ACS PRF | ACS
All e-Annual Reports
42787-G5
Effect of Organic Liquids on the Mechanics and Geometry of Nanoasperity Contacts
This project is looking at how confinement of organic molecules at the edge of a single nanoasperity contact modifies the contact's geometry and mechanical properties. The basic idea is to use nanoindentation to examine model sample materials with organic molecules on their surfaces. 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.
Using SAMs (Self-Assembled Monolayers), which have a very uniform thickness and well defined structure, the role of sub-nanometer thickness organic films in determining mechanical properties has been investigated in a systematic manner. Gold, Au (111), substrates were used. These were grown by evaporation on to flat glass substrates. High purity Au pellets (99.99 % pure) were used. The SAM was grown using 1-Decanethiol (96%) CH3(CH2)9-SH by immersion in 2mM solutions of ethanolic alkylthiols for 19 hours. Excess SAM and ethanol is then washed away to give the final SAM on Au (111) on glass test samples.
The mechanical testing was performed using a Hysitron Triboindenter which is able to perform indents with a maximum indent depth of just a few nanometers. The nanoindentation testing gives a series of data points for the indentation depth and load as a function of time since the start of the nanoindentation. This data can then be analyzed using standard routines to find the hardness, H (sometimes called nanohardness), and the elastic properties in terms of the reduced elastic modulus, Er. The equations used to find these are:
where A is the contact area of the nanoindentation which is related by a polynomial to the plastic depth of the indentation. P is the load, h the depth and S the contact stiffness at the peak load (Pmax).
Arrays of nanoindentations were performed on glass without any surface films, on glass with an Au (111) film and on the SAM-Au-glass combination. The thickness of the gold layer was varied in the 25-50 nm range (checked with ellipsometry), and the method of loading during the nanoindentations was varied. That is different loading rates and different peak loads were used.
The mechanical properties obtained for the thin film systems showed considerable variability with a strong dependence of the measured hardness and reduced elastic modulus on indent depth (displacement). A lesser dependence was also seen on indent rate. Some of the data is shown below.
The results for glass are largely as expected. For Au films the results show increasing substrate effects at large indent displacements. The unexpected results are those for the SAM and Au film when compared to those of just the Au film on its own. Remarkably, even though the SAM layer is <1 nm thick (measured with an ellipsometer) it significantly lowers the reduced elastic modulus (Er) at indent displacements ≥ 50nm. There is no effect on the hardness (H).
One of the most common origins of errors in nanoindentation testing is the incorrect calculation of the contact area, A. Since contact area is used in the analysis of both H and Er it is surprising that only Er shows an effect. This suggests that it is not the contact area that is causing the change in Er. Looking at the equations used in the analysis the only other variable in the equation used to find Er is contact stiffness, S, which is the inverse of the contact compliance.
In order to change the contact compliance the SAM layer in the highly confined geometry at the edge of the contact must be sustaining elastic strains. This is not entirely unexpected since previous simulations using molecular dynamics have shown SAMs with 11 carbons in the chain to significantly modify the repulsive forces acting on sharp AFM tips. Interestingly earlier studies of organic molecules that are not adsorbed to the surface of materials have shown increases in both Er and H. This suggests that the change in the molecules orientation as a result of binding to the surface modifies the compliance of the organic layer.
