Determination of micro-indentation hardness of organic coatings

Many industrial products have to be coated by organic materials not only for aesthetical reasons, but specially to prevent the integrity of the metallic substrate from corrosion attacks during their service life; then, the corrosion resistance of metallic component is often assigned to the physical properties of the coating material. A lot of specific tests are available in the field of surface treatment characterization, in order to assess mechanical, physical and durability properties of paints and varnishes. Among them, indentation hardness allows to investigate on the local properties of a painted layer, not only on the external surface but also into the coating thickness, making it possible to verify hardness at every depth of the layer; non-uniformity in hardness values can be referred to the painting process (curing conditions, coating material, etc.). Hardness measurement technique on the transversal section required particular attention in the specimen preparation; hardness tests were performed by using a Knoop indenter, according to the standard ISO 6441-1. In the present paper are reported the results of several hardness tests performed on the surface and on the transversal section of various aluminium painted samples, some of them partially polymerized to different degrees. The same results have been compared with the results of commonly used tests for coating characterization, as: Taber abrasion test, hardness pencils test, Buchholz indentation test and also impact test.