Coating Analysis via FTIR Spectroscopy

Significance of the Topic

Coating layers are critical for protecting surfaces against wear, corrosion and chemical attack, while also enhancing aesthetic and functional properties. Reliable analysis of these thin films ensures product durability and performance across industries such as electronics, medical devices and consumer goods. Infrared spectroscopy provides a nondestructive approach to verify coating presence, composition, thickness and uniformity, enabling quality control and failure investigation.

Goals and Overview

The application note AN M130 demonstrates the use of FTIR spectroscopy to:

• Detect and identify organic coatings on metal and non-metal substrates
• Determine layer thickness and homogeneity
• Assess surface cleanliness prior to coating
• Support failure analysis and reverse engineering of unknown films

Methodology and Used Instrumentation

Two FTIR systems were employed for distinct sample types:

• ALPHA II FTIR spectrometer with an upward DRIFT (diffuse reflectance) module for macro-scale analysis
• LUMOS II FTIR microscope for microscopic and mapping measurements on features down to a few micrometers

All spectra were acquired and evaluated using OPUS spectroscopic software, with identification assisted by the ATR-COMPLETE spectral library. Measurements included ATR reflection, DRIFT and automated line mapping for thickness determination.

Main Results and Discussion

Example 1: Costume Jewelry Coatings

• Acrylic resin and polyurethane coatings on nickel and brass jewelry were distinguished by their characteristic IR absorption bands.
• Spectrum library search matched unknown film spectra to poly(methyl methacrylate) and polyurethane references, confirming material identity.
• Clean metal surfaces displayed flat baselines, demonstrating absence of coating or contamination.

Example 2: Insulated Copper Wire

• Thermoplastic polyurethane insulation on enamel copper wire was identified via ATR measurements in the LUMOS II microscope.
• Thin-film interference fringes observed in reflection spectra were exploited to calculate local layer thickness using an assumed refractive index of 1.5.
• Automated line scans across 15 positions (60×60 µm each) yielded a thickness map, revealing an average film of ~25 µm with localized defects from mechanical damage.

Benefits and Practical Applications

FTIR spectroscopy offers several advantages for coating analysis:

• Nondestructive testing without sample preparation
• Rapid measurement and automated workflows for high throughput
• High chemical specificity for known and unknown film identification
• Spatially resolved thickness mapping for quality control of uniformity

These capabilities support routine QA/QC, failure analysis in manufacturing and reverse-engineering tasks.

Future Trends and Potential Applications

Emerging directions include:

• Integration of in-line FTIR sensors with process control for real-time coating monitoring
• Advanced multivariate and machine-learning spectral analysis to accelerate identification
• Extension to non-polymeric coatings such as DLC (diamond-like carbon) on tooling
• Higher spatial resolution and hyperspectral imaging for micro-structured surfaces

Conclusion

FTIR spectroscopy—implemented via Bruker’s ALPHA II and LUMOS II systems—proves to be a versatile, accurate and efficient tool for comprehensive coating analysis. It enables reliable identification of coating materials, precise thickness measurement and defect detection, improving product quality and supporting innovation across sectors.

Used Instrumentation

• ALPHA II FTIR spectrometer with upward DRIFT sampling module
• LUMOS II FTIR microscope
• OPUS spectroscopic software
• ATR-COMPLETE spectral library