A comparison of two high spatial resolution imaging techniques for determining carbide precipitate type and size in ferritic 9Cr-1Mo steel

Two high spatial resolution imaging techniques, focused gallium ion beam imaging in conjunction with XeF₂ gas (FIB/XeF₂) and high-speed atomic force microscopy (HS-AFM), were used to analyse 9Cr-1Mo ferritic steel samples, which had been exposed for extended periods to hot CO₂ gas containing traces of CO, H₂, H₂O and CH₄. The carbide precipitates embedded in the metal matrix were observed and their morphology, size and spatial distribution were quantified using these two techniques. The lower resolution of the FIB/XeF₂ imaging technique suggested that small carbide precipitates (<50 nm) may be missed, while the existence of a limited flow layer introduced by sample preparation may influence the HS-AFM results. The gallium ion beam was used to remove a thin oxide layer of approximately 50 nm from sample surfaces prior to FIB/XeF₂ imaging, avoiding the influence of surface contamination. HS-AFM provided higher resolution (~5 nm) than FIB/XeF₂ imaging. A quantitative comparison of the experimental data confirmed the value of both FIB/XeF₂ and HS-AFM for imaging carbide precipitates, while clarifying their strengths and limitations.