1. Adrian B Mann, Department of Materials Science & Engineering
Effect of Organic Liquids on the Mechanics and Geometry of Nanoasperity Contacts
Adrian B Mann, Department of Materials Science & Engineering
Very thin, organic surface layers play a vital role in many tribological processes, but how they modify the perceived mechanical properties of the underlying surface is poorly understood. In this study the effects of organic molecules (self-assembled monolayers, SAMs) on the mechanics of nanoasperity contacts has been investigated. The surfaces examined consist of clean glass, glass coated in gold (Au) and Au covered by a SAM. The mechanical tests were performed using nanoindentation methods. It was expected that the SAM would modify the contact geometry and this would give a perceived change in the hardness (H) and reduced elastic modulus (Er) of the underlying Au layer.
Compression of a SAM under a nanoasperity contact
The results obtained for glass were largely as expected with both H and Er being more or less constant with nanoindentation displacement. For Au the results show increasing substrate effects at large indent displacements. The unexpected results are those for the SAM and Au when compared to those of just the Au on its own. Remarkably, even though the SAM layer is <1 nm thick (measured with an ellipsometer) it significantly lowers Er at indent displacements ≥ 50nm. This is all the more surprising given that the SAM seems to have no effect on H. This indicates that the SAM cannot be merely altering the contact geometry, as expected, since both H and Er depend on the contact area. Rather the results indicate that the SAM actually lowers the elastic compliance of the contact. This maybe due to the SAM molecules being forced to lie at an angle other than their preferred 30° to the normal tilt. Er
(GPa)
maximum displacement (nm) 20 40 60 80
100
120 50
150
200
Glass
Reduced Elastic Modulus Au
Au &
SAM