Adhesion And Wear Of Nanoscale Polymer Contacts

Atomic force microscopy (AFM) is a powerful tool for high resolution surface measurements, nanolithography, and tip-based nanomanufacturing. An understanding of the nanoscale tribological behavior of the tip-sample contact, including adhesion and wear, is critical in these applications. In this dissertation, the adhesion and wear of polymethyl methacrylate (PMMA) in contact with an ultrananocrystalline diamond (UNCD) AFM tip is investigated using a combination of AFM-based nanomechanics experiments and finite element analysis (FEA). A novel AFM-based method, which combines pull-off force measurements and characterization of the 3D geometry of AFM tip, was developed to quantify the properties of the adhesive traction-separation relationship of UNCD-PMMA contacts. Adhesion range and strength, as well as work of adhesion, of the contacts were determined. Characterizing and understanding nanoscale wear of polymers has proven difficult in the past due to experimental complications associated with debris produced in the tests and the failure of traditional empirical wear relations, such as Archard’s law. Here, nanoscale AFM-based line and raster wear experiments were performed on patterned PMMA structures, which had gaps that allow debris to be captured. Results from the line wear tests indicate that the relationship between height loss rate and stress is well described by a transition state theory wear model. The wear parameters obtained from line wear tests were applied to predict the volume loss in raster wear tests. Despite the significant differences in the loading and sliding geometry between line and raster wear experiments, the raster wear behavior was accurately predicted from the parameters obtained from line wear tests. In addition, experiments at varying temperatures were performed to study the temperature dependence of polymer wear. The results suggest that transition state theory overestimates the effect of temperature on wear for PMMA. Modified models considering viscoelastic relaxation of PMMA and atom-by-atom attrition were applied to describe the measured wear behavior. Finally, an iterative FEA method was developed for simulations of wear. These simulations were used to examine the evolution of geometry and stress during the wear process as a function of contact conditions, such as friction and surface roughness