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PENCIL HARDNESS TEST EXPLAINED
How to measure coating hardness using the Wolff Wilborn Pencil Hardness Test, the Wolff Wilborn test is the undisputed go-to standard, so that experts and assessors alike can accurately determine the hardness of the said coating.
If a coating does not dry (or cure) properly, this can affect its performance; resulting in flaws, poor adhesion, and premature coating failures. This is why we test coating hardness, as this indicates how well it has cured – typically the harder the coating, the more complete the cure, and the better its overall performance.
One of the most common methods of determining coating hardness is the pencil hardness test, also known as the Wolff-Wilborn method, where a pencil of a known hardness is pushed across the coating at a specified angle, under a constant force. The pencil hardness is increased from B (soft), though HB (medium) to H (hard), until the coating is scratched. Alternatively, depending on the test method, you may start with the hardest pencil, and then decrease the pencil hardness until the coating isn’t scratched.
The Wolf Wilborn Test explained:
Please note, while this is only a guide to completing the pencil hardness test, certain test methods and Standards differ slightly on how the method should be completed, and how the results should be recorded. So, where possible we recommend you always refer to the test method used by your product provided. This rapid, inexpensive method is not only used by coating manufacturers during developmental work and as an indication of the coating’s performance; but also as a field test during the post-application inspection.
Regardless of which test methods are followed, the preparation is always the same - remove around 5 to 6 mm of wood from the pencil, not including the point, taking care to leave the exposed lead unmarked. Note, there are two different sharpeners to suit the different hardness of lead – quite simply one for soft, the other for hard - extremely important to use the correct one accordingly.
By holding the pencil at a 90° angle to the abrasive paper, the lead is rubbed until it achieves a flat, hard, blunt but smooth, circular cross-section, which is free of chips or nicks ready to perform the test.
The pencil firmly held at a 45° angle to the surface is then pushed away from an examiner only about 6mm (0.25”) at a uniform speed, applying sufficient downward pressure to either cut or scratch the coating or crumble the edge of the lead. Some Standards, however, such as ISO 15184, specify the amount of downward force that should be applied to the pencil during the push.
So under the predefined force of 7.5N. an examiner will at a uniform speed to complete the test.
Even if the test method or standards being worked do not specify a downward force, there are other options other than a manual pencil test that can be considered, ensuring every push during said Pencil Hardness Test will in fact be logged with the exact same downward force so results could & would not be disputed. Electronic testing ensures also that every push, with every pencil, is undertaken using exactly the same angle and downward force. When a test is complete, the examiners will inspect the surface by carefully removing any pencil marks from the surface using either a soft cloth or cotton wool with an inert solvent, or a soft eraser; taking care not to affect the hardness of the coating in the test area or the result of the test.
They can then assess the test area through close visual inspection, by touch using a fingernail, or both. and if agreed, it is even possible to inspect using a magnifier of x6 to x10 magnification. So, what are they looking for? you might ask.
Well, this depends on the test method or Standard you are working on. Some standards, such as ASTM D 3363 for example, state that you should start testing with the hardest pencil, and continue down the scale, testing with softer and softer pencils, until you find either of two results:
you find the hardest pencil that makes a cut which is less than 3mm long out of the 6mm test push – this is known as the gouge or pencil hardness;
you find the hardest pencil that doesn’t scratch the film at all – this is known as the scratch hardness.
When they take a look at this test panel, which has resulted from 6B (soft) all the way through to 9H (hard), and we work down from the hardest pencil, what we can see is that the hardest pencil to make a scratch in the film that’s less than 3mm long is H – so this is the gouge hardness, as all the scratches made by harder pencils are over 3mm in length. As for the scratch hardness, we carry on going from hardest to softest to find the hardest pencil test result that leaves no marking at all – and as you can see then, the result is F. Alternatively, other Standards such as ISO 15184, don’t actually specify which pencil you need to start testing with, although many in the industry tend to start with the softest pencil and continue up the scale until you find the pencil that leaves a mark of over 3mm long out of the 6mm test push. That mark can be defined as either one or a combination of two defects:
a permanent indentation or scratch that does not cut through the coating (known as plastic deformation);
a visible tear or rupture in the surface of the film (known as a cohesive fracture).
If we were to start at the softest pencil and work up, the softest pencil to leave a mark of over 3mm long is 2H. Examining the results a little closer, you can see that the majority of the mark is a cohesive fracture, so it has cut through the film. And on the edges of the results, you can also spot a little bit of plastic deformation.
All tests that are done to determine the Hardness of the coatings & will be done at least twice to ensure repeatability and reproducibility. If the two results differ by more than one unit of pencil hardness, then both results would be discarded, and the test repeated.
So, in conclusion; when you see the H I S C© seal you can trust the product that you are purchasing & you will know it can withstand the rigours of all Goggle/Visor sports. Also, be in no doubt whatsoever that the hardness of its coating will have met our minimum standard.
HIGH IMPACT SURFACE COATING
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