Case 1: Automotive paint inspection

We were approached by a company working in the automotive sector, that were interested in performing inspection on their paints using OCT. In this example the sample was an aluminium plate pre-treated for improved adhesion that was then applied a primer and top coating (see Figure 1.1). The top coating contained ceramic flakes to give it a sparkling look.  

Figure 1.1: Sketch of the automotive paint sample layer structure.


In this case, the company was interested in the following:

  • Layer thicknesses
  • Uniformity
  • Defects 

First, we look at the layer thicknesses. Figure 1.2 shows the maximum intensity signal in the cross-section of a random area on the sample over a 3x3 mm area. This correspond to 1.000 superimposed images. This was done to capture signals from the ceramic flakes in the top coating, since the contrast between top coating and primer in this case was not visible for this type of coating. This allowed us to give an estimate of the boundary between the top coating and the primer, based on the deepest observation of flakes. The line in the top is the boundary between air and top coating, and the bottom line is the boundary between the primer and the aluminium substrate. The thin metal adhesive layer is not observed, so it is assumed to be less than 10 microns thick. 

Figure 1.2: Cross-sectional view of a 3x3 mm scan of a random area on the sample showing the extend of the top coating and primer layers.


The uniformity of the sample is in this example evaluated qualitatively by the difference between the thickness of the top line of a single scan, and that of the superimposed scan. If there was a large variation in thickness, the line in the superimposed scan would be broader than for the single scan. We see from Figure 1.3 that the thicknesses are similar, and therefore we assume that the uniformity is acceptable on the scale of millimeters.

Figure 1.3: Cross-sectional view of a single 3mm scan.


Lastly, we look at defects. In Figure 1.4 we see a part of the sample where clear defects can be identified by the naked eye from the specular reflection of a lamp at the surface. The three images marked A, B, and C, show top view (en face) images of the scanned area at the surface, just below the surface, and at the substrate, respectively, as indicated in the cross sectional image on the top. in A we clearly see the dark area marking the defect, as well as several structural features that could be used to identify the cause of the defect. B shows the ceramic flakes under the surface, which indicate a non-uniform dispersion, possibly due to a void. C shows the substrate, which seem to indicate that there is a scratch or marking in the substrate in the top of the image. Note that because the beam in this case was scanned inside a lens by a galvo-scanner, the corners appear darker because of loss of signal at large angles. This will not be present if mechanical scanning is used.

Figure 1.4: Sample area with identified bubbles/blisters in the surface, and corresponding OCT cross-sectional view and top view (en face) at different depths A, B, and C.


Case 2: Wet and dry spray painted parts inspection (coming soon)



Case 3: Maritime coatings (coming soon)


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