Issue 35

K. Slámečka et alii, Frattura ed Integrità Strutturale, 35 (2016) 322-329; DOI: 10.3221/IGF-ESIS.35.37 323 microstructural imperfections within the top coat, the TGO or at either of its interfaces, according to the relative importance of the misfit stresses experienced upon cooling, the TGO-growth stresses building up during high- temperature exposure and counteracting stress-relaxation processes such as high-temperature creep or sintering of the top coat [2, 3]. Figure 1 : Atmospheric-plasma-sprayed CoNiCrAlY + YSZ thermal barrier coating on Inconel 713 LC substrate. One of the key features significantly influencing the damage development is the ‘‘roughness’’ (or more precisely the ‘‘surface texture’’ or ‘‘surface topography’’) of the bond coat. The roughness affects the oxidation of the bond coat (i.e. the growth of the TGO layer) and the initial as-sprayed microstructure of the top coat (and hence the overall stiffness of the top coat and also the availability of easy-growth crack paths near the interface) [4] thus governing the nature and evolution of stresses and strains and the actual coating failure mode. The modification of the interfacial roughness as a means of enhancing the coating lifetime has been exploited by several authors, e.g. [5-7]. Nowadays, it is agreed that a certain level of bond coat roughness is needed, as top coat bonding is provided primarily via mechanical interlocking, but that too rough coatings would fail prematurely. Experimental studies performed by Liu et al. [4], Dong et al. [5] and Chen et al. [8] also indicate that understanding the relationship between the ‘‘type’’ of roughness and the near-interface TC microstructure could be important in answering on what technological modifications of the bond coat prior to spraying of the top coat are desirable to maximize the performance. Given the complexity of the TBC systems and their operational modes, numerical methods have become important tools both for design of these coatings and for in-service life estimations and optimization. Present models allow the inclusion of many aspects of the degradation process, such as the creep deformation of individual constituents, the actual growth of the TGO layer, or sintering of the ceramic top coat, but they are usually restricted to two-dimensional problems and the continuum representation of the top coat as the realistic modelling of any intricate irregular topography and/or multiple material microstructural defects can impose enormous computational costs. In this contribution, the effect of these two features on stresses in critical locations (the top coat and the interfaces between the top coat, TGO, and the bond coat) and on damage evolution is discu ssed based on Finite Element (FE) and advanced Finite Element Microstructure MEshfree (FEMME) [9] calculations. The performed simulations point to the necessity of incorporating a realistic representation of roughness/microstructure into further studies on possible TBCs lifetime improvements. N UMERICAL MODELLING E calculations on a TBC with an irregular TC/BC interface were done using FE software ANSYS (ANSYS, Inc., Canonsburg, USA). The multilayer infinite-plate model consisted of (i) a nickel-base superalloy substrate, (ii) a generic M CrAlY bond coat, (iii) a single continuous α-alumina TGO layer, and (iv) an YSZ top coat having the following thicknesses: t substrate = 25 mm, t BC = t TC = 200  m and t TGO = 0 and 3  m (the intermediate stable growing state). All layers were assumed to be isotropic homogeneous materials with the ideal elastic (substrate, TGO, TC) or the F