Abstract
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.Graphical
Highlights
Zinc dialkyldithiophosphates (ZDDPs) have been used as antiwear additives for more than 80 years, they remain one of the most critical ingredients in engine lubricants [1]
zinc dialkyldithiophosphates (ZDDPs) needs to decompose into its zinc phosphate-rich active products (ZnPs) [16,17,18] and the remaining sulphur as well as alkyl and aryl groups, whose content in tribo-films tends to be relatively small [18, 19], sulphur may certainly be critical to bind the films to metal surfaces [20, 21]
In this work we studied how a system built of ZDDP model decomposition products—two triphosphate molecules and two zinc phosphate molecules—reacts to different deformations, which included one isotropic and two uniaxial compressions, one of which conserved density, while the other kept all strain-tensor elements constant except for one diagonal tensor element
Summary
Zinc dialkyldithiophosphates (ZDDPs) have been used as antiwear additives for more than 80 years, they remain one of the most critical ingredients in engine lubricants [1]. In this work we want to explore how stress anisotropy, i.e., the presence of shear stress, affects the hydrostatic pressures needed to promote the formation of stiff, simple ZnP networks under the assumption that the ZDDP decomposition had already taken place and an initial soft film was formed. This includes an analysis of how shear stress affects the structure and elastic properties of the stress-modified “films”.
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