Oxidation of simple and Pt-modified aluminide diffusion coatings on Ni-base superalloys - II. Oxide scale failure

The spallation behavior of oxides formed during isothermal oxidation at 1050degrees C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings on the same Ni-base, single-crystalline superalloy (CMSX-4) was investigated by scanning electron microscopy (SEM) and transmission-electron microscopy (TEM). It was found that the main spallation mechanism was the formation of large Kirkendall voids at the oxide-coating interface. It is believed that the void formation was caused by counter-current flow of vacancies to the diffusion of Ni away from the interface as Al is consumed by the oxide. The magnitude of the vacancy current was determined by the oxidation rate. The properties of the void-formation mechanism are discussed in view of previous data on the microstructure of the oxide scales.