Scratch Hardness Test/Abrasion Resistance Test- Pencil Hardness Test
These tests are under mechanical test category. These tests are important for the paint or clear film on furniture, automobile, appliances etc. where the surface is prone to abrasion.
The coating should have resistance against abrasion. Generally scratch hardness is measured by moving a m sharp object under a known pressure over the surface of the material.
The pencil test uses constant pressure and variable hardness of the pencil, Pencils of varying hardness, starting with the hardest lead, are moved over the surface under a fixed pressure of 7.5 Newtons (765 gms) and at a fixed angle of 45 degrees to the surface.
The degree of hardness of the pencil which damages the surface is taken as a measurement for scratch hardness, Fourteen or Twenty pencils in the range of grades from 9B (6B) to 9H (ISO 15184 / BS 3900 – E19 or ASTM D3363 / ECCA T4 specifies only 14 pencils.) are used.
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.
Please note, while this video is a guide to completing the pencil hardness test
Test Method–
Pencil is sharpened to approximately 3/16 to 1/4 in. (5 to 6 mm) of wood from the point of each pencil using a sharpener The pencil is rubbed against abrasive paper at 90° until a flat, smooth and circular cross section is obtained. Insert the pencil in apparatus until its point touches the flat surface. Tighten the pencil clamping screw.
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.
With the push complete, you now inspect the surface and examine the coating for indentation and scratching. If there is none, repeat the test using a harder pencil. If the surface is scratched or indented repeat with softer pencil.
PLEASE NOTE: This method is only applicable to smooth surfaces and coatings.
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.
Hagman Units reading conversion list from Mils to Microns
