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Vast majority of tests for measuring the hardness of organic coatings are based on the following three principles: scratch resistance, indentation resistance and fatigue resistance against periodic deformation.

Widely used scratch resistance test is the pencil hardness test. Pencils with varying hardnesses are used to scratch the paint film. The hardness of the film is reported as the grade of pencil that does not cause any scratching on the surface.

In indentation resistance tests, an indenter harder than the organic coating is forced against the film under a fixed load. The size of the indentation indicates the resistance of the paint. Buchholz hardness test and Tukon (Knoop) hardness test are the most known indentation hardness tests.

Pendulum hardness test is the most common test relateed to fatigue resistance against periodic deformation. The damping time or number of swings made by the pendulum required to make permanent deformation of paint film under periodic load is determined.

See Also Pencil hardness , Buchholz hardness, Knoop hardness, Pendulum hardness