Why do we measure hardness? The main reason is to see how well the coating will stand up to abuse in actual use. Will it resist damage or be dented, scratched, gouged, abraded? We must be careful in interpreting the results because they are dependent on the force applied, how rapidly it is applied, the coating thickness, and the nature of the substrate as well as the properties of the coating. However, very useful comparisons can be made. Hardness also is one indicator of chemical structure and degree of crosslinking and is useful for establishing structure-property relationships.
Unfortunately, there is not just one kind of hardness, there are several and each one is related to a different material property and involves a different test. With regard to organic coatings, the main types are indentation, damping, scratch, and pencil. Indentation hardness refers to resistance to deformation by a loaded indenter, usually one with a shaped tip (pyramid, hemisphere, or wedge) such that the contact area increases with increased indentation. The hardness is defined as the load divided by the depth or area of deformation; the less the deformation, the harder the coating. Indentation techniques include the human thumbnail, Buchholz (mechanical thumbnail, DIN 53153), Pfund (transparent hemisphere, measured while under load, ASTM D 1474), and Tukon/Knoop (diamond-shaped mark, measured after load is removed, D 1474). A newer technique involves use of a nanoindenter instrument. Indentation hardness is related to the elastic modulus of the coating and probably is the most precise method.
Damping hardness involves the movement of a tool on a coating surface, usually with a rocking motion. The tool may be a ring, cylinder, cone or ball, with or without an attached beam or outrigger. The tool deforms the surface slightly and thereby loses a certain amount of its energy. This causes the motion to decrease. A hard surface deforms less and, therefore, loses less energy. It takes a long time for the motion to be damped. Damping can be determined from the total number of swings or, more commonly, by the time necessary for the amplitude of the motion to a certain fraction of the initial amplitude. Damping techniques include the Sward Rocker (ASTM D 2134), the Persoz Pendulum (ISO 1522), and the Konig Pendulum (ISO 1522). The Paul N. Gardner Co., Inc. has developed an automated Sward Rocker that I have not had an opportunity to test, but that looks very interesting. Damping is related to rolling friction and loss modulus.
Scratch hardness is the resistance to scratching by a pointed tool drawn across the coating with a controlled velocity. It is defined as the amount of damage (width or depth of scratch) by a fixed load or the load necessary to initiate a scratch or cause a certain amount of damage. Although scratch hardness may relate to practical conditions, results should be interpreted carefully and scratches themselves examined. Scratch resistance is dependent on adhesion as well as hardness and is as much an adhesion test as a hardness test. Hard, but brittle coatings, may give an apparent scratch line of tiny chips. Several scratch testers exist, with the latest type being a nanoindenter that can move the indenter horizontally to give a calibrated scratch.
Pencil hardness is a very common hardness test for organic coatings (ASTM D 3363). Hardness is expressed as the hardest pencil that does not break or gouge the coating. Pencil hardness is related to the tensile strength and elongation at break of the coating. The film is broken or cut only when the pencil exceeds the tensile strength of the paint film. Most coatings have a hardness that lies between 2B and 4H. The uncertainty in the measurement is at least one pencil classification. Therefore, pencil hardness can only give a coarse division into about six groups or levels of hardness.
All hardness tests should be done with care and with suitable controls. Results always should be treated as relative rather than absolute. Useful references include K. Sato, "The Hardness of Coating Films," Progr. Org. Coat., 8, 1-18 (1980); and P. R. Guevin, Jr., "Hardness," Paint and Coatings Testing Manual: Fourteenth Edition of the Gardner-Sward Handbook, J. Koleske, ed., ASTM, Philadelphia, 1995, pp. 555-584.
"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email firstname.lastname@example.org.
By Clifford K. Schoff
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|Title Annotation:||Coatings Clinic|
|Author:||Schoff, Clifford K.|
|Date:||Aug 1, 2007|
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