MECHANICS OF THE INDENTATION TEST AND ITS USE TO ASSESS THE ADHESION OF POLYMERIC COATINGS

The indentation test provides a simple means by which the adhesion of coatings can be qualitatively assessed. On the way to establishing a q uantitative measurement of the adhesion strength of coatings and films , it is important that the mechanics of this test are clearly understo od. To investigate the influence of factors such as the coating thickn ess, the indenter radius, and friction during the test, numerical simu lations of the indentation of a typical polymeric coating, polymethylm ethacrylate PMMA), bonded to a rigid substrate were conducted by using the finite element method. The stress generated during the indentatio n test were obtained by employing an accurate constitutive model of th e elastic-viscoplastic behaviour of the polymeric coating under consid eration. The results of this analysis illustrate the effects of the fa ctors mentioned above on the deformation of the coating during indenta tion, its confinement, and interfacial shear, and the normal, shear, a nd hoop stress distributions occurring during indentation. These resul ts provide insight into the possible failure mechanisms operative duri ng the indentation of thin coatings and the important effects of the c oating thickness during such tests.