Abstract
By introducing finite size surface and interfacial excess quantities, interactions between interfaces are shown to modify the usual surface premelting phenomenon. It is the case of surface melting of a thin solid film s deposited on a planar solid substrate S. More precisely to the usual wetting condition of the solid s by its own melt l, necessary for premelting (wetting factor Φ<0), is adjoined a new quantity Γ describing the interactions of the l/s interface with the s/S interface. When Γ>0 this interface attraction boosts the premelting so that a two stage boosted surface premelting is foreseen: a continuous premelting, up to roughly half the deposited film, is followed by an abrupt first order premelting. When Γ<0 these interfaces repell each other so that premelting is refrained and the film remains partly solid above the bulk melting point (overheating) what is called astride melting. Elastic stress modifies both types of melting curves. Bulk and surface stresses have to be distinguished. For coherent epitaxial layers the natural misfit determining the strain and the elastic energy density (independent of the thickness of the solid) only shifts the melting curves to lower temperature, up to thicknesses where strain relief happens. Surface stress, as a finite size surface excess quantity, modifies the wetting factor Φ and the coefficient Γ, therefore the wetting properties and thus the melting curves are slightly modified. For perfect glissile epitaxies things are more complex since bulk strain and elastic energy density (now induced by surface stress) varies with the film thickness. The melting curves are thus distorted on their initial part (either in the sense of assisted or refrained premelting) depending upon the set of interfacial stresses. Lastly there is a z-inhomogeneity of stress due to the interactions between the bulk of the various material layers. This leads to measurable strain gradients in the film but only distorts the final part of the melting curve. Some of these theoretical results have been experimentally illustrated in the Γ<0 case where then useful interfacial data, adhesion energies and interfacial stress data have been collected but the Γ>0 case remains fully open to future exploration.
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