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Automotive Paint Layer Analysis with Optical Microscopy, SEM, and EDS

Automotive Paint Layer Characterization
Courtesy of Getty Images

Author: Yi Zhang, PhD. JH Technologies Application Scientist

Abstract

Automotive paint typically consists of multiple layers designed to provide corrosion resistance, color, and a durable glossy finish. In this study, we used optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to characterize the microstructure and elemental composition of the paint layers on a car body.

Materials and Methods: Automotive Paint Sample Preparation

A metal specimen with paint coatings on both sides was sectioned from the vehicle body and mounted in epoxy. The cross-section was ground and polished using a Buehler AutoMet 250 Pro grinder–polisher and mounted in an epoxy puck preparing it for subsequent analysis.

Characterization Methods: Optical Microscopy, SEM, and EDS

  • Optical Microscopy: A Leica DMI8 optical microscope was used for initial microstructural observation.
  • Sample Coating: The prepared surface was sputter-coated with Au/Pd using a Leica EM ACE600 ion sputter coater to improve conductivity and imaging quality in SEM.
  • SEM & EDS: A CIQTEK 5000X FE-SEM combined with a Bruker XFlash 7 EDS detector was employed for high-resolution imaging and elemental analysis.
Grinder/Polisher
Inverted materials Microscope - Leica DMi 8
Field Emission SEM - CIQTEK SEM 5000X
Sputter Coating - Leica Ace 600

Automotive Paint Characterization Results and Discussion

Optical Microscopy of Automotive Paint Layers

Optical microscopy provided an initial examination of the cross-section

  • Figure 1 shows the paint layers on both the outer and inner surfaces of the vehicle body.

    • (a) Outer surface

      (b) Inner surface

  • Figure 2 reveals a region of delamination on the outer surface.

 

Fig. 1 (a) Auto Motive Paint Layers - Outer Surface (Optical Microscope)
Fig. 1 (b) Auto Motive Paint Layers - Inner Surface (Optical Microscope)
Fig. 2 Delamination -The paint layer peels off on the outer surface. (Optical Microscope)

SEM Imaging and Paint Layer Thickness Analysis

Secondary electron (SE) images of the cross-sectional layer structures for both the outer and inner surfaces are shown in Figures 3 and 4.

  • Figure 3. The outer surface exhibits a complex multilayer structure with a total thickness of 136.38 µm. Individual layers (from the substrate outward) measure approximately 10 µm, 21.69 µm, 28.36 µm, 19.35 µm, 18.01 µm, and 36.37 µm.
  • Figure 4. The inner surface shows fewer layers with simpler structures, all anchored to a ~10 µm primer layer directly on the substrate.
Fig. 3 SEM images of the outer surface paint. The lower image shows the layer directly attached to the substrate at 2000x magnification. Layer thickness measured by CIQTEK 5000X FESEM
Fig 4 SEM images of the inner surface paint. The lower image shows the layer directly attached to the substrate at 2000x magnification. Layer thickness measured using CIQTEK 5000X FESEM

Elemental Composition of Automotive Paint

EDS measurement was applied to the outer and inner surface layer respectively.

Outer surface

The EDS identified C, O, Al, Si, Ti, Fe and Zn. Figure 5 shows the SE image and distribution of Ti, Fe, Zn, Si and Al in the paint layers.

Fig. 5a SE image of the outer surface
Auto Paint SE Images of outer surface EDS overlay
Fig 5b EDS overlapped mapping image showing distribution of Ti, Fe, Zn, Si and Al
Auto Paint EDS Elemental Map of outer surface
Fig 6. EDS Elemental mapping images of the outer surface paint
Auto Paint EDS Line scan Image
Fig 7(a) A BSE image of the paint. EDS line scan was performed along a line across the paint layers.
Fig 7(a) Elemental distribution across the layers

Inner surface

The EDS identified C, O, Al, Si, Ti, Zn and Fe in the inner side of the sample. Figure 8 and Figure 9 show the SE image and elemental distribution in the layers.

Auto Paint SE Images of inner surface
Fig 8(a) SE image of the inner surface
Fig 8 (b) EDS overlapped mapping image showing distribution of Ti, Fe, Zn, Al and Si
Auto Paint EDS Elemental Map of inner surface
Fig 9 EDS elemental mapping images of the inner surface paint

Conclusions

The multilayer architecture of automotive paint was confirmed using optical microscopy and SEM, with distinct differences observed between the inner and outer surfaces. EDS verified that the substrate is an iron-based car body, while the paint layers contain:

  • C, O: From organic binders and resins.
  • Al, Si: Associated with pigments, fillers, or extenders.
  • Ti: From TiOâ‚‚ pigments for opacity and UV resistance.
  • Zn: Indicative of anticorrosive primers.

These elements are commonly associated with specific functional components of automotive coatings, such as carbon and oxygen from organic binders, aluminum and silicon from fillers or pigments, titanium from TiOâ‚‚ pigments used for opacity and UV resistance, and zinc from anticorrosive layers. Layer-specific elemental distributions illustrate the functional design of each coating layer, contributing to corrosion protection, mechanical durability, and aesthetic properties.