive in the valley regions. In case of a relatively thick TGO, the stresses turn to be tensile in at valleys region and compressive at the peaks. At high temperatures, the residual stress relaxation induced by plastic or creep deformation can alter the stress state. Despite the popularity of finite element analysis in solving various problems related to performance of TBC systems, there are a number of issues related to adequate modeling of their complex structure (in particularly, a realistic interface shape) and mechanical behavior that need to be addressed. TBC coated turbine blades made of ZhS32 single crystal Ni-based superalloy were supplied after the cyclic oxidation test performed under flowing synthetic air in a purpose-built rig. The thermal cycle consisted of a heating period of 30  C min − 1 up to 1100  C, followed by a dwell and a cooling ramp with an initial rate of 30  C min − 1 . The total duration of one cycle was 23h. The specimens were cross-sectioned and prepared for metallographic analysis using conventional techniques, i.e. grinding, polishing (up to 1200 grit SiC paper), and fine polishing (up to 1 μm diamond paste). Before mounting, the samples were coated with a thin epoxy resin layer to prevent ceramic layers damage during metallographic preparation. The microstructure of the TBC cross-sections was characterized using optical metallography, scanning electron microscopy (SEM) using a FEG-SEM 7001F JEOL equipped with an energy dispersive analyzer Oxford Instruments (Inca Penta FET-x3 EDS spectrometer). The oxide and substrate phase structures were identified by X-ray diffraction (XRD) using a D8 Advance diffractometer with CuK  radiation.In order to evaluate the residual stresses the finite element model coded with a parametric design language was used within a commercial package (ANSYS, 2013). 2. Materials and methods The TBC coated system considered in this study consists of four components: the 2 mm thick substrate, ZhS32 single crystal Ni-based superalloy with the following chemical composition (in wt.%): 4.9 Cr, 9.0 Co, 1.0 Mo, 8.5 W, 5.9 Al, 4.0 Ta, 1.6 Nb, 4.0 Re (Eliseev J.S., 2008), the 30  m thick NiCoCrAlY-bond coat and 60  m thick EB PVD 7YSZ top coat. According to SEM-EDS characterisation confirmed by XRD analysis, the TGO consists mainly of  -Al 2 O 3 . The average thickness of TGO is 7  m. The typical irregularities of microstructure were (i) localized penetration of the TGO into the bond coat along the TGO/BC interface, (ii) localized penetration of the TGO into both the top coat and the bond coat. Representative back-scattered electron (BSE) images of the TBC cross-section and zoom in TGO irregularity (ii) which is critical in terms of crack initiation and propagation are presented on Fig.1. In this domain the oxide consists of two clearly distinct zones. The inner dark zone of TGO in contact with the metal (labeled by (1) in Fig.1b) is predominantly  - Al 2 O 3 and an outer relatively light zone (labeled by (2) in Fig. 1b) was identified as Cr-rich spinel type oxide Ni(Cr,Al) 2 O 4 . The bright spots near the TGO/TC interface labeled by (3) on BSE images Fig. 1b are Cr-rich oxide particles. The particles rich in Ta-oxide embedded in alumina were also identified by EDS mapping. 3. Microstructure analysis and model description