Mechanics of the Peel Test for Thin Film Adhesion

The mechanics aspects of the peel test are analyzed. In this analysis, the limitation and applicability of the peel test for thin film adhesion have been investigated. Firstly, the effect of plasticity in the peel test of thin metallic films is analyzed. The analysis has given a closed form solution for the partitioning of peel strength between the energy expenditure rate for the decohesion process and the internal work expenditure rate caused by plastic deformation (bending) of the film. The results predicted the variation of peel strength as a function of the film thickness, the film yield stress and the compliance of the substrate. This analysis shows that the peel strength is very sensitive to the thickness of the film and the yield stress of the film. As observed in the experiment, the peel strength has a peak value at a certain thickness of the film. This is explained for both L and T peel tests. Unlike the L peel test, the peak phenomenon in the T peel test (of metallic films) is due to the hardening of the adhered film. Secondly, the effect of plasticity and viscoelasticity in the peel test of thin polymer films is analyzed. Unlike the peel test of metallic films, tension and shear effect of the film deformation is very important in the analysis of the polymer peel test. An integral equation is formulated for the viscoelastic models of the adhered film. The solution of the integral equation reveals the important parameters in polymer peeling and their contribution to the peel force as well as to the viscoelastic dissipation. In addition, the meaning of the interface toughness is reexamined and the interface toughness is evaluated by the peel test. Based on these analyses, a Universal Peel Diagram has been constructed, which accounts for the hardening behavior of the film. On this diagram, objective interfacial decohesion toughness can be readily obtained using very simple peel tests. The analysis includes not only the 900° peel test but also the general angle peel test. Comparison between the theory and experiment has been made with the experimental data provided by J. Kim of IBM. The comparison shows good agreement.