Nanoindentation of thin organic films and self-assembled monolayers
It has been established for some time now that interfaces between polymers and stiff substrates contain an interphase region (10 nm thick) where the properties of the polymer differ from those of the bulk. It has also been observed that interfacial cracks actually grow in these interphase regions. Thus it is important to understand the mechanical behavior of materials in such thin materials. Self-assembled monolayers (SAMs) provide well-controlled models for such studies. They are also important materials in MEMS devices where they are used for reducing friction and stiction. The SAM considered in this study was octadecyltrichlorosilane (OTS), which has a chain length of 2.6 nm. The OTS was probed with an interfacial force microscope (IFM). This relatively new device provides unambiguous force-displacement profiles in tension (adhesion) and compression (indentation). The IFM was also used to probe thicker films of (3-aminopropyl)triethoxysilane (γ-APS) so that a range of thickness could be considered. Thin γ-APS films and self-assembled OTS monolayers were deposited on Si(100) surfaces and characterized. A new approach for making OTS monolayers with nano-scale uniformity was developed. Some guidelines for the analysis of IFM experiments were drawn from a parametric study of layer and substrate interactions. Its high resolution in both force and displacement allows the IFM to be used to determine both the elastic mechanical and adhesive properties of thin films. Due to the different molecular structure of thin γ-APS films and selfassembled OTS monolayers, different analytical approaches were required. Combined experimental and continuum analyses of IFM nanoindentation of thin films, which accounted for the layer/substrate and adhesive interactions, were developed. A continuum analysis was used for γ-APS films, while a hybrid continuum-molecular analysis was required for OTS monolayers.