XPS depth profiling of coated nitrided steel using femtosecond laser ablation

Significance of the topic

Accurate compositional and chemical-state information as a function of depth is essential for understanding and optimizing coated, nitrided steels used in demanding tribological applications. Conventional ion sputter depth profiling often alters stoichiometry and chemical states through preferential sputtering and beam-induced chemistry, compromising interpretation of buried interfaces. Femtosecond laser ablation (fs-LA) coupled with X-ray photoelectron spectroscopy (XPS) offers a promising alternative that can preserve chemical integrity while enabling faster, deeper profiling for multi-layer systems such as MoS2:Ti coatings on nitrided steel.

Objectives and scope of the study

The study aims to evaluate fs-LA/XPS depth profiling as an alternative to monoatomic Ar+ ion sputtering for a heat-treated, gas-nitrided steel (EN31CrMoV9) coated with a 1.2 µm Ti-doped MoS2 film. Key goals were to compare stoichiometry retention, chemical-state preservation, achievable depth and interface resolution, and overall experiment duration between fs-LA and conventional ion sputtering approaches.

Experimental approach and methodology

Analyses were performed on a nitrided EN31CrMoV9 sample with a PVD-deposited 1.2 µm MoS2:Ti top layer. XPS spectra were acquired in SnapShot mode with a 30 µm X-ray spot. Two depth-profiling methods were compared:

• Ion sputtering: 500 eV monoatomic Ar+ from a MAGCIS Dual Beam Ion Source for sequential sputter–analyze cycles.
• Femtosecond laser ablation: 1,030 nm laser pulses at 160 fs pulse length; pulse energy ramped from ~83 µJ in the coating to ~250 µJ in the bulk to maintain material removal and interface access.

Complementary techniques referenced for cross-validation included glow-discharge optical emission spectroscopy (GDOES) and X-ray diffraction (Rietveld analysis) for phase and stoichiometry benchmarks.

Used instrumentation

• Thermo Scientific Hypulse Surface Analysis System for XPS acquisition (SnapShot mode, 30 µm X-ray spot).
• Thermo Scientific MAGCIS Dual Beam Ion Source for 500 eV Ar+ sputtering.
• Femtosecond laser ablation system: 1,030 nm wavelength, 160 fs pulse duration, adjustable pulse energy (reported 83–250 µJ).

Main results and discussion

Comparative profiling revealed marked differences attributable to the removal mechanism:

• Coating stoichiometry: Ion sputtering produced a distorted composition for the MoS2:Ti layer ((Ti,Mo)0.5), indicating sulfur loss and chemical damage (also evidenced by altered Mo3d, Ti2p, and S2p peak shapes/positions). In contrast, fs-LA preserved coating stoichiometry near (Mo,Ti)S1.8 and maintained consistent binding energies for Mo3d, Ti2p and S2p, implying minimal chemical modification.
• Iron nitride layer: fs-LA yielded Fe:N ratios consistent with compound-nitride phases (Fe2.5–3.2N), closely matching GDOES and Rietveld-derived expectations (Fe2.8–3.3N and identification of ε- and γ’-phases). Ion sputtering produced artificially high Fe:N ratios (Fe6.4–7.9N), reflecting strong preferential removal of nitrogen during Ar+ bombardment.
• Depth and resolution: fs-LA produced depth profiles extending to ~11 µm (profile shown) while preserving interface contrast comparable to ion profiles; ion sputtering in this study reached only ~5–6 µm for similar or worse resolution.
• Throughput: The fs-LA profile to ~11 µm required approximately 7 hours overall (predominantly XPS acquisition time). The Ar+ ion approach took over 30 hours to reach about half that depth, with ~65% of time consumed by sputtering.

Benefits and practical applications

Fs-LA coupled with XPS provides distinct advantages for multi-layer, chemically sensitive systems:

• Improved stoichiometry fidelity and chemical-state preservation compared with low-energy ion sputtering, especially for chalcogenide and nitride systems prone to preferential sputtering.
• Greater effective sampling depth and faster total experiment time, enabling characterization of thick coatings and deep interfaces within practical laboratory timeframes.
• Tunable laser parameters (pulse energy) that can be adapted across layers to optimize ablation rate and preserve interface resolution.
• Applicability to industrial and research settings where reliable buried-interface chemistry is critical—examples include tribological coatings, corrosion-resistant nitrided layers, and complex PVD/CVD multilayers.

Future trends and opportunities

Adoption and further development of fs-LA/XPS depth profiling will likely follow several directions:

• Method standardization and development of best-practice protocols (e.g., laser energy ramp strategies, crater characterization) to ensure reproducibility across materials and instruments.
• Integration with quantitative calibration (using GDOES, RBS or standard reference materials) to enable absolute concentration depth profiles and validated stoichiometry reporting.
• Combined multimodal workflows that correlate fs-LA/XPS with microscopy (SEM/TEM) and diffraction for comprehensive interface characterization.
• Instrumental improvements to increase throughput further, refine depth resolution, and reduce redeposition or matrix-dependent artefacts.

Conclusions

Femtosecond laser ablation XPS depth profiling is a robust alternative to conventional low-energy Ar+ sputtering for coated, nitrided steels. In this application, fs-LA preserved coating and nitride stoichiometry, maintained chemical-state information, extended profiling depth, and reduced total analysis time. These attributes make fs-LA particularly attractive for studying chemically delicate layers and deeply buried interfaces where ion-beam methods introduce artefacts.

References

1. Baker MA, et al. Femtosecond laser ablation (fs-LA) XPS – A novel XPS depth profiling technique for thin films, coatings and multi-layered structures. Applied Surface Science. 654 (2024): 159405.