Meaning
Structural degradation in bimaterial joints or coated metals occurs due to the unequal diffusion rates of different atomic species across an interface. The formation of Kirkendall vacancy porosity involves the accumulation of atomic-scale vacancies into macroscopic voids on the side of the faster-diffusing element. This phenomenon weakens the physical bond between the materials.
Void Formation
Atoms do not diffuse at identical speeds. When a coating and a substrate exchange atoms, the faster migration of one species creates a net deficit of atoms on its original side. This deficit results in Kirkendall vacancy porosity, which appears as a line of microscopic holes parallel to the interface.
Coating Degradation
Coated components exposed to high-temperature environments are particularly vulnerable to this form of decay. Over time, the Kirkendall vacancy porosity can grow to the point where the protective coating delaminates from the underlying superalloy. This delamination exposes the turbine blade to rapid oxidation and hot corrosion.
Quality Control
Industrial compliance audits require metallographic examination of coated parts to detect these boundary voids. Suppliers must limit the growth of Kirkendall vacancy porosity by utilizing diffusion barrier layers like platinum or rhenium. These barriers are applied using specialized electroplating equipment before the protective aluminide coating is deposited.
Regional quality centers assess the effectiveness of these barriers by performing thermal cycling tests on sample coupons, ensuring that only parts with stable coating interfaces are cleared for assembly.
