SURFACE-ENERGY AND STRUCTURE EFFECTS ON SURFACE CRYSTALLIZATION

The shape of a critical cluster formed at a planar surface of a solid was determined taking into account both interfacial contributions and elastic terms in the expression for the free enthalpy of cluster formation. The shape of critical clusters may change continuously from a sphere in the ambient solid tangent to the interface via a hemisphere to a circular nearly planar layer depending on the values of the specific surface energies (cluster-air, ambient phase-air and ambient phase-cluster phase). The work of formation of critical clusters formed at or near planar interfaces is significantly reduced compared with cluster formation in the bulk of the matrix. This reduction is even more significant for solids with a rough surface structure. As an application of the outlined theoretical analysis, the preferential surface crystallization of glasses, often observed experimentally, can be explained. For isoconcentration crystallization of glass-forming melts, nucleation catalysis on surfaces is caused mainly by the relative decrease of the elastic strains for crystallization at the surface compared with the bulk both for planar and rough surfaces.