How to measure coating hardness using the Wolff-Wilborn Pencil Hardness Test

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.

CONTENTS
0:29 - What is the pencil hardness test?
1:17 - The Elcometer 3080 & Elcometer 501
2:20 - Completing the pencil hardness test
4:34 - How to read the result of a pencil hardness test

Why do we measure coating hardness?

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.

The pencil hardness test can be completed free-hand using the Elcometer 3080, which includes a set of 14 pencils (grades 6H to 6B), two specialised pencil sharpeners, and an abrasive paper block to prepare the pencil for testing.

Alternatively, some Standards state you should use a specially designed tester, such as the Elcometer 501; which also includes a set of 14 pencils, the two specialised sharpeners and abrasive paper block, and a setting block of a predefined size for preparing the tester.

Please note, while this video is a guide to completing the pencil hardness test, as you’ll discover certain test methods and Standards differ slightly on how the method should be completed, and how the results should be recorded. So, we recommend you always refer to the test method or Standard you are working to directly before testing.

To begin, prepare the pencil by using the special sharpener provided, to remove around 5 to 6 mm of wood from the pencil, not including the point, taking care to leave the exposed lead unmarked. Please note, there are two different sharpeners – one for softer pencils, the other for harder pencils - so use the correct one accordingly.

Next, hold the pencil at 90° to the abrasive paper provided, and rub the lead until you achieve a flat, smooth, circular cross-section, which is free of chips or nicks. You are now ready to perform the test.

If you are completing the test free-hand, using the Elcometer 3080, with the test panel on a level, firm, horizontal surface; hold the pencil firmly at a 45° angle to the surface, and push it away from you 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. This is where the Elcometer 501 Pencil Hardness Tester comes in.

With the tester resting on the setting block provided, simply fit the pencil into the body with the tip resting on the surface, lock it in place using the thumbscrew, and your pencil is instantly held at 45°
under the predefined force of 7.5N. Simply remove the setting block, place the tester down carefully ensuring not to damage the surface or pencil lead, and push the tester away from you 6mm (0.25”) at a uniform speed to complete the test.

Even if the test method or Standard you are working to does not specify a downward force, using the Elcometer 501 Pencil Hardness Tester ensures that every push, with every pencil, is undertaken using exactly the same angle and downward force, ensuring repeatable and comparable results.

With the push complete, you now inspect the surface.

Carefully remove any pencil marks from the surface taking care not to affect the hardness of the coating in the test area or the result of the test.

You then assess the test area through close visual inspection, by touch using a fingernail, or both. It is even possible to inspect using a magnifier of x6 to x10 magnification, such as the Elcometer 137.

How you assess the result is dependent on the test method or Standard you are working to. For example ASTM D 3363 and ISO 15184 differ greatly on how you report the result. Make sure you watch from 4:34 to find out more.

To find out more about the Elcometer 137 LED Illuminated (x10) Magnifier, click here: https://www.elcometer.com/en/coating-...

For more information on Elcometer's range of Hardness and Scratch testers, contact Elcometer at [email protected] or visit our website at https://www.elcometer.com/en/laborato...

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Click here to download the video script in English: https://www.elcometer.com/pub/media/P...

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STANDARDS

Elcometer 3080 & Elcometer 501

ASTM D 3363, BS 3900-E19, ECCA T4, EN 13523-4, ISO 15184, JIS K 5600-5-4