Effects and measurement of internal surface stresses in materials with ultrafine microstructures

Variations in lattice parameters for materials that form very thin lamellae are analyzed using a thermodynamic model that incorporates surface stress effects. It is predicted that lattice spacing variations should be proportional to the reciprocal of the lamella thickness, in agreement with experimental data for polyethylene, n-paraffins, and a copolymer of tetrafluoroethylene and hexafluoropropylene. The model is then used to calculate the surface stress associated with lamella interfaces in these crystalline materials. The calculated surface stress has the same order of magnitude as a surface tension, but is negative. The model is extended so that surface stresses associated with grain boundaries can be measured in very fine-grained metals and ceramics.