Surface Inspection

Properties such as hardness, elasticity of thickness or coatings are often decisive for the performance of important functions. They can qualify an entire component for a special application. FISCHER offers easy-to-use measurement devices that precisely determine the properties of paint applications and varnish layers on rough base materials.

Surface inspection

Application notes

Measuring lacquer coatings on rough surfaces

Irregular surface structures, or “roughness” – as typically found on grey cast iron or sand-blasted steel – complicate the process of measuring the thickness of overlying paint layers. The unevenness of the substrate’s surface can cause large fluctuations in the measured values. This leads to uncertainties in the interpretation of the results and makes it difficult to monitor the coating process.

The underlying surface roughness always affects an overlying paint layer. However, it is difficult to make a quantitative assessment of the degree of interference because this depends on several parameters, such as the geometry of the roughness and the thickness of the lacquer. When measuring with a single-poled probe, measurement positions over roughness peaks or valleys may lead to different readings despite uniform paint thickness. The magnetic field lines are affected differently by the valleys and the peaks. In dual-poled probes this effect, and thus the influence of the roughness is significantly reduced – which consequently reduces the number of measurements required to assure an acceptable mean and standard deviation.

Fig.1: Left: single-poled probe; right: dual-poled probe

The dual-poled probe V7FKB4 used in combination with one of the FISCHER FMP handheld instruments or with the table-top FISCHERSCOPE® MMS® PC2 (module PERMASCOPE®) is especially developed to accurately measure paint layers atop rough surfaces. Compared to those attained with single-poled probes, significantly lower coefficients of variation and higher repeatability precisions are achieved, as shown in Table 1.

Fig.2: Measuring lacquer on a rough surface with the V7FKB4 probe

 

Standard probe

(single-poled)

Probe V7FKB4

(dual-poled)

Measurement number

sample 1

even

sample 2

rough

sample 1

even

sample 2

rough

1

126.4

241.6

125.8

237.2

2

125.2

263.0

125.6

245.4

3

125.7

232.6

125.2

248.7

4

125.3

250.3

126.2

241.6

5

126.2

252.3

126.0

252.4

6

125.5

244.3

125.6

251.3

Mean value (µm)

125.71

247.35

125.71

246.11

Standard deviation (µm)

0.47

10.38

0.36

5.90

Coefficient of variation (%)

0.37

4.20

0.28

2.40

Tab.1: Comparative measurements: single-poled standard probe vs. the dual-poled V7FKB4 probe, on even and rough surfaces

The measurement accuracy is very dependent on careful calibration. The V7FKB4 probe reduces not only the calibration effort on original pieces but also the number of measurements required to verify the results.

For precise measurement of paint layers on rough surfaces, the dual-poled magnetic induction probe V7FKB4 is ideal when used together with a device from the FMP family of handheld instruments or the FISCHERSCOPE® MMS® PC2. The expansion of the magnetic field lines between the two poles mini-mises the measurement variations induced by the roughness, providing the levels of precision and accuracy you have come to expect from FISCHER. For more information, contact your local FISCHER representative.

Determining the Surface Hardness of Paint Coatings – Pencil Testing vs. Instrumented Indentation Testing

Until recently, quick scratch testing with pencils to determine the hardness of paint coatings has been commonplace. However, the reliability and reproducibility of this method is questionable. Because of the stringent quality standards in the coating industry, it is necessary to be able to test the hardness of paint coatings reliably.

Determining the ‘pencil hardness’ – or better put, the scratch resistance by means of marking with pencils – according to Wolff Wilborn or DIN ISO 15184 is a method commonly used in the coating industry. With this method, pencils of different hardnesses are moved at a certain angle and with a certain force across the paint surface to be tested. The ‘pencil hardness’ of the coating is defined by two consecutive levels of pencil hardness, where the softer one leaves only a writing track, while the harder one actually causes a tangible deformation of the paint coating.

Fig. 1: FISCHERSCOPE® HM2000 S for the determination of the Martens Hardness

The shortcomings of this procedure lie in the poor reproducibility of the measurements. For one, the material under test will not always manifest the same properties, since pencil hardness is not clearly defined in any standard and there are distinct differences between individual manufacturers. Furthermore, the operator influence is significant. Thus, it is often impossible to interpret the results unambiguously.

Fig. 2: Comparison of the Martens Hardness of pencils of different hardneses, shown with the standard deviation of the measurements

If one correlates the various pencil hardnesses with their Martens Hardness, the limitations of the method become even more obvious. Fig. 2 shows the results of multiple measurements on pencils of various hardness levels. Broad overlapping is apparent when one considers the standard deviations of the individual test series. In fact, especially in the upper range, the nominal hardness (B, HB, F, H, etc.) of pencils is not a dependable indicator of their actual hardness.

The FISCHERSCOPE® HM2000 S can measure the hardness of paint coatings directly and accurately. In addition, other characteristics can be determined, such as creep and relaxation behavior, as well as the modulus of elasticity. All of these hardness parameters provide a true indication of the paint quality. 

FISCHERSCOPE® hardness measurement systems demonstrate that the actual hardness of a pencil can vary significantly from its nominal hardness, meaning the pencil is not a dependable measuring device. Therefore, a method employing a pencil as its key instrument cannot be expected to reliably assess the hardness of anything. For directly determining the surface hardness of e.g. paint coatings, the FISCHERSCOPE® HM2000 S, for example, will give you the same accurate, precise results – every time. Your local FISCHER partner will gladly provide additional information.

Microhardness measurements of paint coatings shorten weathering tests

Paint for architectural coatings is not only used to give surfaces an attractive appearance, but also plays a very important role in protecting facades against external damage and corrosion. To avoid waiting years to see if the coating really protects the surface, simulating and measuring weathering influences is necessary.

Paint coating systems are exposed to severe environmental influences like strong temperature variations, moisture and aggressive media such as acid rain, insect residue or strong cleaning agents. Facade coatings should withstand such influences and have quality characteristics such as light fastness, weathering resistance and easy cleaning.

The characteristics of such coatings depend not only on the thickness, but also on hardness, elasticity, degree of polymerisation and resistance to UV radiation. These parameters can be determined using the instrumented indentation test.

To demonstrate weathering influences, measurements were performed on samples with original surfaces (reference), on samples after 400 hours of QUV radiation (equipment weathering) and after 1 year Florida exposure test (outdoor weathering).

Fig. 1: Influence of weathering on the Martens Hardness of polyester powder coating.

The reference sample (green plot) without weathering does not show a hardness increase at the surface. The sample exposed to weathering outdoors for 1 year in Florida shows a slight increase of hardness near the surface. The sample exposed to QUV irradiation for 400 hours shows the largest hardness gradients. Reason therefore is a change in the molecular structure of the paint. Cross-linking of the paint molecules lead to an increase in hardness caused by the repeated alternation of drying, moistening and irradiation. As outdoor weathering often spans a number of years and involves very expensive sample holders and large standing areas, artificial weathering is used to simulate such outdoor weathering.

With the FISCHERSCOPE® HM2000 hardness meas­uring instrument, the effects of weathering tests can be measured easily and accurately, therefore saving costs and shortening time compared to outdoor testing significantly. Ask your local FISCHER repre­sentative for further information.

Your contact to FISCHER

Fischer Technology Inc.
Windsor/United States

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Phone: (860) 683-0781
E-Mail: info@fischer-technology.com
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