The thermal etching of silver

The mechanism whereby grain boundaries are delineated and striations formed on polished surfaces of heated metal specimens has been examined. Experiments on electrolytically polished silver show that grooves form at grain boundaries at temperatures as low as 300° C and striations at 500° C in air. Striations only appear in the presence of oxygen and may be removed by heating in nitrogen. A furnace for high-temperature photomicrography, suitable for specimen temperatures up to about 950° C, is described. Previous theories are found inadequate to explain the effects observed in silver, and a theory which regards the surface etching as an approach to equilibrium by the reduction of surface free energy is suggested. Thus the equilibrium condition of the boundary is a groove whose shape is determined by the relative magnitudes of the free energy per unit area of the boundary and the surface energies of the crystalline faces meeting the boundary in the surface of the specimen. The striations are caused by the development of those crystalline planes having the lowest free energy; the relative surface energies of different planes being modified by the presence of oxygen. It is suggested that the chief means whereby the boundary grooves are formed is that of surface migration of ions: both surface migration of ions and evaporation of silver oxide molecules may be expected to play major parts in the formation of the striations.