The Effect of Surface Contamination on Adhesive Forces as Measured by Contact Mechanics
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The Effect of Surface Contamination on Adhesive Forces as Measured by Contact Mechanics John A. Emerson, Rachel K. Giunta, Gregory V. Miller, Christopher R. Sorensen, and Raymond A. Pearson1 Department of Organic Materials, Sandia National Laboratories, Albuquerque, NM 87185-0958, USA 1 Materials Science and Engineering Department, Lehigh University, Bethlehem, PA 18015-3195, USA ABSTRACT The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to mate the two surfaces and subsequently applying a tensile load to peel the surfaces apart. The contact region is assumed the superposition of elastic Hertzian pressure and of the attractive surface forces that act only over the contact area. What are the effects of the degree of surface contamination on adhesive forces? Clean aluminum surfaces were coated with hexadecane as a controlled contaminant. The force required to pull an elastomeric hemisphere from a surface was determined by contact mechanics, via the JKR model, using a model siloxane network for the elastomeric contact sphere. Due to the dispersive nature of the elastomer surface, larger forces were required to pull the sphere from a contaminated surface than a clean aluminum oxide surface. INTRODUCTION We are evaluating work of adhesion measurements to facilitate our understanding of adhesive failure mechanisms for systems containing encapsulated and bonded components. One issue under investigation is the effect of organic contamination on the adhesive strength of several polymer/metal interface combinations. We are interested in quantifying the level of contamination at which adhesive strength decreases. We have applied contact mechanics, the JKR method, to study this question. The model contaminant is hexadecane – a non-polar, medium molecular weight hydrocarbon fluid. We choose hexadecane because it replicates typical machining fluids, is nonreactive with aluminum oxide surfaces, and should not dissolve readily into the adhesive systems of interest. The application of a uniform, controllable and reproducible hexadecane layer on aluminum oxide surfaces has proven to be difficult [1]. A primary concern is whether studies of model systems can be extended to systems of technological interest. The JKR theory, developed by Johnson, Kendall, and Roberts [2], is a continuum mechanics model of contact between two solid spheres. The JKR theory is an extension of Hertzian contact theory [3] and attributes the additional increase in the contact area between soft elastomeric hemispheres to adhesive forces between the two surfaces. The JKR theory allows a direct estimate of the surface free energy of the interface as well as the work of adhesion (Wa) between solids. Early studies performed in this laboratory involved the determination of Wa between silicone (PDMS) and aluminum surfaces in order to establish the potential adhesive failure mechanisms. A model PDMS elastomer and polymer treatments were dev
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