The scratch test has long been used to study the adhesion of coatings. In this test an indenter is drawn across the surface of a coating under an increasing (continuous or stepwise) load. The load (normal to the surface) at which 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 parameters including the friction coefficient. In this paper a review of theoretical models applicable to scratch adhesion testing is given. Experimental data is used to compare the ability of these theoretical models to describe the effect of friction between the indenter and the coating on the critical load. We applied the scratch test to a model system consisting of a (hybrid) sol-gel coating deposited on polypropylene. The friction coefficient between indenter and coating was varied by a short plasma modification of the surface of the coating, while all other relevant parameters (i.e. interfacial adhesion, layer thickness, E-modulus of the coating, etc.) remained constant. The critical load (normal to the surface) showed a pronounced decrease of more than an order of magnitude with increasing friction coefficient. Several models are discussed and compared to the experimental data. In addition, the effect of substrate pretreatment on coating adhesion was studied. The adhesion of the sol-gel coating induced by microwave oxygen plasma modification of polypropylene is considerably better than the adhesion obtained by wet-chemical modification in chromo-sulfuric acid at room temperature. The adhesion induced by immersion in chromosulfuric acid is shown to be independent of the immersion time between 1 and 10 min.