Characterization of microstructural defects in plasma-sprayed thermal barrier coatings

Thermal barrier coatings with a NiCrAlY bond coating and a 1.5 mm thick zirconia top coating were air plasma sprayed onto a nickel-base substrate. The top coatings were deposited with the same spraying parameters except for the amount of external cooling, which varied from no cooling to the maximum available. This resulted in four sets of samples produced with different cooling conditions where substrate temperature varied from 100 to 830 °C. The coatings were examined by electron microscopy on polished surfaces and on fracture surfaces. The crack structure in the top coating was correlated to the substrate temperature. The density both of horizontal delaminations and of vertical microcracks was shown to decrease at higher substrate temperatures. The grain structure was columnar, and smaller grains were found at lower temperatures. Explanations for the differences in defect densities are discussed.