Abstract No.:
1318

 Scheduled at:
Tuesday, June 03, 2008, Auditorium 2 2:00 PM
Gasturbines 3
Coatings for mobile and stationary turbines, protection against wear, high temperature corrosion and thermal stresses, clearance control coatings for a better efficiency


 Title:
Failure mechanism for thermal fatigue of thermal barrier coating systems

 Authors:
Carlo Giolli* / Turbocoating SpA, Parma
Andrea Scrivani / Turbocoating SpA, Italy
Gabriele Rizzi/ Turbocoating SpA, Italy
Francesca Borgioli/ Dipartimento di Ingegneria Civile, Università di Firenze, Italy
Luca Lusvarghi/ Università degli Studi di Modena e Reggio Emilia, Italy
Giovanni Bolelli/ Università degli Studi di Modena e Reggio Emilia, Italy

 Abstract:
Failure Mechanism for Thermal Fatigue of Thermal Barrier Coating systems.
C. Giollia,c, A. Scrivanic, G. Rizzic, F. Borgiolib, Giovanni Bolellid and Luca Lusvarghid

aUniversità di Firenze, Dip. di Chimica, Via della Lastruccia 3 Sesto Fiorentino 50019 Firenze
bDipartimento di Ingegneria Civile, Università di Firenze, via S. Marta 3, 50139 Firenze, Italy
cTurbocoating S.p.A., via Mistrali 3, 43010 Rubbiano di Solignano, Italy
dUniversità degli Studi di Modena e Reggio Emilia, Department of Materials and Environmental Engineering, Via Vignolese 905, 41100 Modena (MO), Italy

Abstract
High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. Due to the difference in thickness and microstructure between thick TBCs and traditional thin TBCs, they cannot be assumed a-priori to possess the same failure mechanisms. Thick TBCs, consisting of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat with different degrees of porosity, were produced by Air Plasma Spray. Thermal fatigue resistance limit of TBCs was tested by Furnace Cycling Tests (FCT) according to the specifications of two important Original Equipment Manufacturers (OEMs). TBC systems were analyzed before and after FCT. The morphological and chemical evolution of CoNiCrAlY/TGO microstructure was studied. Sintering effect, residual stress, phase transformation and fracture toughness were evaluated in the ceramic Top Coat. All the tested samples passed FCT according to both OEM specifications. The limit of thermal fatigue resistance increases with the amount of porosity in the Top Coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/Top Coat interface. The failure mechanism of thick TBCs subjected to thermal fatigue is similar to the failure mechanism of thin TBC systems made by APS.


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