Definition - What does Fingernail Test mean?
A fingernail test is simple test of the hardness and abrasion resistance of a surface coating. As the name implies, the test is performed using a fingernail, which is gouged at the surface of the cured coating. In its simplicity, it is not limited to a particular finger as it is also referred to as a thumbnail test. The same test is also commonly performed by knives or coins. All together, these techniques are hand tests for hardness and abrasion resistance.
While fingernail tests are useful for quick on the spot tests at a job site, they are impractical for industrial testing and lack scientific robustness. Thus, controlled testing equipment is available that replicates the interaction of fingertips with surfaces.
Corrosionpedia explains Fingernail Test
One of the central purposes of paints and coatings is protecting the underlying material and preventing abrasive damage. It is therefore important to have methods to evaluate if a particular coating is doing its intended job. Fingernails provide both a convenient quick tool for such evaluation and also act as a way to emulate the interaction of surfaces with human fingers, as occurs with everyday contact by human fingers. The test simply involves placing a fingernail on the surface and attempting a quick scratch. A properly cured and abrasive resistant coating should not scratch.
The way a fingernail test is performed can be categorized as indenting, shearing and grazing actions, which vary by the way the nail interacts with the surface. Each method gives insight into the hardness of the surface and how resistant it may be to abrasive actions. While these tests are conveniently performed with an actual fingernail, they are often replaced with a nail or other object.
It is noteworthy that similar tests exist that involve the finger’s action against a surface rather than the nail. While such tests like thumb tests are similar to fingernail tests, they evaluate adhesion and hardness in a different way.