Mechanisms of surface faceting and coarsening

The progression of surface faceting on ceramic substrates is explored using the {<10(1)over bar 0>} and (0001) surfaces of Al2O3 (alpha-Al2O3, corundum structure) as model systems. Atomic-forte microscopy and electron microscopy techniques are used to monitor the progression of faceting as a function of annealing time. Whether the surface facers into a hill-and-valley structure, such as the {<10(1)over bar 0>} surface, or into a terrace-and-step structure, such as the (0001) surface, faceting starts with the nucleation and growth of individual facets. The growth of individual facets is found to promote the nucleation of adjacent facets, and thus leads to the formation of facet domains. As domains that are out of phase with each other coalesce regions with a high density of facet junctions are formed. Facet junctions, points where two facets merge to form one, are found to have an important role in facet coarsening. The mechanism for the coarsening of the completely faceted surface involves the motion and elimination of facet junctions. Both the facet wavelength and density of facet junctions are monitored as a function of annealing time for the {<10(1)over bar 0>} and (0001) Al2O3 surfaces. (C) 1997 Elsevier Science B.V.