Film thickness

1 Definition film thickness

Film thickness is defined by the DIN EN ISO 2808 [1] as the distance between the surface of the coating and the surface of the substrate. Film thickness is the measurement result, which is obtained from a coating according to an agreed, standardized, precisely described and carefully applied measuring method (Fig. 1). 
The DIN EN ISO 2808 [1] describes several methods for determining wet film thickness, dry film thickness, and the thickness of unbaked powder coatings. For each method described, the scope of application, existing standards and precision are given.  
 

Figure 1 Figure 1:Film thickness sample

2 Measuring principles for testing wet film thickness

In order to control the process variables during application, it is often desirable to determine the film thickness of the wet film. The determination of the wet film thickness is advantageous for systems whose dry film thickness can only be measured destructively. The wet film thickness measurement is a quick and easy test. With the help of the technical information of the paint manufacturer, the dry film thickness can be predicted approximately with the following calculation:
Dry film thickness=(Liquid paint film in µm x Solid content)/100

As a basis, paint systems can be classified as follows:
The measured wet layer thickness of 80µm corresponds to a dry layer thickness of:    
Paint System    Solid Content    Dry film thickness
LS    Low solid    < 50%    < 40µm
MS    Medium solid    < 60%    < 48µm
HS    High solid    < 70%    < 56µm
UHS    Ultra High Solid    > 70%    > 56µm
Depending on the shape and measuring range of the surface to be tested, wet layer measurements are carried out with different devices.
2.1    Wet Film Thickness Gauges with notches
The measuring comb can be used on flat surfaces and for a wide variety of substrates (Fig. 2). In the case of slight curvatures, it should be checked that the coating thickness gage can be placed parallel to the axis of curvature. Before use, make sure that the teeth of the measuring comb are clean and undamaged. The measuring comb is placed vertically on the measuring surface and lifted vertically again. The coating must be given enough time to wet the teeth of the measuring comb. With the first wetted tooth, the wet layer thickness is read on the scale (Fig. 3). The measurement must take place as soon as possible after the application of the paint, so that not too many solvents have already evaporated [1, 2, 3]. 
2.2    Wet Film Thickness Gauges with wheels
The wet film double wheel, also called Inmont wheel, is a proven standard measuring device in the coatings, printing inks and processing industries. Three instrument models with a total of 12 measuring ranges are available for a range of coating materials. The accuracy of this test method is ± 2.5 µm or 2.5% of full scale [1, 2, 3].   
When using the measuring wheel, the wheel must be wetted by the coating material. The disc-shaped measuring instrument is unrolled onto the wet and freshly applied layer of paint. In this case, two rollers move on the paint base, while the eccentrically arranged the measuring rib is only wetted at the point of paint. The rotational speed of the wheel should be uniformly slow to give the coating time to wet (Fig. 4).
 

Figure 2 Figure 2: Comb Gage skala

Figure 3 Figure 3: Comb Gage

Figure 4 Figure 4: Gauges with wheels

3 Measuring principles for testing dry film thickness

The determination of the dry film thickness can be carried out non-destructively or, for example in the case of multi-layer systems, destructively.
3.1    Destructive film thickness measurement with cutting tools
In order to be able to check the layer thickness on different substrates (metal, wood, plastic, etc.) or to check the single layer thickness of multi-layer systems, a simple method with different cutting tools can be used which is also described in detail by DIN EN ISO 19399 and ASTM D4138 standards [4, 5] and illustrated by figure 5.
With specially manufactured cutting tools, a small incision is made in the coating. A V-shaped cut is created through the coating to the substrate (Fig. 6 and Fig. 7). Each of the precision cutting edges has two bevels with different angles of inclination. Since the inclination of the cut is known, the horizontal width over the entire bevel (from the substrate to the top edge of the coating) is a measure of the layer thickness. In order to precisely determine the width of a' (b'), which is proportional to the layer thickness a (b), LED illumination and a microscope with 50x magnification are integrated in the byko-cut measuring device. 
 

3.2    Destructive film thickness measurement with dial gauge (IG clock)
The dial gauge is usually used for higher paint layer thicknesses, for example for heavy corrosion protection with zinc dust paint.
For measuring, the coating must be removed at the measuring point (a scratch stylus 1mm is well suited), set the zero point of the device on the calibration plate (glass plate). Then place the dial gauge vertically on the exposed area and read result (Fig. 8).

Figure 5

Figure 6

Figure 7

Figure 8

4 Non-destructive testing of dry film thickness

For this purpose, electronic measuring devices are used that measure and digitally display the layer thickness of insulating coatings on non-magnetic, metal substrates (NFe) and of non-magnetic coatings on steel or iron (Fe). Two different measuring principles are used:
•    Magnetic-inductive measurement on Fe substrates such as steel and cast iron
•    Eddy current measurement on NFe substrates such as aluminum, copper, brass, non-magnetic steel, bronze, magnesium, zinc.
Examples of insulating and non-magnetic coatings: paints, plastics, enamel, chrome, copper, zinc, powder coatings, galvanic coatings, rubber, hard chrome plating, injection metal, ceramics.
When measuring, make sure that at least 3 measuring points are distributed on the sample. On wood at least 10 measuring points. The measurements should not be located directly at the edge and should have sufficient distance to each other (Fig. 9).

Figure 9 Figure 10: Uniform distribution of measuring points the sample width - not near the edges and plate ends

5 Measuring principle for testing dry film thickness with magnetic-inductive method (Fe)

This measuring principle allows non-magnetic coatings such as paints, plastics, enamel, chrome, copper, zinc, powder coatings, galvanic coatings, rubber, hard chrome plating, injection metal, ceramics to be tested on magnetic substrates.
This method is based on the use of two magnetic coils whose magnetic field is altered by approaching a ferromagnetic substrate. The magnetic field change depends on the distance of the probe to the substrate and thus on the layer thickness. The second coil absorbs the magnetic current. This magnetic coupling between the two magnetic poles is a measure of the layer thickness. Electromagnetic induction uses alternating magnetic fields produced by a ferromagnetic coil (Fig. 10 and Fig. 11).
The magnetic field can be influenced by the following factors:
• Geometry of the substrate (curvature, thickness)
• Material properties of the substrate (e.g. conductivity, pretreatment) 
• Roughness of the substrate
• Other magnetic fields (residual magnetism of the substrate, external magnetic fields)
The thicker the coating, the weaker the induced measurement signal. The coating must be designed in such a way that the contact of the device on the surface of the measured value is not distorted and does not leave dents/attachment marks. 

Figure 10

Figure 11

6 Measuring principle for testing dry film thickness with eddy current method

This method is used for the measurement of non-conductive layers (NFe) on non-ferromagnetic base metal (NFe), such as aluminum.
The eddy current method is based on the principles of electromagnetic induction (Fig. 12). Through a high-frequency alternating current, a magnetic field is built up in a coil of fine wire, which changes its direction according to the applied alternating current. If the probe is brought close to a conductive carrier, eddy currents are generated in this carrier, which act back on the magnetic field of the coil. The size of the feedback depends on the nature of the carrier and the distance – and thus the layer thickness – between the probe and the substrate (Fig. 13).

Figure 12

Figure 13

7 Test conditions and sample preparation

The sample to be tested must be free of dirt and cooled to room temperature. Changes in the environment such as temperature, humidity, altitude, and intensity of surrounding electromagnetic fields can lead to measurement errors.
The measuring instrument must also be calibrated regularly according to the manufacturer's instructions.
7.1    Measuring on rough surfaces 
On rough surfaces, the result of the paint layer thickness measurement can be falsified, depending on whether the probe is placed on a tip or in the valley of the roughness profile (Fig. 14). In general, layer thickness measurement for rough surfaces only makes sense, if the layer thickness is at least twice as high as the roughness peaks. It is recommended to perform multiple measurements (at least 3) in order to determine a reliable average value.
7.2    Measuring on curved surfaces
Measurement errors often occur due to the shape of the test piece. With curved surfaces, the proportion of the magnetic field passing through the air changes. For example, if a measuring instrument was calibrated on a flat sheet, a measurement on a concave surface would lead to a decrease – on a convex one to increased result. The errors can add up and give distorted results of the actual layer thickness (Fig. 15).
 

Figure 14 Figure 18: Measuring on rough surfaces

Figure 15 Figure 19: Measuring on curved surfaces

8 Accuracy of measurement results

In all measuring methods, the substrate surface is touched by a part of the measuring instrument. The test method used must be suitable for the coating surface being wet or dry, destructive or non-destructive and dependent on the substrate being hard, soft, metallic or non-metallic.

8.1    Accuracy of wet film thickness tools

In general, the agreement between the measuring combs and the measuring wheels is good because they are insensitive to smaller film differences in the paint layer thickness, i.e. the step intervals of the measuring comb and measuring wheel are relatively large.
The accuracy and precision of the wet layer thickness measuring method depend on the method of paint application and the time when the measurement is performed. Some paint systems quickly loose volatile solvents during spray application, which can falsify the measurement result. 

8.2    Accuracy of destructive methods for dry film thickness

For some colors, it is helpful to apply a contrast marker with a black or white marker. This is used as a guide to cut through the substrate for easier reading of the value with the microscope.
 

8.3    Accuracy of non-destructive methods for dry film thickness

Accuracy and precision is primarily dependent on device type and is affected by the type of calibration, substrate thickness and alloy. Information on repeatability and precision are provided by the manufacturer.

9 Standards and Literature

[1]     DIN EN ISO 2808       Paints and Varnishes — Determination of Film Thickness
[2]     ASTM D4414              Standard Practice for Measurement of Wet Film Thickness by Notch Gages
[3]    ASTM D1212               Standard Test Methods for Measurement of Wet Film Thickness of Organic Coatings 
[4]     DIN EN ISO 19399    Paints and varnishes - Wedge-cut method for determination of film thickness
[5]     ASTM D4138             Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means