A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system. Part II: Life prediction model

A parametric study is conducted to quantify the effects of different microstructural variables and service conditions on the local stresses induced by oxidation, sintering processes and thermal cycling in a typical plasma sprayed thermal barrier coating (PS-TBC) system. The study relies on the numerical results obtained from a continuum mechanics-based mechanistic study of the oxidation-induced degradation of the PS-TBC system. Analytical expressions are presented for the peak out-of-plane stress component which promotes the nucleation and growth of mesoscopic cracks within the top zirconia-based ceramic coating in terms of thermal cycle parameters, and accumulated oxidation time. Based on the results of the parametric study, a damage mechanics-based life prediction methodology for the failure of the PS-TBC under thermal fatigue loading conditions is proposed. The model assumes that PS-TBC failure occurs by a cleavage-type mechanism within the top zirconia coating, in agreement with experimental evidence, and that the accumulation of damage with thermal cycling is linked to the gradual degradation of the intrinsic cleavage strength of the zirconia. The model is shown to be capable of predicting consistently a broad range of thermal fatigue data. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.