Abstract
Residual stresses play an essential role in determining the failure mechanisms and life of an electron beam-physical vapour deposition thermal barrier coating (EB-PVD TBC) system. In this paper, a new transitional roughness model was proposed and applied to describe the interfacial roughness profile during thermal cycles. Finite element models were implemented to calculate residual stresses at specific positions close to the interface of TBCs using temperature process-dependent model parameters. Combining stresses evaluated at valleys of the topcoat (TC) and excessive sharp tip roughness profiles, positions where the maximum out-of-plane residual stresses occur were identified and used to explain possible cracking routes of EB-PVD TBCs as interfacial roughness evolves during thermal cycling.
Highlights
The residual stresses that develop close to the interface between layers play a significant role in determining the life span of an electron beam-physical vapor deposition (EB-PVD) thermal barrier coating (TBC) system
A few analyses focused on the pattern variation of interfacial roughness profiles, where typical roughness parameters were measured from the observed microstructures and used to characterize the roughness profiles versus temperature and exposure duration
Less effort has been made on how a change in interfacial roughness affects the development of residual stresses [8,9,10], both of which could be critical in determining the possible positions where cracks nucleate at the maximum out-of-plane residual stresses estimated from the stress model
Summary
The residual stresses that develop close to the interface between layers play a significant role in determining the life span of an electron beam-physical vapor deposition (EB-PVD) thermal barrier coating (TBC) system. Different failure mechanisms of EB-PVD TBCs were developed based on the roughness level measured in the thermal cyclic experiments with the respective energy release rate (ERR) calculated [11]. A set of geometrical parameters used in this parametric study are obtained from the failure time (177 h on 1151 ◦C), while the temperature process-dependent material properties are fully integrated into FE calculations. It was experimentally found that the coating experiences a phase transformation and even crystallization process that would changes the material properties during the high-temperature annealing period [17] This effect is introduced in the current FE model where the elastic modulus of the TC is significantly affected by the sintering process during the isothermal exposure period, and the modulus undergoes a drastic change during thermal cycles.
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