Cleavage force, tribology and bond breaking in some transition metals

The idealised cleavage force F(z) as a function of interplanar separation z for Fe, Ni and Cu has been calculated using many-body interatomic potentials. The separation z, measured from the equilibrium cleavage planar separation a, at the maximum F(z)max is taken as a “criterion for bond breaking” and zmax is found to be ∼0.2a for these metals. The results are also compared with earlier studies on covalently bonded Si and it is suggested that even in these metallic systems, cleavage may be associated with the breaking of bonds. The position of zmax is found to depend on the number of broken bonds at the cleavage plane. From the results on Cu, it would appear that the main feature of F(z) is not very dependent on the form of the potential. For fixed cleavage separations, 0max, energy curves are presented for the low velocity rubbing of two halves of a cleaved crystal in a parallel configuration, for non-defective solids. Bond breaking and bond bending aspects are discussed. It is concluded that bond charge or bond order force fields may be appropriate for describing the idealised tribology of the transition metals. The model presented should be viewed as a precursor, based on the assumption of ideal materials, to more realistic calculations of sliding adopting, for example, a dislocation mechanism