ABSTRACT
Thin films deposited on polymers are widely used in various industrial applications. One of the most important factors is that the coating should not delaminate from the substrate during long-term usage, and thus good adhesion is required. However, adhesion measurement of coatings on flexible polymers is generally achieved by fixing the polymer onto a rigid substrate using an adhesive, and by performing a test valid for rigid substrates. The use of adhesive has several limiting factors:
(i) it must have a strength exceeding the adhesion of the coating; (ii) the adhesive must not penetrate and chemically affect the interface; and
(iii) it must not introduce additional stress during its setting. The adhesion of coatings onflexiblepolymers is generally assessed means of a peel-off
test. This method requires the use of two adhesives and is also limited to low-adhesion coatings (the coating must be completely removed from the substrate). A more suitable test consists of the tensile test, but little work has been done on this and the mechanics of the test are not well understood. The laser spallation test seems to be easily applicable to polymers without using an adhesive, but this technique is very sensitive and may be
expensive. The scratch test method is generally considered as a standard test for rigid substrates. We have studied the application of the scratch test to flexible polymers, and more specifically the influence of the adhesive layer. The results show that the critical load alone is not a good indication of adhesion, but semi-quantitative results can be obtained if the composite hardness of the sample is taken into consideration.
