Out-of-sync co-evaporation in atom probe tomography: Shifting perspectives on compositional biases in TiC and TiN coatings

Atom probe tomography (APT) offers unparalleled three-dimensional insights into the nanoscale composition of advanced materials. However, the presence of multiple detection events, charge state distributions as well as molecular ions and consequent compositional biases complicate quantitative analyses. In this study, the phenomenon of out-of-sync co-evaporation in binary transition metal carbides and nitrides is explored through a comparative study of TiC and TiN coatings. These coatings, synthesized by chemical vapor deposition on cemented carbide substrates, were investigated using a reflectron-based LEAP 5000 XR and a straight-flight-path LEAP 5000 XS atom probe in laser-assisted mode. For validation, time-of-flight elastic recoil detection analysis, Rutherford backscattering spectrometry and elastic backscattering spectrometry were employed to obtain accurate reference compositions.
Ion correlation histograms and out-of-sync co-evaporation plots were used to analyze compositional biases and field evaporation behavior. TiC exhibited pronounced thermal tails in the mass spectrum and structured ion patterns in the ion correlation histogram. Co-evaporation plots revealed tracks for both synchronous and out-of-sync evaporation, predominantly involving C-containing ion pairs. In contrast, TiN showed minor thermal tails, but a variety of vertical tracks, indicating out-of-sync co-evaporation. Additionally, its stronger tendency for molecular ion formation further complicates the evaporation analysis.
This work unveils the complex interplay of field evaporation mechanisms in transition metal carbides and nitrides, while introducing an analysis tool for more accurate characterization of compound materials. By utilizing the commonly neglected background, crucial insights into APT analyses are gained and thus, could play a pivotal role in paving the way toward increased measurement accuracy.