Application Note | Coating Finish
Powder Coating Finish Measurement Using 3D Profilometry
Replacing Visual Inspection with Roughness, Peak-to-Valley and Surface Area
Research & Experimental Testing
Craig Leising
Visual Design & Editorial
Andrew Shore
Introduction
Powder coating, known for its durability, is a coating applied electrostatically as a free-flowing, dry powder and then cured under heat. This forms a hard and durable coating typically used on metal household appliances, automotive and aerospace parts. Traditionally, powder coatings have been visually inspected. The intended or accepted finish of a powder coating varies greatly from application to application and depends on the dry powder particle size, in the µm range.
Importance of Profilometry Inspection for R&D and Quality Control
Because of the large variation possible and the unreliability of visual inspection, the surface finish of powder coatings should be properly inspected for quality control. Understanding surface features can lead to the best selection of surface finish and control measures. Ensuring quality control of such parameters relies heavily upon quantifiable, reproducible and reliable inspection. The NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology with unmatched capability to measure powder coatings. Where other techniques fail to provide reliable data due to probe contact, surface variation, angle, absorption or reflectivity, NANOVEA Profilometers succeed.
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Measurement Objective
In this application, the NANOVEA ST400 is used to measure and compare the surface finish of four different powder-coated samples. Several surface parameters are automatically calculated from the coating profile. Here we focus on surface roughness, peak to valley and surface area for comparative evaluation.
NANOVEA ST400 Standard
Optical 3D Profilometer
Results
The 3D view and false color view of each sample are shown in Figs. 2 to 5, and the ISO 25178 height parameters and developed surface area of the four samples are compared in Table 1.
| ISO 25178 parameter | Sample 1 | Sample 2 | Sample 3 | Sample 4 |
|---|---|---|---|---|
| Sa (µm) | 27.927 | 17.371 | 11.443 | 22.149 |
| Sq (µm) | 41.306 | 21.427 | 13.992 | 27.725 |
| Ssk | 2.1221 | −0.066279 | −0.020995 | −0.59415 |
| Sku | 8.7157 | 2.7247 | 2.5702 | 3.1891 |
| Sp (µm) | 257.93 | 75.953 | 45.409 | 55.517 |
| Sv (µm) | 153.39 | 68.661 | 45.322 | 108.14 |
| Sz (µm) | 411.31 | 144.61 | 90.731 | 163.66 |
| Sdar, developed area (mm²) | 173.34 | 91.964 | 62.521 | 61.035 |
| Spar, projected area (mm²) | 60.000 | 60.000 | 60.000 | 60.000 |
Table 1: Height parameters and surface area of the four powder coatings (from the PDF panels).
Conclusion
In this application, we have shown how the NANOVEA ST400 3D Profilometer can precisely characterize the surface finish of powder-coated samples (many other measurements could have been made besides those shown here). By looking at the three highlighted parameters, surface roughness, peak to valley and surface area, we can easily quantify differences between the finish and quality of the four samples that may not be obvious by visual inspection. With this information, powder coatings can be investigated for R&D or quality control procedures. To view further detail, a 2D cross section can quickly be chosen to analyze special areas of interest at the nanometer range, which could also be further analyzed with an integrated AFM or microscope module.
NANOVEA 3D Profilometer speeds range from 20 mm/s to 1 m/s, from laboratory and research use to high-speed inspection; they can be built with custom sizes, speeds and scanning capabilities, Class 1 clean room compliance, an indexing conveyor and inline or online integration.
Frequently Asked Questions About Powder Coating Finish
Why measure powder coating finish instead of inspecting it visually?
Powder coating finishes vary widely with application and powder particle size, and visual inspection is unreliable; quantitative 3D measurement makes finish comparable and controllable.
Which parameters were compared?
Surface roughness (Sa), peak to valley (Sz) and developed surface area (Sdar), from ISO 25178 analysis of each scan.
How different were the four coatings?
Sa ranged from 11.443 µm (Sample 3) to 27.927 µm (Sample 1), and Sz from 90.731 µm to 411.31 µm.
What does surface area tell you?
For the same 60 mm² projected area, the developed area ranged from 61.035 mm² (Sample 4) to 173.34 mm² (Sample 1), a measure of how much texture the finish has.
Can specific features be examined further?
Yes. A 2D cross section can be chosen to analyze areas of interest down to the nanometer range, and integrated AFM or microscope modules can look closer.
Can this run in production?
NANOVEA profilometers scan from 20 mm/s to 1 m/s and can be built with custom sizes, indexing conveyors and inline or online integration.
