Adhesion, Friction and Mechanical Properties of CF3- and CH3-Terminated Alkanethiol Monolayers: A Comparative Study

Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS).[1] Of particular interest as tribological films have been SAMs terminated by fluorocarbon groups, because of their inert nature and enhanced thermal stability. Surprisingly however, fluorocarbon films were shown to actually produce higher coefficients of friction (relative to CH3-terminated films) in atomic force microscopy (AFM) studies. Subsequent work has concluded that the increased van der Waals radius of the fluorine groups (∼45%) causes a steric disruption of the order of the molecular surface giving rise to an increased friction.[2] We present results from a direct comparison of the adhesive, mechanical and frictional properties of SAMs terminated by CF3 and CH3 groups using both Interfacial Force Microscopy[3] (IFM) and the AFM. IFM results are shown for a two micron tungsten tip interacting with C16 alkylthiol molecules assembled on Au(111) single-crystal surfaces. AFM results involve a ∼20 nm tip interacting with the same two molecules assembled on Au films deposited on mica surfaces. A direct AFM comparison is accomplished by using a “nanografting technique”.[4]

Interfacial Force Microscopy and its Application in Metal Matrix Composites

Metal matrix composites (MMCs) combine the properties of metal and ceramic or intermetallic materials. Common examples of metal matrix composites are Cu-Al2O3, SiCw-Al, Al-Al2O3, Al-B4C, and Ni-NiAl3. Mechanical or thermal properties, such as strain-stress behavior, or thermal expansion coefficient can be tailored by changing the content of the reinforcing phase. The most common techniques of measuring mechanical properties of composite materials rely on macroscopic approach. During the past fifteen years, a significant effort has been made to develop various techniques of measuring mechanical properties on a microscopic level. These techniques include atomic force microscope (AFM) and depth sensing indentation techniques, based on Hertzian contact mechanics. However, it is still a challenge to measure reliably and quantitatively the Young's modulus and Poisson's ratio of individual phases as well as properties at the interfaces. This presentation will focus on fundamental aspects of measuring of mechanical properties of metal matrix composites at nano-scale using Interfacial Force Microscopy (IFM). The IFM is a scanning probe microscope, which utilizes a unique self-balancing capacitance force sensor. Force-displacement curves obtained with the IFM are analyzed using Hertzian contact mechanics to extract the Young's moduli of the individual phases and interface region with high spatial resolution. The properties of Cu-Al2O3, Al-SiCp composites will be discussed in detail. Furthermore, a comparison of experimental data with mechanical properties calculated from first principles will be discussed.

Quantitative nanoscale mechanical properties of a phase segregated homopolymer surface

Crystallization of poly(ethylene terephthalate) (PET) is accompanied by significant changes in surface topography, easily detected by atomic force microscopy (AFM). Phase imaging by AFM qualitatively indicates contrast in mechanical properties of nanometer scale areas of an annealed PET surface, but cannot provide quantitative data. Using interfacial force microscopy (IFM), we have, for the first time, made quantitative measurements of the elastic moduli of such nm-scale areas on a homopolymer surface. Values of 2.2 GPa, 4.3 GPa, and 11.8 GPa, were found, respectively, for amorphous PET and for phase segregated regions on the surface of an annealed homopolymer PET sample. The method is applicable to any phase segregated surface with nm-sized domains of differing elastic moduli.