Effect of conformational change on nanoscale friction behavior of organic thin films

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Kanaga Subramanian, Kanaga Karuppiah
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Sriram Sundararajan
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Mechanical Engineering
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The overarching theme of this dissertation is to probe relationships between structure of organic thin films and their specific functional property of friction in the context of various engineering applications. Two specific thin film systems were studied - biological macromolecules in total joint replacements and self-assembled alkanethiol monolayers for microdevice applications. Before delving into the actual systems, a thorough understanding of friction at small length scales was required. To address this, a friction study of two material pairs (Si3N4/mica and Si3N4/ultra-high molecular weight polyethylene) was conducted using a microtribometer and atomic force microscope (AFM) at the micro- and nanometer length scales respectively, while keeping the environmental and counterface conditions same at both scales and thereby evaluating contact area dependence in the absence of surface damage and contact area independence when damage occurs.

Biological macromolecules such as proteins and lipids are important constituents of the synovial fluid which is the natural lubricant present in all of our human joints. The effect of adsorbed films of proteins and lipids on the micro/nanoscale tribological response of the polymeric materials used in total joint replacements (TJRs) were investigated. The friction and wear response of UHMWPE samples with different crystallinities was studied in the presence of bovine serum albumin protein and phospholipids. The observed friction increase upon exposure to proteins was attributed to the formation of a layer of denatured proteins on the surface. Changing the crystallinity and surface energy of UHMWPE affected the protein adsorption mechanism and the resulting increase in friction behavior. It was also found that increased crystallinity lowered the friction response and increased the scratch and wear resistance at both micro and nanoscales. It was also found that higher crystallinity increased the adsorption of the phospholipid and acted as an effective lubricant reducing the friction response and increasing the wear resistance of the interface.

The surface stress generation during the formation of a self-assembled monolayer (SAM) of alkanethiols on a macroscale domain was investigated in order to exploit this effect for sensing systems. To that effect, a curvature interferometry technique was used to study the surface stress generated during the formation of octadecanethiol SAM on a 25 mm x 25 mm mica sample. It was seen that the magnitude of surface stress measured on macroscale domain compared well with previously reported measurement on micron sized domains.

The possibility of utilizing a SAM system as a means to achieve active friction modulation of a surface was also investigated. A low-density SAM system, shown to exhibit conformational changes in the presence of an electric field, was synthesized and its friction response was studied using an AFM. Friction experiments showed that in the presence of a positive bias, the film showed a higher friction response (up to 300%) than when a negative bias was applied. The difference in the friction responses was attributed to the changes in the structural and crystalline order of the film between the two bias conditions.

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Tue Jan 01 00:00:00 UTC 2008