Issue 35

K. Slámečka et alii, Frattura ed Integrità Strutturale, 35 (2016) 322-329; DOI: 10.3221/IGF-ESIS.35.37 328 temperature decay and the damage growth are approximately linear with external temperature change, and between 820 ºC and 1020 ºC the temperature of the points above the TGO layer increases due to the increment of the damage between those points and the external surface. Up to 820 ºC the damage is developed along the TGO layer, Fig. 8, increasing the insulation of those points, which is shown in the decay of the temperature ratio. Between 820 ºC and 1020 ºC the damage propagates to reach the surface, reducing the insulation and increasing the element temperature relative to the external temperature. The boundary conditions are applied only in the external nodes, so the insulating effect of damage on the conductivity of the damaged finite element is apparent. To model the hotspots created by the cracks in a more representative manner, more complex boundary conditions could be implemented to address the penetration of hot gases through the open cracks. C ONCLUSIONS he performed calculations show that both the waviness of the BC/TC interface and the microstructure of the ceramic top coat have a significant impact on predicted stresses and the damage evolution. The stresses due to waviness are superimposed with stresses due to the form and with those locally induced by roughness, increasing the chances of an early damage formation near the large BC peaks. The particular crack path is strongly influenced by thermal history, the local waviness and the microstructure of the top coat. A system approach to TBC design and performance improvements thus clearly requires that a realistic representation of both the interfacial geometry and the microstructure is introduced into numerical models. A CKNOWLEDGEMENTS . Slámečka, P. Skalka, L. Čelko, and J. Pokluda acknowledge the financial support provided in the frame of the projects (GA 15-20991S) by the Czech Science Foundation project and "CEITEC - Central European Institute of Technology" (CZ.1.05/1.1.00/02.0068) by European Regional Development Fund. L. Saucedo-Mora and T.J. Marrow acknowledge the support of the UK EPSRC project “QUBE: Quasi- Brittle fracture: a 3D Experimentally- validated approach” (EP/J019992/1). R EFERENCES [1] Besmann, T., Interface science of thermal barrier coatings, J. Mater. Sci., 44 (2009) 1661-1663. DOI: 10.1007/s10853-009-3323-0. [2] Rabiei, A., Evans, A., Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings, Acta mater., 48 (2000) 3963-3976. DOI: 10.1007/s10853-009-3323-0. [3] Evans, A.G., Mumm, D.R., Hutchinson, J.W., Meier, G.H., Pettit, F.S., Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater Sci., 46 (2001) 505-553. [4] Liu, D., Seraffon, M., Flewitt, P., Simms, N., Nicholls, J., Rickerby, D., Effect of substrate curvature on residual stresses and failure modes of an air plasma sprayed thermal barrier coating system, J. Eur. Ceram. Soc., 33 (2013) 3345-3357. DOI: 10.1016/j.jeurceramsoc.2013.05.018. [5] Vaßen, R., Kerhoff, G., Stöver, D., Development of a micromechanical life prediction model for plasma sprayed thermal barrier coatings, Mater. Sci. Eng., A303 (2001) 100-109. [6] Eriksson, R., Sjöström, S., Brodin, H., Johansson, S., Östergrenc, L., Li, X., TBC bond coat–top coat interface roughness: Influence on fatigue life and modelling aspects. Surf. Coat. Technol., 236 (2013) 230-238. DOI: 10.1016/j.surfcoat.2013.09.051. [7] Dong, S., Song, B., Zhou, G., Hansz, B., Liao, H., Coddet, Ch., Multi-layered thermal barrier coatings fabricated by plasma-spraying and dry-ice blasting: Microstructure characterization and prolonged lifetime, Surf. Coat. Technol., 236 (2013) 557-567. DOI: 10.1016/j.surfcoat.2013.10.066. [8] Chen, W., Wu, X., Marple, B., Lima, R., Patnaik, P., Pre-oxidation and TGO growth behaviour of an air-plasma- sprayed thermal barrier coating. Surf. Coat. Technol., 202 (2008) 3787-3796. DOI: 10.1016/j.surfcoat.2008.01.021. T K