Application of image analysis and image simulation for quantitative characterization of scale spallation during cyclic oxidation of a Pt-aluminide coating

An image analysis and simulation of the spalled alumina areas at the surface of a Pt modified b-NiAl aluminide coating RT22 during cyclic oxidation in air is presented. The size distribution of the freshly spalled areas and of the unspalled zones is studied for the same coating deposited on three different nickel base superalloys which were exposed to various numbers of 300 h cycles at 1050 8C (up to 10,500 h) in a previous study [Vialas N, Monceau D. Oxid Metals, submitted for publication]. It is shown for this coating that the distribution of the freshly spalled area depends on the nature of the coated substrate and on the number of cycles. It is also shown that spallation of these three systems can be simulated by a twoparameter model which describes the spalling during the cooling step as successive microspalling events through nucleation and growth processes. This approach is consistent with previous in-situ observations and allows the introduction of a time-dependent damaging process in the models of cyclic oxidation kinetics