Characterization of Paints

Mechanical properties such as surface hardness are a reliable indicator of the quality of paint coatings. The smallest variations can be accurately verified with a precise measurement of microhardness by means of nanoindentation. Even in the case of extremely thin protective coatings such as those on glass and lenses, the high-performance microhardness measurement devices from FISCHER deliver exact results.

Characterization of lacquers

Application notes

Hardness of Complex Coating Systems for Optical Components

The demands placed on the performance of optical components have skyrocketed and, in response, highly complex coating systems have been developed to produce surfaces that are scratch-resistant, dirt-repellent, anti-static and reflective. Various curing processes are integral to the production of optical coatings, making it difficult but important to find the decisive balance between coating hardness and elasticity.

Quality control therefore requires correspondingly powerful measurement methods and systems. For the standard-compliant determination of such material parameters as hardness and elastic modulus the instrumented indentation test can be used, even thin coatings of less than 100 nanometres in thickness can be measured accurately.

With the load/indentation depth method according to DIN EN ISO 14577 and ASTM E 2546, the indenter, typically a Vickers or Berkovich pyramid is pressed with continuously increasing test load into the material and then reduced in the same manner while simultaneously measuring the respective indentation depths. Important technological characteristics can be calculated from the resultant load/unload cycle, for example the Martens hardness. The elastic modulus of indentation can be determined when the test load is reduced. 

Fig. 1: Depth-dependent profile of the Martens hardness (HM) of two differently composed optical coatings. Marked in blue is the area where already an influence from the base material is given according to Bückle’s rule.

The figure 1 presents the measurement of Martens hardness and the associated standard deviation on two plastic lenses, samples courtesy of Rodenstock GmbH, Munich. The samples were produced under the same process conditions but exhibit differences in the composition of the coating system. As result a significant change of the hardness from one coating to the other can be seen.

At a certain indentation depth, the substrate material starts to become detectible. In order to avoid that influence while measuring the coating, the indentation depth must be limited to no more than 1/10 of the coating thickness (Bückle's Rule). The coefficients of variation for the two samples, 1.73 and 1.60 percent, respectively, as achieved using the FISCHER PICODENTOR HM500, demonstrates the potential for accuracy.

Fig. 2: The principle of instrumented indentation test: a designates the load increase, b the load decrease.

Although only the Martens hardness can be measured depth-dependent using standard methods, additional mechanical properties such as the Vickers hardness or the elastic modulus of indentation can be determined via the ESP (Enhanced Stiffness Procedure) method, which employs partial loading and unloading.

Conclusion: If the right balance between coating hardness and elasticity for coatings on optical components has to be determined the FISCHER PICODENTOR® HM500 is the suitable instrument to evaluate these parameters. For further consultancy please 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.

Mechanical characteristics of conformal coatings

In the electronics industry, two-component conformal coatings are often used to minimize current leakage on PCBs and as protection against humidity and other environmental stressors. Because the exact composition of the polymer determines its final mechanical properties, quality control using a reliable measurement technology is mandatory.

The conformal coatings used on PCBs often consist of two components: an alcohol and an isocyanate group. For production the dosage is calculated stoichio-metrically such that a hydroxyl group of the alcohol forms a bond, or cross-link, with an isocyanate group. If there is an excess of alcohol (called “under cross-linking”), the cured polymer is not as hard and can become hygroscopic; it can also grow sticky, which causes problems further down the assembly line. If there is an excess of isocyanate (called “over cross-linking”), it can lead to reactions with humidity from the air, which generates CO2, causing bubble build-up within the lacquer.

To ensure that such problems do not develop over time, it is important to test that the composition of the conformal coating is correct. With the instrumented indentation method, the quality of the polymer can be quickly determined immediately after curing. The measurement results are not influenced by the substrate material and sample preparation is minimal. Beside the plastic and elastic deformation measurement (hardness), other parameters can also be determined, such as creep.

For technical applications the so-called “cross-link density” of the polymer is taken under consideration; Figures 2a and 2b show the results of the hardness measurement for five conformal coating polymers of different composition, as measured using the FISCHERSCOPE® HM2000. Figure 2a shows the Martens hardness over depth. The Martens hardness changes drastically depending on the cross-link density and is therefore a good indicator for the composition of the lacquer. The creep at maximum force, shown in Figure 2b, is related to the brittleness of the material and indicates an excess of isocyanate.

Fig.1a: Martens hardness (HM) of differently cross-linked polymers

Fig.1b: Creep (constant force at maximum force level) as indicator for the proportion of isocyanate

Using the instrumented indentation method, the FISCHERSCOPE® HM2000 is the optimal choice to determine the quality of two-component conformal coatings on PCBs. For further information please contact your local FISCHER representative.

Thickness of protective coatings on wristwatch dials

In today's hectic world, there are few people who can do without a wristwatch. The importance of this "cultural companion" has also changed in recent decades. For some, it just tells the time, helping its wearer to structure daily routines. For others, however, it is jewelery made of precious metal, some even set with diamonds and other gemstones. But what most of them have in common is a dial: a signature feature of any watch, and a very delicate part exposed to significant stressors. 

In Switzerland – the land of clocks and watchmaking - the production of this component is taken very seriously. Basically, a clock face is made from a metal disk of ferrous alloys, in which the various recesses for numbers, hands, date window, etc. are stamped according to designer specifications. They are then often plated with gold, silver, copper or palladium, depending on the effect desired.

Fig.1: Dial with coating of precious metal

Because it is worn directly on the wrist, a watch is continuously exposed to a wide range of temperature stresses, for example, abrupt changes between body heat and cold outside temperatures in winter, the shock of being dipped into the pool, or baking in the sun – to name just a few. So that the beautiful metal plating retains its brilliant polish without surface discoloration (especially silver), the finished coating is sealed with a thin layer of varnish. Usually around 10-20 microns thick, this protective top-coat is applied by hand with a spray gun, covering the many fine edges of the recesses with the airtight and moisture-proof fixative, thus preventing oxidation and tarnish.

Fig.2: The handheld ISOSCOPE® FMP30, eddy current probe FTA3.3-5.6HF and convenient support stand V12 BASE ensure precise and highly effective measurements of varnish atop thin metallic layers

Although in principle a challenging measurement task, the thickness of this lacquer over thin metallic coatings can be determined precisely using the eddy current probe FTA3.3-5.6HF from FISCHER, along with the handheld instrument ISOSCOPE® FMP30. A further aid to improve the measurement accuracy and ease is the support stand V12 BASE. The measurement sequence is then straightforward for the operator: The probe is placed on the dial with the assistance of the support stand, which enforces level and even positioning of the probe. Because operator influence is thus minimized, extremely uniform measurements can be executed, ensuring high repeatability – the standard deviation can be even below hundred nanometers for measuring the thin layer of varnish.

Thin layers of lacquer protecting the dials of valuable watches can be easily and precisely measured using the combination of ISOSCOPE® FMP30, probe FTA3.3-5.6-HF and support stand V12 BASE. For more information, please contact your local FISCHER representative.