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A standard method of determining the relative hardness of dried paint films is the ASTM Test for Film Hardness by Pencil Test [D 3363-74 (1980)]. Although the pencil test is useful in comparing the film hardness of paint systems, it does not yield specific numerical hardness data. Pencil hardnesses are alphanumerical and therefore pseudoquantitative. In order to quantify hardness results from the pencil test, a correlation was sought between pencil hardness and Knoop microindentation hardness. First, the Knoop microindentation hardness of a set of drawing leads meeting the requirements of ASTM Test D 3363-74 (1980) was determined. A linear correlation was found between pencil lead hardness numbers and Knoop microindentation hardness values. The Knoop microindentation hardness was next determined on a series of painted panels of known pencil hardness. A useful correlation was found between the film hardness as determined by the pencil test and that found by the Knoop microindentation hardness test.
paint films, hardness, pencil test, Knoop microindentation hardness, pencil lead hardness, microindentation hardness testing
Senior staff engineer, Hughes Aircraft Company, Tucson, AR