Micro- and nano-scale analysis of passivated stainless-steel landing gear with XPS, SEM, and TEM

Importance of the topic

Landing-gear components demand exceptional strength, toughness, and oxidation/abrasion resistance. Traditional protective coatings such as cadmium or chromium offer excellent performance but pose environmental and regulatory concerns. Replacing coated carbon-steel components with passivated stainless-steel alternatives requires rigorous characterization of surface chemistry, inclusions, and nanoscale precipitates to ensure corrosion resistance, mechanical integrity, and manufacturability. Multi-scale analysis that links surface condition to subsurface microstructure and nanoscale precipitates is therefore critical for qualification and process optimization.

Objectives and study overview

This application study aimed to diagnose a surface staining problem on a GKN Aerospace stainless-steel landing-gear cylinder and to evaluate whether a passivated stainless-steel component could be a viable, environmentally preferable replacement for coated carbon-steel parts. Specific objectives were to:

• Identify the nature and origin of the surface stain;
• Characterize near-surface chemistry and passivation effectiveness;
• Quantify and classify micro- and nano-scale inclusions and precipitates that influence mechanical properties;
• Recommend process adjustments to prevent staining and preserve precipitate-driven hardening.

Materials and composition summary

The landing-gear alloy was produced by vacuum induction melting and vacuum arc re-melting. Key compositional highlights (as measured by optical emission spectroscopy) include:

• Chromium ~11.6 wt% and nickel ~11.2 wt%;
• Titanium ~1.6 wt% (notably high for this family of alloys);
• Light elements: C ~24 ppm, N ~18 ppm, S ~6 ppm; dissolved oxygen ~2 ppm due to aluminum deoxidation;
• Balance iron (~74 wt%).

Low C, N, and S (<25 ppm) and controlled oxygen indicate tight refining targeted to minimize formation of TiC, TiN, TiS or TiO2 during processing.

Used instrumentation

This study combined complementary instruments for depth-resolved chemistry and multi-scale imaging:

• Optical emission spectrometer: Thermo Scientific ARL iSpark 8860 for bulk composition;
• Scanning electron microscopy (SEM) with integrated EDS: Thermo Scientific Axia ChemiSEM and Phenom ParticleX Steel Desktop SEM for surface imaging, ChemiSEM compositional mapping, and automated inclusion analysis;
• X-ray photoelectron spectroscopy (XPS): Thermo Scientific Nexsa G2 Surface Analysis System for top ~10 nm chemical analysis and depth profiling to 80 nm;
• Focused ion beam (FIB) lift-out: Thermo Scientific Helios 5 Laser PFIB for TEM sample preparation;
• Transmission electron microscopy (TEM) with STEM-EDS: Thermo Scientific Talos F200X equipped with Automated Particle Workflow (APW) for unattended high-resolution imaging, mapping, and particle sizing.

Methodology

The diagnostic workflow proceeded from surface to nanoscale:

1. Macroscopic surface inspection and SEM (secondary electron and backscattered electron imaging) to locate stains, machining marks, and fractured particles;
2. ChemiSEM/EDS to identify fractured surface particles (e.g., TiN) and map elemental contrast from the top micrometer;
3. XPS depth profiling (0–80 nm) on both a clean and a stained cylinder to quantify passivation layer composition and depth distribution of Cr, Fe, O, Ni, and Ti states;
4. Automated SEM inclusion mapping (ParticleX) over 50 mm², characterizing all inclusions >1 µm by size, chemistry (TiN, TiS), area fraction, and spatial distribution;
5. PFIB lift-out of lamellae and TEM/STEM-EDS mapping to detect and size nanoscale η-Ni3Ti precipitates implicated in precipitation hardening; APW collected tile-by-tile images and EDS maps over ~1 µm² and performed automated particle sizing.

Main results and discussion

Surface chemistry and passivation:

• XPS depth profiles for a properly passivated (clean) cylinder showed a chromium- and oxygen-rich top layer concentrated in the top ~20 nm consistent with formation of Cr2O3, with metallic Fe, Cr and Ni beneath — indicating an effective chemical passivation barrier.
• The stained cylinder displayed oxidized iron and chromium throughout the full 80 nm profile, indicating inadequate or compromised passivation and explaining visible discoloration.
• XPS Ti2p spectra confirmed titanium existed in multiple states near the passivation region: TiO2, TiN, and some metallic Ti — corroborating SEM findings of Ti-containing particles at the surface.

Surface and subsurface inclusions:

• ChemiSEM imaging revealed fractured TiN particles on the machined surface; fracture was attributed to machining/grinding rather than subsurface degradation, since cross-sectional inclusions showed no deformation.
• Automated SEM over 50 mm² quantified inclusions >1 µm; the TiN + TiS combined area fraction was ≈2.278 × 10⁻⁴ for the clean sample and ≈2.275 × 10⁻⁴ for the stained sample — essentially identical, indicating that overall inclusion populations were not the cause of the staining difference.
• TiS inclusions were present in the bulk cross section but were absent from the surface on the passivated component, consistent with acid-based passivation dissolving sulfides at the surface.

Nanoscale precipitates and hardening phases:

• TEM STEM-EDS mapping identified η-Ni3Ti nanoscale precipitates associated with precipitation hardening. Using the Talos F200X APW, >4,000 precipitates were measured with an average length of ~12 nm, confirming a nanoscale distribution consistent with strengthening by η-Ni3Ti.
• Automated TEM particle sizing validated the precipitate size distribution and provides a repeatable metric to compare heat-treatment or compositional changes.

Interpretation and corrective action:

• The staining was linked to inadequate surface passivation (oxidation through 80 nm) and the presence of fractured TiN particles at the surface; machining-induced particle fracture likely exposed TiN and other reactive phases to oxidation.
• Revisions to surface cleaning and passivation steps (improved removal of deposited particles, adjusted acid/passivation parameters) eliminated future staining in production cylinders.

Benefits and practical applications

Key practical outcomes from the multi-technique investigation:

• Established a robust, multi-scale diagnostics workflow to resolve surface failures (XPS) and correlate them with microstructure (SEM) and nanoscale strengthening phases (TEM).
• Enabled process changes that prevented staining without altering base alloy chemistry or precipitate strengthening, supporting stainless steel as an environmentally preferable replacement for cadmium/chromium coatings on certain landing-gear components.
• Provided quantitative inclusion and precipitate metrics (inclusion area fractions, size distributions, precipitate average length) that can serve as quality-control benchmarks for production and heat-treatment consistency.

Future trends and potential applications

Building on these findings, anticipated directions and opportunities include:

• Broader adoption of integrated, automated multi-scale workflows (SEM ChemiSEM + XPS + TEM APW) for routine qualification of critical aerospace components.
• Use of advanced surface analytics (angle-resolved XPS, ToF-SIMS) and in-situ or low-damage FIB methods to better resolve thin oxide chemistry and particle chemistry at pristine surfaces.
• Applying data-driven correlations between inclusion/precipitate statistics and mechanical/fatigue performance to enable predictive quality control and targeted alloy or process modifications.
• Evaluating alternative, lower-emission coating or surface-engineering approaches (e.g., HVOF thermal spray on stainless substrates) coupled with the presented analytical workflow to balance environmental, corrosion, and wear requirements.

Conclusions

A coordinated XPS–SEM–TEM characterization campaign resolved the root cause of surface staining on a stainless-steel landing-gear cylinder and demonstrated that adequate passivation is essential to prevent oxidation-driven discoloration. Fractured TiN particles produced by machining were identified on the surface, while TiS and TiN populations in the bulk were quantified and shown not to differ between clean and stained parts. TEM confirmed the presence and size distribution of η-Ni3Ti precipitates responsible for precipitation hardening (average length ≈12 nm). Process modifications to cleaning and passivation successfully prevented subsequent staining, supporting stainless steel as a viable, more environmentally conscious alternative to chromium- or cadmium-coated carbon steel for selected landing-gear components.

Reference

• Thermo Fisher Scientific, Application Note AN0217-EN-01-2023: Micro- and nano-scale analysis of passivated stainless-steel landing gear with XPS, SEM, and TEM.