Surface melting of copper with (100), (110), and (111) orientations in terms of molecular dynamics simulation

Surface melting of copper having the (100), (110), and (111) orientations has been investigated with the use of molecular dynamics (MD) simulation. The interaction between copper atoms was expressed by the approximation of second-moment tight-binding scheme potential. The structures of copper were determined at temperatures between 500 and 1390 K by constanttemperature MD, where calculation was conducted on bulk and surface models of copper having the (100), (110), and (111) orientations. The position and velocity of atoms calculated led to the internal energy, the number density of atoms, and the mean square amplitude of thermal vibrations of atoms. The (110), (100), and (111) surface models melted at temperatures of about 1270, 1290, and 1310 K, respectively; these temperatures are lower than the melting point of copper. The surface internal energies for the (110), (100), and (111) surface models, derived as the difference between the internal energies for the bulk and surface models having the same plane orientation, displayed steep increases at temperatures of about 1100, 1200, and 1300 K, respectively. In addition, the distribution of the number density of atoms in the direction normal to the surface indicated the presence of a structurally disordered layer near the surface of each surface model. Lindemann's law on melting has suggested that surface melting occurs in the surface models at temperatures lower than the melting point of copper, from the profiles of the mean square amplitude of atomic vibrations. The surface melting temperatures have been determined as 800 K for (110), 1000 K for (100), and 1300 K for (111). It has also been concluded that the steep increase in the surface internal energies originates from the formation of the surface-melting layer. The dependence of the surface-melting temperature on the plane orientation would be dominated by the ease with which atoms can move in the plane or the number density of atoms in the plane.