The effect of friction on scratch adhesion testing: application to a sol-gel coating on polypropylene

The scratch test has long been used to study the adhesion of coatings. In t his test an indenter is drawn across the surface of a coating under an incr easing (continuous or stepwise) load. The load (normal to the surface) at w hich detachment of the coating occurs is termed the critical load. Usually, the magnitude of the critical load is related to the adhesion between the substrate and the coating by some theoretical model. It is well. known that apart from the adhesion the critical load depends on several other paramet ers including the friction coefficient. In this paper a review of theoretic al models applicable to scratch adhesion testing is given. Experimental dat a is used to compare the ability of these theoretical models to describe th e effect of friction between the indenter and the coating on the critical l oad. We applied the scratch test to a model system consisting of a (hybrid) sol-gel coating deposited on polypropylene. The friction coefficient betwe en indenter and coating was varied by a short plasma modification of the su rface of the coating, while all other relevant parameters (i.e. interfacial adhesion, layer thickness, E-modulus of the coating, etc.) remained consta nt. The critical load (normal to the surface) showed a pronounced decrease of more than an order of magnitude with increasing friction coefficient. Se veral models are discussed and compared to the experimental data. In additi on, the effect of substrate pretreatment on coating adhesion was studied. T he adhesion of the sol-gel coating induced by microwave oxygen plasma modif ication of polypropylene is considerably better than the adhesion obtained by wet-chemical modification in chromosulfuric acid at room temperature. Th e adhesion induced by immersion in chromosulfuric acid is shown to be indep endent of the immersion time between 1 and 10 min. (C) 2000 Elsevier Scienc e S.A. All rights reserved.