Effect of Residual Stresses and Prediction of Possible Failure Mechanisms on Thermal Barrier Coating System by Finite El
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JTTEE5 19:1054–1061 DOI: 10.1007/s11666-010-9512-1 1059-9630/$19.00 ASM International
Effect of Residual Stresses and Prediction of Possible Failure Mechanisms on Thermal Barrier Coating System by Finite Element Method M. Ranjbar-Far, J. Absi, G. Mariaux, and S. Shahidi (Submitted December 16, 2009; in revised form March 16, 2010) This work is focused on the effect of the residual stresses resulting from the coating process and thermal cycling on the failure mechanisms within the thermal barrier coating (TBC) system. To reach this objective, we studied the effect of the substrate preheating and cooling rate on the coating process conditions. A new thermomechanical finite element model (FEM) considering a nonhomogeneous temperature distribution has been developed. In the results, we observed a critical stress corresponding to a low substrate temperature and high cooling rate during spraying of the top-coat material. Moreover, the analysis of the stress distribution after service shows that more critical stresses are obtained in the case where residual stresses are taken into account.
Keywords
finite element model, residual stresses, thermal barrier coatings system
Analytically, the magnitude of tensile quenching stress rq can be estimated from (Ref 4): rq ¼ ac ðTm Ts ÞEc
1. Introduction Air plasma sprayed (APS) thermal barrier coatings (TBC) protection is widely used to prolong the lifetime of turbine components. Because of the low thermal conductivity of the top-coat layer, the substrate temperature can decrease by some hundred degrees (Ref 1). The functionality and reliability of plasma spray coatings are strongly related to their microstructure, porosity (Ref 2), and residual stresses of thin films and coatings (Ref 3, 4). Among these factors, the effect of residual stress generated after coating process on the final stress appearing in the TBC system is the main objective of this study. Residual stresses in the plasma sprayed TBC are generated through three events (Ref 3-5):
Stresses associated with volume changes at solid-state phase transformation in a typical top coat. Stresses originating from rapid contraction of sprayed splats from the deposition temperature to that of the underlying materials named ‘‘quenching stresses.’’
M. Ranjbar-Far and J. Absi, GEMH, Ecole Nationale Supe´rieure de Ce´ramique Industrielle, 47, Avenue Albert Thomas, 87000 Limoges Cedex, France; G. Mariaux, SPCTS, University of Limoges, 16 Rue Atlantis, 87068 Limoges Cedex, France; and S. Shahidi, LIUPPA, University of Pau, Avenue de lUniversite´, 64013 Pau Cedex, France. Contact e-mail: [email protected].
1054—Volume 19(5) September 2010
ðEq 1Þ
where ac, Ec, Tm, Ts are coefficient of thermal expansion, elastic modulus of coating, melting point of the sprayed material and substrate temperature, respectively.
Stresses originated from differences in thermal expansion coefficients between those of underlying materials and coatings. Thermal stress at the surface of coating rtc can be estimated by: rtc Ec Da DT
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