STUDIES OF MICROHARDNESS AND MAR RESISTANCE USING A SCANNING PROBE MICROSCOPE

Scanning probe microscopy (SPM) is in a period of rapid development. I t shows great promise for characterizing coating surfaces. This paper describes modification of an SPM so that it can be used to mar the sur faces of coatings under controlled conditions and to characterize the mars. Mar resistance of coatings is analyzed in terms of a 'three resp onse, two mechanism model.' The three responses (fracture, elastic, an d plastic) can be measured quantitatively using the SPM. Of the three responses, only two (fracture and plastic deformation) are marring mec hanisms - elastic deformations recover instantaneously. In some cases mars resulting from plastic deformation may recover slowly with time o r with immersion in water; this phenomenon is attributed to viscoelast ic creep. Microhardness is also measured with the modified SPM. Some t hermoset coatings appear to be substantially harder near their surface s than in the mass of material, and such materials may respond quite d ifferently to stress applied at different levels near the surface. Thi s finding has important implications for all coating properties that a re strongly influenced by the surface. A quantity called 'micro mar re sistance' is defined. It may be useful for comparing different coating s under specified conditions of marring. However, there can be no sing le quantity that expresses 'mar resistance' of a coating under all con ditions. (C) 1998 Elsevier Science S.A. All rights reserved.