The Influence of the Laser Energy on the Thermal Diffusivity Evaluation of TBC by Laser Flash
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Federico Cernuschi and Paolo Bison (Submitted December 10, 2007; in revised form March 10, 2008) Laser Flash is considered the standard technique for measuring the thermal diffusivity of solids. The interaction between TBC and the laser energy is studied because very low thermal effusivity and thermal diffusivity of TBC can produce very high temperature increase on the surface and temperature gradient within the sample. In such a case, microstructural modifications of TBC can be generated. In this work, such phenomena are studied experimentally on free standing TBC samples.
Keywords
laser flash, microstructure evolution, thermal barrier coatings
1. Introduction Ceramic thermal barrier coatings (TBCs) are widely applied for protecting hot path components of gas turbines from combustion gases. The state-of-the-art TBC is represented by Yttrium oxide partially stabilized Zirconium (YPSZ) oxide (7-8 wt.% Y2O3 + ZrO2) deposited onto the components either by Air Plasma Spray (APS) or by Electron Beam Physical Vapor Deposition (EB-PVD) (Ref 1). TBC microstructural features being related to the deposition process parameters and service conditions, to estimate the effective insulation performances of TBCs, the thermal conductivity of either new or aged/serviced TBC samples is often investigated. In fact, exposure to high temperature promotes sintering phenomena within the TBC by microcrack healing, by neck formation, and by reducing the very fine porosity, making the TBC less strain compliant and more thermally conductive. Nowadays, Laser Flash (LF) is considered the standard technique for measuring the thermal diffusivity a of solids. This method consists in heating the front face of a sample by a short laser pulse and in detecting the temperature rise on its rear surface (Ref 2). This technique has been extensively studied in the past, especially for all those effects that limit the reliability and the accuracy of measurements, such as finite pulse duration (Ref 3), heat radiation losses (Ref 4), nonuniform heating (Ref 5), sample thickness (Ref 6).
Federico Cernuschi, CESI RICERCA, Via Rubattino, 54, 20134 Milano, Italy; and Paolo Bison, ITC – CNR, C.so Stati Uniti, 4, 35127 Padova, Italy. Contact e-mail: federico.cernuschi @cesiricerca.it.
Journal of Thermal Spray Technology
Heterogeneous refractory materials such as ceramic coatings, composites, and foams represent critical cases according to the aforementioned uncertainties sources. This is particularly true for TBCs which are low conductive and usually thin (100-1000 lm). Taylor et al. report the errors in thermal conductivity of TBC deposited onto metallic substrate calculated on the basis of the thermal diffusivity data, as given by LF measurements, considering the effects of errors in the input parameters (i.e., thickness, density and specific heat of coating and substrate, half-time, and substrate diffusivity) (Ref 7, 8). Many authors reported the effects of IR detector nonlinearity as a function of temperature. In particular, linearity could be usually assum
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