Nanoindentation studies of the chemomechanical effect in sapphire
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For the first time, nanoindentation techniques have been used to study chemomechanical effects due to water, alcohols and myristic acid on the detailed surface mechanical response (e.g. elastic flexure, stiffness, dislocation nucleation and plasticity) of (10–12) single-crystal sapphire. Because the magnitude of surface tension forces at these small scales can be comparable with the applied load, the experiments have been performed on samples annealed, quenched into various environments and tested after the liquid had been allowed to evaporate. At low loads and indentor displacements (i.e. ⩽ 60 nm), two separate effects have been identified: the occurrence of a soft surface layer together with significant differences in the elastic-plastic deformation transition. The experiments were repeated after a 4 week exposure to ambient laboratory atmosphere and in each case, the behaviour had moved towards that displayed by the original water-quenched sample. The results are explained in terms of physisorption and chemisorption on dislocation nucleation behaviour and subsequent plasticity. At higher loads, the results showed that the previously conflicting evidence from microhardness testing concerning these effects is not surprising, because no differences in the load-displacement responses were seen. However, scanning electron microscopy of the indentations showed significant differences in the deformation behaviour. Low-load creep experiments were also performed but, in contrast to reported microhardness responses, showed no real evidence for time-dependent deformation behaviour. The results presented clearly show the advantages and potential of nanoindentation techniques for investigating these effects.
KeywordsSapphire Myristic Acid Surface Tension Force Creep Experiment Dislocation Nucleation
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