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Microstructural Characterization of Porous Thermal barrier Coatings by Laser Flash Technique

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Microstructural Characterization of Porous Thermal barrier Coatings by Laser Flash Technique

by Laser Flash Technique Federico Cernuschi*, Paolo Giulio Bison**, Alessandro Moscatelli*** 36th International Conference On Metallurgical Coatings And Thin Films ICMCTF 2009 San Diego, California, USA, 27 Aprile 1 Maggio 2009 PRESENTAZIONE POWER POINT * CESI RICERCA ** CNR-ITC *** POLITECNICO DI MILANO In the case of materials with open porosity such as thermal barrier coatings, the environment where the thermal diffusivity measurement is carried out (i.e. vacuum or gases) significantly affects the thermal diffusivity value. In fact, most of the more common gases have higher thermal diffusivity than that of YPSZ even if gas thermal diffusivity within the pores is lower than in large volumes ("bulk" value), when the pore dimension is comparable to the mean free path of gas molecules (Knudsen effect). The contribution of the measurement atmosphere to the thermal diffusivity strictly depends on the specific microstructural features of the porous sample under investigation such as porosity content, orientation and morphology. Thus, if thermal diffusivity measurements would be performed by varying gases a non destructive estimation of microstructural parameters would be possible. This approach can be used also to characterise sintering phenomena typically consisting in the healing of very fine crack-like pores which are difficult to be properly detected by conventional image analysis techniques. In this work an inversion procedure has been developed to obtain microstructural parameters describing the porosity morphology of porous thermal barrier coatings (TBC) from the thermal diffusivity measured in different environments by a Laser Flash technique. A simplified approach in the inversion procedure has been proposed, and the reliability has been checked by simulating different microstructures within the TBC. The inversion procedure has been also applied to experimental thermal diffusivity values of a TBC which were obtained by filling pores with He, N2, Ar and in vacuo. This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between CESI RICERCA and the Ministry of Economic Development – General Directorate for Energy and Mining Resources stipulated on June 21, 2007 in compliance with the Decree n.73 of June 18, 2007.

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