The Wilborn-Wolff surface hardness test is used to assess the scratch resistance of coatings and paints.
The surface hardness test, also known as the scratch resistance test, is performed using the hardness of a pencil.
Pencil leads, which have a well-graded hardness ranging from 6B to 9H (a set of 17 pencils), act as a “thermometer” for the resistance of the coating.
The pencil is inserted into a special instrument, simply called a Wolff-Wilborn scratch test apparatus, which places it in a position with an inclination of 45°; at this point, the pencil is pressed onto the coating with a force slightly less than that necessary to break it.
Hardness and scratch resistance will then be indicated by the type of lead that will not damage the tested surface.
The standard governing this is ISO 15184.