FAILURE ANALYSIS AND LIFE PREDICTION of EB-PVD AND APS THERMAL BARRIER COATINGS

Thermal-barrier coatings (TBCs), which consist of a thermally insulating ceramic top coat (TC) and an oxidation-resistant metallic bond coat (BC), have long been used in high-thrust gas turbine engines in combination with internal cooling schemes to protect hot section engine components against hot combustion gases. These TBCs can be deposited by plasma spray (PS) or electron beam physical vapour deposition (EB-PVD) techniques, the latter being more often used for aero-engine applications, because the EB-PVD coating microstructure is more tolerant to thermal cycling. In this presentation, newly proposed thermal residual stress models were used to analyze the degradation behaviour of TBCs under cyclic loads and implement a life prediction approach into both PS and EB-PVD TBC system design. Based on the identified failure mechanisms, the residual stress models were formulated for the TC and the interface between a thermally grown oxide (TGO) and the BC naturally formed on its surface, for both PS and EB-PVD systems. The sintering effect of the TC, the mismatch of coefficients of thermal expansion (CTE) between the TGO and the substrate, the thickness of the TGO and the morphology of the interface were examined to evaluate failure characteristics and durability of TBCs. Specifically, the rumpling effect at the TGO/BC interface in EB-PVD TBCs due to thermal cycling was examined and its effect on coating delamination was investigated. Attempts were also made to study interactions of residual stresses distributed in TGO, TC and BC, upon which the effect of these interactions on the durability and the life of TBCs can be derived.