Damage Accumulation Mechanisms in Thermal Barrier Coatings

摘要

The primary focus of the investigation was to conduct a chronological evaluation of damage initiation and propagation in Electron Beam - Physical Vapor Deposition (EB-PVD) thermal barrier coated (partially stabilized zirconia) Rene N5 single crystal superalloy. Damage initiation and failure events due to thermal cycling were investigated. Early crack initiation was rationalized through interfacial asperity induced high stresses between the TBC and the PtAl bond coat . Analytical solution was forwarded for transient and steady-state residual stresses in the EB-PVD system. Oxide scale development as a function of time was predicted using Fick' S Law. The deviation of experimental results from Fick's Law was attributed to microcrack interaction and coalescence. Oxide scale induced internal pressure on crack surfaces was shown to be a plausible mechanism for microcrack growth. Buckling analysis was used to estimate the critical size of delamination necessary (sixteen times the TBC thickness) for spallation. It was shown that for various thermal cycles, spallation life can be estimated based on critical oxide layer thickness. Thermal wave imaging technique was used to track damage condition as a function of thermal cycles. It was found that saturation of thermal wave amplitude corresponds to spallation life of the sample. This work provides a mechanism-based framework for life prediction in TBC systems.