Ceramic Coated Pencil Hardness Test

Coating on Substrate

Coatings are usually very thin layers of material supported by a substrate. Anything laid on top of a substrate is a coating, and this includes waxes, lacquer/acrylic/enamel paints, and other types of materials that are applied to a substrate.

Coatings applied to hard substrates will naturally increase the hardness of the coating. The ceramic coating will form a layer over the car's clear coat, helping to protect the surface from harmful UV rays, pollen, bird droppings, acid rain and other external elements that damage the car's paint.

What is hardness?

Hardness is the resistance of a solid material to deformation when a compressive force is applied. Some materials, such as metal, are harder than others, such as plastic. Macroscopic hardness is usually characterized by strong intermolecular bonds, but the behavior of solid materials under force is much more complex; therefore, there are different methods of measuring hardness:

Scratch hardness

Indentation hardness

Rebound hardness.

Hardness is highly dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity and viscosity.

Scratch hardness scale type

These tests are used to determine the scratch and abrasion hardness of a material.

Generally, there are 3 scales used to determine the hardness of a mineral:

Mohs Hardness: It is based on relative scratch hardness with a value of 1 for talc and 10 for diamond. There are two limits to Mohs hardness; it is not linear and most modern abrasives are between 9 and 10.

Ridgway Hardness: A modification of the Mohs scale, giving garnet a hardness of 10 and diamond a hardness of 15.

Wooddale Scale: Extending the Ridgway scale by using abrasion resistance, and extrapolating the scale from 7 for quartz and 9 for corundum, resulted in a South American Brown Diamond Porter value of 42.4.

How a Pencil Hardness Tester Works

Mineral flakes (Mohs, Ridgway and Wooddale) are less suitable for use with coatings or films. Therefore, a standardized ASTM method for measuring thin films using the pencil hardness scale was created. Pencils are made of graphite with a Mohs hardness of only 1-2H. Think of the pencil hardness scale as a detailed hardness scale for graphite.

For many years, the paint industry has used pencil hardness measurements to determine the hardness of clear and pigmented organic paint films. This test method has also been used to determine the cure of these coatings, especially when heat forced drying is used.

This method of testing is especially useful in development work and production control testing in individual laboratories. It should be recognized that results obtained by different laboratories may vary when different pencils and panels are used. Every effort should be made to standardize the hardness of the lead used and the technique followed.

It is also important to note that the pencil hardness test is only one of many tests used to evaluate coatings. There's more to consider than just the hardness of any coating, so don't judge a finish solely on its hardness.

Graded pencils are available in hard and soft pencils and can be found in most art or office supply stores, ranging in hardness from 10H to 9B. "H" stands for hardness, "B" stands for black, and HB stands for hard and black pencils.

The hardest is 10H, followed by 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H and HF is the middle of the hardness scale; then HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B and 9B, which is the softest. A commonly used writing pencil is #2 (Grade HB), which is quite soft, contains more graphite, and leaves darker marks.

How to perform a pencil hardness test?

Coatings typically tested were 25.4-38.1 microns thick and dried for 7 days. Ambient temperature during testing may also be a factor in drying time and affect the hardness of the coating.

Pick a pencil and make a line about 1/2 inch long. If the pencil scratches the surface of the coating, softer pencils are used until we get the first pencil that does not scratch the coating.

Repeat the test to ensure reproducible results. There are also coatings that are so hard that a 10H pencil won't scratch them - these are rated 10H to designate their hardness.

Use this chart as a guide only. It is also important to understand that the hardness of any generic coating may not always be the same; in other words, if one company has a 3H hardness for polyurethane, do not assume that all other polyurethanes are 3H. They won't; they vary by manufacturer and product.

As mentioned above, the pencil hardness test is just one of many tests used to evaluate coating performance. Other tests like abrasion resistance, reverse impact resistance, direct impact resistance, cross-hatch adhesion, oxidation, gloss retention, UV resistance, yellowing, blistering, drying time, chemical/solvent resistance, Salt spray resistance, moisture resistance, acid and corrosion resistance, VOC and HAP content, etc. are also critical to the performance of the coating.