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.
Measuring the thickness of micaceous iron oxide coatings
Micaceous iron oxide (MIO) coatings are called for wherever maximum corrosion protection is essential: Steel bridges, power poles, and even famous structures like the Eiffel Tower or Sydney Harbour Bridge are shielded against the elements with a layer of this specific type of paint. To ensure the coating actually lasts as long as foreseen by the manufacturer, a certain layer thickness must be applied and checked.
Micaceous iron oxide (MIO) is not – as often assumed – ground iron but is rather a form of the naturally occurring mineral hematite; mixed as an additive into paint, MIO forms an additional, protective barrier against corrosion. A crystalline iron oxide mineral consisting mainly of iron III oxide, powdered MIO is flaky in texture; when suspended in viscous epoxides, the minerals align themselves parallel to the surface as the paint dries, forming a dense, nearly impenetrable shield of overlapping plates that repels water and other rust-forming elements. Known as "scale armour paint", MIO coatings are used for extremely heavy-duty applications.
Typical for MIO paint is its red-brown tint. Famous structures such as the Eiffel Tower in Paris, the Giant Ferris Wheel in Vienna, the Sydney Harbour Bridge and Istanbul’s Bosphorus Bridge are all protected with it.
While found primarily on galvanized steel parts, MIO coatings can also be used on non-galvanized steel, iron and aluminium. However, the reliability and longevity of the corrosion resistance depend on the thickness of the coatings, which are generally applied about 80-120 microns thick.
Just like with normal paint layers, the thickness of MIO coatings can be measured using the magnetic induction method, as hematite itself is antiferro-magnetic; thus, the MIO coatings are usually not magnetic either. Instruments from FISCHER’s FMP handheld series are ideally suited for this task. Due to the paint’s relatively high surface roughness, FISCHER also recommends the use of one of its robust probes with carbide metal tips, which have particularly long service life: the F20H.
The addition of platelet-shaped minerals to so-called MIO coatings boosts the corrosion protection for structures that are constantly exposed to weather. Using the handheld instruments from the FMP series and the durable F20H probe, the thickness of MIO coatings can be measured quickly and accurately. For more information, please contact your local FISCHER representative.
Measuring anti-fouling coatings on marine structures
The related costs of biofouling are so high that even expensive prevention technologies quickly pay for themselves: some sources put the savings in fuel consumption alone at 40%. Properly applied anti-fouling systems can significantly reduce a variety of operating costs, including downtime at dry dock. High-tech inspection instruments equipped to handle the wide variety of materials and thickness ranges typical of anti-fouling paints help ensure that the finished coatings can indeed fulfil their expected service lifetimes.
Any part of a marine structure – whether ships or offshore rigs or piers – that is permanently submerged under water will fall victim to biofouling, or marine organisms attaching to its surface. This affects performance by adding to overall weight, increasing drag in the water, and contributing to corrosive processes.
The solution used to be an easy decision: the marine bottom paint of choice contained highly effective tributyltin (TBT) which prevented the growth of a variety of biofouling agents. But due to toxic effects on marine life, the International Maritime Organization (IMO) banned it as of 2003. In response, a variety of alternatives has come on the market, each employing very different approaches to the problem.
One feature that the new multi-layer anti-fouling coating systems all have in common is that they tend to be rather thick, sometimes even more than 1 mm. Indeed, the principle behind some anti-fouling paints is ablation or sloughing, meaning the controlled and sustained loss of material over time: coatings of this sort cannot start out thin! Others are partially soluble, still others self-polishing.
Since there is no “silver bullet” anymore that addresses all potential biofouling scenarios, marine service providers and dry docks need to be able to monitor a wide range of coating types, on both magnetic and non-magnetic substrates – without having to change instruments all the time.
For just such measurement tasks, FISCHER developed the FD13H probe with an especially robust hard-metal probe tip for longer life. When used with the mobile DUALSCOPE® FMP handhelds, the underlying substrate material is automatically detected and the correct measurement principle applied; this greatly simplifies the task of inspecting anti-fouling coatings because the probe and gauge are suited for measuring such a wide variety of coating materials on exactly the kinds of substrates typically found in marine installations.
To inspect typical anti-fouling coatings on variable substrates FISCHER offers the FD13H probe together with the DUALSCOPE® FMP instruments, which can measure a wide variety of non-conductive coatings over magnetic and non-magnetic substrates like steel or aluminium. For more information, please contact your local FISCHER representative.