Strategies for Preparing and Analyzing Thin Passive Films With Atom Probe Tomography

Atom probe tomography (APT) provides a unique, three-dimensional map of elemental and isotopic distributions over a wide range of materials with near-atomic scale resolution and is particularly strong at analyzing buried interfaces within materials. However, it is much more difficult to apply atom probe to the analysis of nanoscale surface films, such as those formed during alloy passivation, where unique challenges persist for sample preparation and data collection. Here, we present sample preparation strategies involving the deposition of a <100 nm capping layer that enables reliable characterization of thin passive films ∼2-5 nm thick formed on binary and multiprincipal element alloys via APT. Several capping layer materials (Pt, Ti, and Ni/Cr bilayer) and deposition methods are contrasted. Our results indicate a sputtered Ni/Cr bilayer enables the characterization of the entire passive film and concentration profiles that can easily be interpreted to clearly distinguish base alloy/passive film/capping layer interfaces. Lastly, we highlight ongoing challenges and opportunities for this experimental approach.