Measuring Films and Coatings on Curved Surfaces

Traditionally, the accurate measurement of coatings deposited on a curved surface has been difficult without the use of witness samples. Difficulties arise as coatings are frequently found on curved surfaces – such as the coatings found inside syringes (and other tubes), polymer coatings on medical devices, or the protective polymer layer found on automotive headlights.

The MProbe system facilitates the easy and fast measurement of films on curved surfaces using either a microscope or manual probe (CSH).

Using a microscope is recommended for smaller parts (of less than 25 mm size), as the small spot size of the microscope, which is less than 40 μm, means the surface curvature has no significant impact on measurement. Microscopes are frequently used for testing the coatings of medical devices.

There are two different types of manual probe which can be used, which differ in their spot size – CSH, which has a measurement spot of 2 mm, and CSHF, which has a measurement spot of 200 μm. Which probe should be used depends on the sample size and degree of curvature. CSH and CSHF probes conform to the samples own curvature,, with soft rubber bottoms that can be placed directly on a curved part, making it simple to take precise, accurate measurements.

Measuring Coatings on Automotive Lights

During the manufacturing of automotive lights, there are several stages at which the thickness of coatings must be evaluated for QC.

Coatings are employed to protect the light casing from external damage and scratches, and to protect reflectors (hard-coatings), while polycarbonate lenses must be coated in anti-fog coatings. For each coating application, there are measurement challenges, such as colored surfaces beneath the films, reflective surfaces, and a low optical contrast between polycarbonate and the coating itself.

The MProbe Vis system can help researchers solve all of these unique problems when used alongside Semiconsoft’s TFCompanion software, which runs FFT (Fast Fourier Transform) algorithms. The FFT algorithms can be taught to accurately measure difficult and unconventional samples using an uncomplicated process that can be used by operators with minimal experience.

The MProbe Vis makes it easy for operators to quickly measure components immediately after the coating process.

Clear headlight with hardcoat: Reflectance spectra measurement.

Figure 1a. Clear headlight with hardcoat: Reflectance spectra measurement. (see results Fig. 1b)

Results of the measurement of clear headlight with hardcoat. Hardcoat thickness- 15.2 μm; IPL (interface layer) thickness - 1.85 μm (First peak corresponds to hardcoat layer; second peak corresponds to the total thickness hardcoat+IPL)

Figure 1b. Results of the measurement of clear headlight with hardcoat.
Hardcoat thickness- 15.2 μm; IPL (interface layer) thickness - 1.85 μm
(First peak corresponds to hardcoat layer; second peak corresponds to the total thickness hardcoat+IPL)

Red headlight with hardcoat (on textured surface): Reflectance spectra measurement.

Figure 2a. Red headlight with hardcoat (on textured surface): Reflectance spectra measurement. (see results Fig. 2b)

Results of the measurement of the red headlight with hardcoat. The hardcoat thickness - 8.7 μm, IPL (interface layer) thickness - 1.4 μm

Figure 2b. Results of the measurement of the red headlight with hardcoat. The hardcoat thickness - 8.7 μm, IPL (interface layer) thickness - 1.4 μm

Measuring Coatings inside Transparent Tubes

For many applications, coatings must line the inside of a transparent tube. For example, medical syringes use an internal coating to reduce friction with the plunger, or drinks bottles contain an inert coating to preserve the beverage held inside.

Whilst tubing applications each have their own challenges, some of these problems are common to all coatings inside tubes; such as low levels of reflectivity, and the scattering of light due to the roughness and imperfections of the material.

Figure 3 shows a measurement taken using the MProbeVis-Micro (measurement spot of 40 µm) of a polymer coating deposited on the internal surface of a syringe. The syringe was made of a transparent polymer, with a milky appearance and a diameter of approx. 15 mm.

Measurement of the polymer layer inside the syringe tube. Measured vs. model data, scattering and surface roughness correction is applied. Thickness: 288 nm

Figure 3. Measurement of the polymer layer inside the syringe tube.
Measured vs. model data, scattering and surface roughness correction is applied. Thickness: 288 nm

Measuring Coatings on Medical Devices

To protect medical implants from degradation in the body, and to prevent rejection, medical implants must be coated with biocompatible materials and/or polymers.

Common challenges in measuring these coatings include scattering from the metallic surface of the implant and the small size and steep curves of the devices. When measuring non-polymer coatings, the optical coefficients, n and k, must also be measured, as they are impacted by the deposition conditions.

To avoid problems with the steep curvature, measurement is carried out using a small spot of 40 µm, and light scattering is corrected for metallic surfaces.

Figure 4 shows the results for coating measurement of a Ti/Al oxide coating on a Ti nail of approx. 6 mm diameter.

Results of the measurement (measurement vs. model) of the coating on Ti nail. Oxide coating material dispersion is represented using Tauc-Lorentz approximation. Light scattering is corrected. Thickness: 115 nm

Figure 4. Results of the measurement (measurement vs. model) of the coating on Ti nail. Oxide coating material dispersion is represented using Tauc-Lorentz approximation. Light scattering is corrected. Thickness: 115 nm

Measuring Coatings In-Line

As well as being ideal for QC processes in the lab, the MProbe Vis can also be used for in-line coating measurement.

The system uses a fiber optical probe, allowing accurate measurements to be taken at a distance. Semiconsoft provides a Modbus (TCP) system, which can integrate with factory controllers to make in-line measurement processes easy to set-up, with custom integration available if needed. This system facilitates quality control on a pass/fail basis.

This information has been sourced, reviewed and adapted from materials provided by SemiconSoft.

For more information on this source, please visit SemiconSoft

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