QMT Features: October 2017
Choose the right method – enhance coating inspection
Markus Fabich of Olympus Europa talks about techniques that make industrial coating inspections faster, simpler and more precise.


In many industries coatings are used as protection against air, moisture and physical damage – or for decorative purposes. Adding a coating can improve the appearance of a product, but also its function and its lifetime. The thickness of the coating often determines the level of protection it offers and therefore the longevity of the component.

During manufacturing and while a component is in operation, inspection is essential in assessing the thickness and the quality of a coating. Inspections help to predict the strength of a coating as well as its remaining lifetime. To inspect the wide range of coatings available, different inspection techniques, including microscopy, ultrasonic testing, eddy current testing and X-ray fluorescence, can be used.

In order to determine the best technique for a specific component, it is important to establish which parameters provide the most useful information. Furthermore, when a component is coated with more than one layer of coating, it is important to use a technique that can measure each layer. Commonly used criteria for determining the optimal inspection technique include the maximum number of layers that can be analysed, accuracy, thickness limit, ease of use and whether the method is destructive or nondestructive.

This article takes a closer look at the commonly used coating inspection techniques, explaining the benefits and drawbacks – and gives examples of industrial applications where each of these techniques is particularly well suited.
Microscopy – get the full picture

Microscopy is a precise and versatile technique to analyse coatings; light can be used in a variety of ways – both destructively and nondestructively – to give information about coatings.

The classic microscopy method examines a coating by analysing polished cross sections. This technique has a limit of detection below 1 µm and can cope with a high number of layers on top of each other. However, this is also a destructive technique that requires detailed sample preparation before analysis.

One application where polished cross section microscopy can reveal fine details is in the inspection of printed circuit boards (PCBs). PCBs consist of a resin coated with thin layers of copper that can be formed into highly intricate patterns. As the electrical resistance depends directly on the thickness of the coating, inspecting coating thickness is essential to confirm the functionality of PCBs.
Figure 1 shows the complex internal structure of a PCB revealed by polished cross section imaging. The cross section gives a detailed view of the different layers without the need for expensive scanning equipment, such as computer tomography scanners.

When a nondestructive test is essential for an inspection workflow, confocal microscopy can be used instead. Confocal microscopes, such as Olympus’ LEXT OLS 4100, can reveal precise details of transparent coatings in three dimensions without the need for elaborate calibrations.

The inspection of semiconductors, where coatings of SiO2 are applied on top of Si wafers, is an example of a coating where confocal imaging is suitable. It provides a detailed, easy-to-use, nondestructive test, in which different wafers or different areas of a wafer can be inspected quickly (figure 2).

In addition to the two microscopy techniques described above, many other procedures exist in which microscopes are used for coating measurements. Microscopy imaging is used, for example, in the so-called calotest, which is done on very hard coatings used in milling and grinding tools. Also, microscopes with 3D measurement capability can carry out direct step measurements to determine the thickness of a structured coating.

Scan large surfaces faster with ultrasound

When large areas of a coated surface need to be inspected nondestructively and in a short space of time, ultrasonic (UT) inspection is often a suitable technique. UT inspection relies on the reflection of sound waves at a boundary between materials, for example between a coating and the bulk material. It is a portable solution that is well suited for measuring large structures and thick coatings as there is no upper limit on the travel distance through the coating.

The sensitivity of UT measurements depends on the efficiency of waves reflecting off a boundary and therefore on the acoustic properties of the different materials. There are also limits to the ability of UT technology to measure thin coatings, such as those below 80 µm.

UT inspection is frequently used to inspect metal plates and pipelines that have been coated with corrosion-resistant alloys. Figure 3 shows the intensity of the UT beam reflected from the metal–coating interface and from the back wall of the component. It demonstrates its dual function as a coating measurement tool and as a thickness gauge, while also detecting the effects of corrosion and other flaws in the material.

Add thickness measurements by eddy current

Another measurement technique that can be easily integrated into an existing inspection workflow due to its dual function as a flaw detector and coating inspection tool is eddy current detection. An eddy current probe works by using an electric current in the coil of the probe to generate so-called eddy currents in the material to be inspected. Flaws in the material – as well as the presence of a coating – will affect the phase or the amplitude of these eddy currents (figure 4).

Eddy current probes can inspect even the thinnest coatings, offering a lower thickness limit below 0.5 µm. Furthermore, it is a nondestructive technique done on a portable instrument. The main limitation of eddy current measurement is the range of materials it can be used on; ferromagnetic materials may not be suitable. Some coatings may also require calibration; however, Olympus’ N600C contains a comprehensive library of commonly used materials. Olympus also supplies easy-to-use calibration software for when calibration using a microscope is required.

In the aerospace industry, flaw detection using eddy current probes is commonplace, for example when looking for microfractures in the wings or fuselage. The dual function of eddy current flaw detectors means that coating measurement can easily be added to the inspection workflow without significant increases in labour and cost.

Fast results with portable XRF
A final method to analyse the composition and thickness of a coating quickly and precisely is to use X-ray fluorescence (XRF). XRF detects the presence of metals in a coating by irradiating the coating with X-rays and detecting secondary (fluorescent) photons coming back (figure 5). Once calibrated, an XRF spectrum can be used to calculate the thickness of a coating from the amount of fluorescence detected.

The main benefit of XRF is that it is a fast technique (typical scanning time: 10 s) that detects coatings in the range of 50 nm in a small, portable device (figure 6). It does, however, only detect the presence of metals. For this reason, the detection of a layer or layers of coating relies on unique metals being present in each layer.
Inspection of galvanisation is a good example where XRF excels in terms of speed, reliability and simplicity. Galvanisation involves applying a thin coating of zinc on a steel or iron compound to prevent rusting. The high amount of zinc in the coating, combined with the absence of zinc in the bulk material, makes these systems ideal for XRF measurements.

Summary

Coating thickness measurements provide valuable information about the quality of a coating, the quality of the product itself and its estimated lifetime. The materials used in coatings vary enormously depending on the function of the coating, which means that there is not one single technique that is suitable for all types of inspection.

The four techniques described here – microscopy, ultrasound, eddy current and X-ray fluorescence – are all commonly used in industrial inspection to look at a wide range of different coatings (table 1). Comparing the benefits and drawbacks of each of these techniques helps to make informed decisions when choosing a technique for accurate, fast and reliable measurements.
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