Epoxy/Aluminum Adhesion as Measured by Contact Mechanics (JKR) in the Presence of an Organic Contaminant
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Epoxy/Aluminum Adhesion as Measured by Contact Mechanics (JKR) in the Presence of an Organic Contaminant Dara L. Woerdeman1, John A. Emerson, Rachel K. Giunta Department of Organic Materials, Sandia National Laboratories, Albuquerque, NM 87185-0958, USA 1 Department of Chemical Engineering and Center for Bioelectronics, Biosensors and Biochips (C3B) Virginia Commonwealth University Richmond, VA 23284-3028 ABSTRACT The JKR contact mechanics approach is employed to analyze the effects of surface contaminants on adhesive bonding, as well as quantify the level of contamination at which adhesive strength decreases. 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 join the two surfaces, subsequently applying a tensile load to assess the energy dissipation mechanisms involved in the debonding process. In the present work, we monitor the interactions between a diglycidyl epoxy elastomer and an aluminum oxide substrate in the presence of an organic contaminant. Furthermore, we present a method by which surface contamination can be quantified using a single number, referred to as the adhesion hysteresis parameter, H. INTRODUCTION Surface contamination is a common problem in the adhesive bonding of microassemblies, and therefore calls for a convenient method to assess the cleanliness of the adherends before introducing the adhesive to the microsystem. Photoelectron surface analytical techniques are routinely employed to characterize substrates, however they are not applicable to the assessment of surface contamination, since organic contaminants are vaporized in high vacuum. The JKR contact mechanics technique, on the other hand, has a number of unique advantages for examining micron-scale areas in a processing environment, as it is relatively inexpensive, versatile, and easy to operate. We, therefore, apply the JKR method to probe the adhesive interactions of our model epoxy/aluminum oxide system. The model contaminant is hexadecane, a non-polar medium molecular weight hydrocarbon fluid. We chose hexadecane because it replicates typical machining fluids, is nonreactive with aluminum oxide surfaces, and should not readily dissolve into the adhesive system of interest. The JKR theory, developed by Johnson, Kendall, and Roberts [1], is an extension of Hertzian contact theory [2] and attributes the additional increase in the contact area between soft elastomeric hemispheres to adhesive forces between the two surfaces. In particular, it allows a direct estimate of the work of adhesion, W, between the two solids. In this approach, a low elastic modulus material in the shape of a hemispherical lens is brought into contact with a solid surface of interest and the resultant contact area is monitored as a function of applied loading. The contact area is found to vary with applied loading according to the JKR equation, Equation 1, which describes the system behavior as a function of its geometry, material
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