Abstract
The oxidation of the bond coat in a thermal barrier coating results in the formation of a protective alumina scale. Such scales experience large gradients in oxygen partial pressure (PO2) during formation, but it is unclear how PO2 affects grain-boundary (GB) transport in alumina. To study the effect of PO2 on GB kinetics in alumina, the rate of oxygen penetration was measured at 1400 °C over a wide range of PO2. This was accomplished using a novel reduction experiment using nickel aluminate spinel particles. The results indicated that the dependence of the boundary transport on PO2 is mainly due to the driving force, namely the gradient in PO2 across the alumina. Point-defect models were then invoked to interpret the measurements, and it was found that transport in an alumina scale is controlled by extrinsic defects.
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