Year

2020

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

Inorganic sodium phosphate glass has been proposed as a potential lubricant in hot rolling processes. Previous experimental works have demonstrated the outstanding tribological performance of this glass lubricant under the extreme condition of temperature, load and shear. The lubricity mechanism of sodium phosphate glass has also been revealed in the laboratory tests. However, the detailed picture of the mechano-tribochemical reaction of sodium phosphate glass lubricant on iron oxide surfaces is still not complete due to a number of missing pieces, for example, how the role of elements/compounds in reducing friction, lubrication and wear. This thesis applies various theoretical methods to unveil the tribochemical behavior of different sodium phosphate compounds with iron-based interfaces at the atomistic scale and the resulting lubrication effect of this inorganic glass. Firstly, the bond nature of the system, effect of surface and effect of chain length on depolymerization of phosphate-based lubricant have been analyzed with density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations at 1100K. In general, the tribo-system contains medium covalent P-O bond, pure ionic Na-O interaction and moderate Fe-O ionic/covalent bond. The bridging P-Ob is the weakest bond targeted for the depolymerization which is induced by sodium - bridging oxygen interaction. The iron oxide surface plays a dual role in promoting/inhibiting the phosphate depolymerization. On one hand, the phosphate adsorption of oxide surface generates stable configurations and the partial anchoring of phosphate chain on substrates supports P-Ob depolymerization due to the effect of temperature. On the other hand, the oxide surface captures sodium cation which reduces the sodium attack on P-Ob bond and obstructs the depolymerization. The monodentate complex (one atom such as O, P linked to one atom of Fe) structures is dominant in all adsorption cases regardless of the chain length. The chain length of phosphate has little effects on the P-O bridging dissociation. The monodentate structures create the deformed FeO4 tetrahedra on the oxide surface which allow the flexibility of phosphate network but still maintains the strong adherence lubricant film. Moreover, the small chain length structure is preferable at the lubricant-surface interfaces for the full phosphate coverage and all linkages are monodentate, which are related to the good lubricity of short-chain Na3PO4 and the depolymerized products of long-chain NaPO3 in some experimental studies.

Secondly, DFT calculations and AIMD simulations are also used to investigate the surface transformation and interactions of iron oxide in glassy lubricant. Among three main interlayer interactions between phosphate networks with iron oxide, Fe-Oglass is the most stable linkage which can weaken the outermost Fe-O layer of oxide surface. Osurface-P interaction is observed under high load conditions and Fe-P direct bond occurs under severe conditions of high temperature, with an exposure of considerable number of iron atoms and a presence of under-coordinated phosphorus atoms. The Fe-P linkage can strengthen the Fe-O bonds of the iron oxide surface but has low probability to form in the system. Sodium cations in the glass network also reduce the Fe-Osurface stability through generated O-terminated iron oxide surfaces. The iron oxide structure deformation can occur at normal temperatures with an excess concentration of sodium. In addition to thermal and mechanical factors, phosphate glass itself has a combined chemical/electronical effect on the deformation of the iron oxide surface, which supports the abrasive particle digestion theory in anti-wear mechanism of phosphate lubricant.

Last but not least, a comprehensive reactive force field (ReaxFF) has been developed for sodium phosphate/iron oxide system using a robust genetic algorithm (GA) and a consistent reference data from quantum mechanics (QM) calculation. This force field shows a significant improvement in the prediction of the heat of formation, mechanical properties, lattice constants, bulk modulus, and density of Fe, Na, and P, as well as their binary oxides compared to previous ReaxFFs. Additionally, the new parameters of ternary and quaternary oxides of NaxPyOz, FexPyOz, and NaxFeyPzOn were also developed and validated against QM calculation at static and elevated-temperature dynamic conditions. This new ReaxFF not only predicts well the crystalline properties of these oxides, but it also predicts the most stable configuration and the order of energies of the intermediate states. The application of the new developed ReaxFF for the system of Na4P2O7 lubricant confined between Fe2O3(0001) surface reveals a hierarchical tribochemical layers in which a sodium layer was formed at lubricant-surface interface to improve the system tribological performance which is in agreement with previous experimental work.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.