Modeling the Influence of Reactive Elements on the Work of Adhesion between Oxides and Metal Alloys
A method is presented that allows determination of the work of adhesion between oxides and metals using a macroscopic atom model. The method allows complex interfaces to be modeled easily and the influence of additives and impurities to be assessed. The model is used to study the work of adhesion between α-Al2O3 and -NiAl. This interface is of importance for the performance of thermal barrier coatings as applied to jet turbines. The model shows that the work of adhesion is not significantly altered by so-called reactive element additions. Reactive elements are known to improve the durability at the alloy/oxide interface and include elements such as Zr, Y, and Hf added in concentrations of less than 1 at.%. A significant weakening of the interface is predicted when impurities such as sulfur and carbon are present. The model also predicts a large interaction enthalpy between the reactive elements and impurities. It is proposed that the primary effect of reactive element additions is impurity scavenging. The impurities are fixed in the bulk of the alloy by the reactive elements and cannot diffuse to the oxide/metal interface to weaken it.
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