1. Crystallographic and geometric approach to a surface

If we consider a crystalline material, i.e. one with a periodic arrangement of atoms, the surface introduces a discontinuity into it. In the case of a centered cubic structure (that of ferritic steel, for example), if we "cut" the crystal by a {100} plane passing through the vertex common to eight adjacent cubes, we find that the N _c coordination number (number of nearest neighbors which, in our case, are at the center of these eight cubes) falls from eight to four (figure 1 ). To re-establish the equilibrium of the force fields to which they are subjected, the atoms of the newly-created surface will tend to modify the bonds with their nearest neighbors, either on the...