Nondestructive Thickness Determination of Coatings and Layers from Sub-micron to Tens of Microns Using Step-Scan FT-IR Photoacoustic Phase Spectroscopy

Importance of the topic

Layered and coated polymeric materials are ubiquitous across packaging, medical devices, electronics and surface treatments. Accurate, nondestructive measurement of coating and layer thickness is critical for quality control, functional performance and failure analysis. Conventional spectroscopic depth-profiling methods (microtoming for IR microscopy, ATR, or confocal Raman) suffer from destructive sample prep, limited probing depth or optical-property dependence. Step-scan FT-IR photoacoustic phase spectroscopy (S2ΦM PAS) provides a robust, nondestructive alternative able to measure thicknesses from sub-micron to tens of microns even in optically challenging, strongly scattering or opaque samples.

Objectives and study overview

The application note evaluates a simplified phase-difference model applied to S2ΦM PAS for layer-thickness determination in polymer laminates and thin coatings. The goals were to (1) demonstrate the practical probing range and resolution of the method, (2) validate a reduced model that simplifies thickness extraction, and (3) apply the approach to two representative samples: a multilayer food-packaging laminate and a sub-micron coating on a textured elastomer.

Methodology

S2ΦM PAS measures the photoacoustic (PA) response as two interferogram components referenced to the modulation phase: the in-phase (I) component, dominated by near-surface contributions, and the quadrature (Q) component, which contains deeper, time-delayed thermal-wave information. The magnitude spectrum M = (I2 + Q2)1/2 and the phase spectrum Φ = arctan(Q/I) are used to locate the phase maxima of characteristic vibrational bands belonging to different layers. Depth sensitivity is set by the thermal diffusion length µ = (α/πf)1/2, where α is thermal diffusivity and f the phase-modulation frequency; thus the probing depth is frequency controlled and approximately constant across the spectral range in S2ΦM PAS.

The authors previously derived a general model linking phase differences between PA signals from layers to optical and thermal material properties; in the optically/thermally thick limiting case the relationship simplifies. When the dominant absorption bands of each layer are used, absorption-related terms become large and the model reduces to a simple proportionality between measured phase difference and layer thickness. This simplification permits straightforward experimental thickness determination provided layer-specific bands can be identified and appropriate thermal diffusivities are known or estimated.

Used instrumentation

The measurements reported used a Thermo Scientific Nicolet 8700 FT-IR spectrometer with step-scan phase-modulation capability and an MTEC 300 photoacoustic accessory. A Ge-on-KBr beamsplitter and an air-cooled IR source were fitted; the spectrometer bench was purged with dry CO2-free air and the PA cell purged with high-purity helium. A 1975 cm-1 long-wave-pass filter restricted the spectral range to improve step-scan collection efficiency. Phase referencing used a glassy carbon standard and data handling employed Thermo OMNIC software.

Main results and discussion

1) Food-packaging laminate: The sample consisted of four polymer layers (polypropylene, polyethylene, a pigmented intermediate layer, and polypropylene). S2ΦM PAS magnitude spectra were collected at modulation frequencies from 300 to 100 Hz. As f decreased (longer thermal diffusion length), bands from deeper layers emerged in the spectra. Using characteristic bands (e.g., 1382 cm-1 for polypropylene and 1732 cm-1 for the pigment layer), phase maxima were located and phase differences calculated. Applying the simplified model with an assumed thermal diffusivity for polypropylene (α = 0.78 × 10-3 cm2 s-1) gave a polypropylene layer thickness of 21.0 ± 0.5 µm, in excellent agreement with an optical thickness measurement of about 22 µm from a sectioned sample. The authors note signal saturation at lower modulation frequencies can reduce apparent phase differences and bias thickness estimates downward if not accounted for.

2) Coating on elastomer: A highly filled, opaque elastomer with a delicate, sub-micron coating was analyzed. ATR (Ge, single-bounce, penetration depth ≈ 0.6 µm) indicated the coating was thinner than ~0.5 µm because substrate features remained visible. S2ΦM PAS at 500 Hz provided optimal probing depth and signal-to-noise for this system. Phase differences between coating and substrate bands were measured on multiple pieces; applying the simplified model with the coating thermal diffusivity provided by the manufacturer (α = 1.86 × 10-3 cm2 s-1) yielded a mean thickness consistent with prior gravimetric estimates. Replicate variation was attributed to coating non-uniformity over the textured elastomer surface.

Benefits and practical applications of the method

• Nondestructive thickness measurements spanning ~0.1 µm to tens of microns without sectioning or contact-based removal.
• Operates on optically challenging, scattering or opaque substrates where ATR or confocal Raman are limited.
• Frequency-controlled probing depth provides tunable sampling of different depths; phase analysis isolates signals from different layers.
• Relatively simple data processing when the simplified phase-difference model applies (dominant bands present and optically/thermally thick conditions approximated).
• Useful for industrial QA/QC of multilayer packaging, thin functional coatings, and layered polymer constructions where rapid, nondestructive checks are required.

Limitations and practical considerations

• Accurate thickness extraction requires known or estimated thermal diffusivity for the layer material; uncertainties in α propagate to thickness errors.
• The simplified model assumes strong, layer-specific absorption bands and regimes where absorption terms are large; for weak bands or thin optical layers the full model may be required.
• Low modulation frequencies can induce PA signal saturation from deep absorbers, reducing phase contrast and biasing results.
• Surface roughness and non-uniform coatings introduce measurement scatter; replicate sampling is recommended.

Future trends and potential applications

Further development could focus on applying more comprehensive multilayer models to extract thicknesses of multiple subsurface layers simultaneously and to handle optically thin regimes. Potential directions include frequency sweeps or multi-frequency mapping to improve depth resolution, integration with imaging PA accessories for lateral mapping, calibration protocols to reduce dependence on a priori thermal diffusivity values, and coupling with data-driven algorithms for automated layer identification and thickness prediction. Industrial deployment could leverage rapid, in-line PA sampling for production QA of laminates and coated products.

Conclusions

The simplified phase-difference approach applied to step-scan FT-IR photoacoustic phase spectroscopy is an effective, nondestructive method for measuring coating and layer thicknesses from sub-micron to tens of microns, including in optically challenging samples. Validation on a multilayer food-packaging laminate produced thickness results that matched optical sectioning, and sub-micron coatings on an elastomer were resolved consistent with independent gravimetric expectations. The method’s tunable probing depth and phase-based discrimination make it a valuable tool for research and industrial quality control, with further opportunities for model refinement and multispectral/frequency approaches to broaden applicability.

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