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Title: Adhesion, friction & mechanics of nanoscale contacts between coated surfaces studied by atomic & friction force microscopy
Author: Nikogeorgos, Nikolaos
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2011
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Friction force microscopy was employed for the tribological investigation of human head hair in: a dry atmosphere and in de-ionized water. The effects of bleaching, conditioning, and immersion in methanolic KOH were quantified using the relative coefficient of friction (u). The coefficient of friction in both environments increased in the order: virgin < virgin-conditioned < damaged-conditioned < bleached < KOH-damaged hair. All categories of hair exhibited higher friction coefficients in the aqueous environment, attributed mainly to the higher elastic compliance of the fibres due to water absorption. Secondary ion mass spectroscopy was used as a complementary technique to examine the presence 0 f fatty acids on the cuticular surface 0 r the different categories of hair as well as the conditioner distribution. Conditioner species were detected along the whole cuticular surface. The combination of SIMS with FFM data suggests that the main reason for the different tribological properties of the categories of hair examined is the altered chemistry of the surface, i.e., partial removal of the covalently bound lubricant layer of 18-MEA and, also, of the unbound fatty acids (stearic, palmitic, myristic, etc.) by the damaging agents. Atomic and friction force microscopy was employed to investigate friction and adhesion between polar and between non-polar self-assembled mono layers in pure solvents as well as in heptane/acetone mixtures of varying polarity. The two polar interfaces examined were l l-rnercapto-l-undecanol vs l l-rnercapto-l-undecanol (MUT), and II-mercapto-l- undecanol vs Oiethoxy-Phosphatoethyl-Triethoxysilane (OPTS), while the non-polar interface examined was l-dodecanethiol vs l-dodecanethiol (000). For the MUT and OPTS interfaces the pull-off forces were found to decrease with increasing static dielectric constant of the medium (e), while for the 000 interface the opposite trend was observed. A simple model based on functional group H-bond parameters was found adequate to describe the adhesive interactions for the MUT and OPTS interfaces in terms of the degree of so lvation of the functional groups of the mono layers by the medium. In the heptane/acetone mixtures the pull-off force was observed to correlate excellently to the free energy of interaction as predicted by the model. For the 000 interface an approximation of the Lifshitz theory predicted satisfactorily the pull-off forces in media of weak hydrogen bond donor ability. The friction-load relationship at the 'wearless' regime (L < 20 nN) was found to be dependent on the strength of adhesion as well as to the molecular properties of the medium. Systems of lowest adhesion obeyed Arnonton's law, while as adhesion increased their offset displaced towards tensile loads and their slope increased, but strongly adhering systems provided a sublinear Friction - Load relation best fitted by the Derjaguin-Muller-Toporov (DMT) model of contact mechanics. Exceptionally, in n-octanol a two-sloped linear relation was observed for all interfaces, attributed to its ability to strongly physisorb on them, decreasing the friction coefficient considerably and eliminating the frictional differences of the three interfaces at loads lower than about 3 nN. These observations were rationalized by friction being considered to be the sum of an interfacial and a plowing term. The Fr-L data for all interfaces were successfully modeled, under the assumption that DMT contact mechanics is obeyed, with an adhesion-independent friction coefficient due to plowing, and an adhesion-dependent shear strength due to interfacial friction. For all interfaces the shear strength was found to increase as the pull-off force increased, thus explaining Amontori's law as the limit of zero shear strength at very weakly adhesive systems. For the MUT and OPTS interfaces in the mixtures, the shear strength was found to correlate closely to the free energy of interaction predicted by the H-bonding model used. The OPTS interface exhibited lower adhesion but higher friction than the MUT one, while the DOD interface exhibited the lowest friction. Analysis with the model used resulted in very close friction coefficients for the DOD and MUT films, but considerably higher one for the OPTS films. The shear strength of the MUT and DPTS was similar, but that of the DOD monolayer was clearly lower. These findings demonstrate the importance of packing effects and hydrogen bonding to friction.
Supervisor: Leggett, Graham J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available