Damage Accumulation Mechanisms in Thermal Barrier Coatings

Progressive damage evolution leading to spallation was investigated in Electron Beam—Plasma Vapor Deposition (EB-PVD) partially stabilized zirconia thermal barrier coating (TBC) applied to Nickel-based single crystal superalloy, Rene N5 with PtAl bondcoat. Thermal cycles were between 200-1177C. Progressive damage evolution was monitored using microscopy on samples subjected to a series of thermal cycles. Fick’s law can describe the thermally grown oxide (TGO) thickness for early cycles. However, at higher number of thermal cycles, damage in the form of microcracks and their link-up results in the development of a larger delamination crack through the TGO layer and monitoring oxide thickness becomes difficult. Thus, both oxidation kinetics and damage appears to play significant roles as they relate to spallation. As the early microcracks coalesce to form a major delamination crack, the susceptibility for TBC buckling is increased. The damage thickness continues to increase with number of thermal cycles indicating a progressive buckling condition. Estimation shows that a delamination crack length of about sixteen times the TBC thickness is needed for the current material system to cause buckling. Progressive microcrack linking to form a large delamination crack followed by progressive buckling of the TBC layer appear to promote spallation. Physical evidence of microcrack link-up and progressive buckling was found in specimens prior to complete spallation.