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The results of a study on two thermal barrier coating systems, oxidised isothermally in thermobalances at 1000°C, show that the surface roughness of the bond coat and its density have a significant effect on the oxidation behaviour. The particular system examined is composed of the Haynes 230 alloy substrate with CoNiCrAlY bond coat and yttria stabilised zirconia ceramic outer layer. Coatings were applied either using air- plasma spraying for both layers in the sample type with the coarsest interfacial roughness or electroplating deposition for the bond coat in the sample with the smoothest interface. Weight gain per unit geometric area is higher for the plasma-sprayed bond coat due both to its larger effective surface area and because of gas access into the structure and consequent internal oxidation. This enhanced oxidation results in a much more rapid depletion of aluminium within the bond coat and an earlier transition into chemical failure. The consequences of this on the formation of delamination cracks is discussed.