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    Strength by atomic force microscopy (AFM): Molecular dynamics of water layer squeezing on magnesium oxide

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    Authors
    Kendall, K.
    Dhir, Aman
    Yong, Chin W.
    Issue Date
    2009-11
    
    Metadata
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    Abstract
    Localised strength testing of materials is often carried out in an atomic force microscope (AFM), as foreseen by Kelly in his book Strong Solids (Clarendon Press, Oxford, 1966). During AFM indentation experiments, contamination can strongly influence the observed strength and theoretical interpretation of the results is a major problem. Here, we use molecular dynamics computer modelling to describe the contact of NaCl and MgO crystal probes onto surfaces, comparable to an AFM experiment. Clean NaCl gave elastic, brittle behaviour in contact simulations at 300 K, whereas MgO was more plastic, leading to increased toughness. This paper also considers the strength of an oxide substrate contaminated by water molecules and tested by indentation with a pyramidal probe of oxide crystal. Recent theory on the effect of liquid contaminant layers on surface strength has been mainly focussed on Lennard Jones (LJ) molecules with some studies on alcohols and water, described by molecular dynamics, which allows the molecules to be squeezed out as the crystal lattice is deformed. In this work, we have focused on water by studying the forces between a magnesium oxide (MgO) atomic force microscope (AFM) probe and an MgO slab. Force versus separation has been plotted as the AFM probe was moved towards and away from the substrate. Simulation results showed that the water layers could be removed in steps, giving up to four force peaks. The last monolayer of water could not be squeezed out, even at pressures where MgO deformed plastically. Interestingly, with water present, strength was reduced, but more in tensile than compressive measurements. In conclusion, water contaminating the oxide surface in AFM strength testing is structured. Water layer squeezing removal can be predicted by molecular modelling, which may be verified by AFM experiments to show that water can influence the strength of perfect crystals at the nanometre scale.
    Citation
    Strength by atomic force microscopy (AFM): Molecular dynamics of water layer squeezing on magnesium oxide 2010, 90 (31-32):4117 Philosophical Magazine
    Publisher
    Taylor and Francis
    Journal
    Philosophical Magazine
    URI
    http://hdl.handle.net/2436/621458
    DOI
    10.1080/14786430903520732
    Additional Links
    http://www.tandfonline.com/doi/abs/10.1080/14786430903520732
    Type
    Journal article
    Language
    en
    ISSN
    1478-6435
    1478-6443
    ae974a485f413a2113503eed53cd6c53
    10.1080/14786430903520732
    Scopus Count
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    Faculty of Science and Engineering

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