Stress Anisotropy Severely Affects Zinc Phosphate Network Formation

Using density-functional theory based simulations, we study how initially disconnected zinc phosphate molecules respond to different externally imposed deformations. Hybridization changes are observed in all cases, in which the coordination of zinc atoms changes irreversibly from tetrahedral to seesaw and square pyramidal, whereby the system stiffens substantially. The point at which stiff networks are formed does not only depend on the hydrostatic pressure. Stress anisotropy generally reduces the required hydrostatic network formation pressure. Moreover, networks obtained under isotropic deformations turn out stiffer, elastically more isotropic, and lower in energy after decompression than those produced under anisotropic stresses. We also find that the observed stress-memory effects are encoded to a significant degree in the arrangement of atoms in the second neighbor shell of the zinc atoms. These findings refine previously formulated conjectures of pressure-assisted cross-linking in zinc phosphate-based anti-wear films.

Automated summary

Through density-functional theory simulations, it was found that initially disconnected zinc phosphate molecules undergo hybridization changes in response to externally imposed deformations, resulting in the formation of rigid networks. Stress anisotropy reduces the pressure required for network formation, while networks formed under isotropic deformations are stiffer, more isotropic, and lower in energy after decompression. Stress-memory effects were found to be encoded in the arrangement of atoms in the second neighbor shell of the zinc atoms.