Depolymerization of sodium polyphosphates on an iron oxide surface at high temperature

Abstract

Density functional theory (DFT) and first principles molecular dynamics (FPMD) studies of pyrophosphate cluster NaPO and triphosphate cluster NaPO absorbed and decomposed on an FeO(0001) surface have been conducted. Comparative analyses of the structure properties and adsorption processes during the simulation at elevated temperature have been carried out. The results depict the key interactions including the covalent P-O bonds, pure ionic Na-O or Fe-O interactions. The iron oxide surface plays an important role in the bridging bond decomposition scheme which can both promote and suppress phosphate depolymerization. It is found that the chain length of polyphosphates does not have considerable effects on the decomposition of phosphate clusters. This study provides detailed insights into the interaction of a phosphate cluster on an iron oxide surface at high temperature, and in particular the depolymerization/polymerization of an inorganic phosphate glass lubricant, which has an important behavior under hot metal forming conditions.

Tribochemistry and Lubrication of Alkaline Glass Lubricants in Hot Steel Manufacturing

“…The chain length also has an influence on the depolymerization behavior. In fact, Na 4 P 2 O 7 indicated no bond breaking for the P-O b bond at 1100 K, while there was a coexistence of bond breaking and reforming during simulation for Na 5 P 3 O 10 on the iron oxide surface [32]. This simultaneous depolymerization and polymerization of phosphate chains from AIMD simulations affirmed the previous XPS experimental characterizations [1].…”

“…Unlike organic phosphite/phosphate additives used in automotive applications, where the direct Fe-P bonding was found on iron surfaces because of the chemical dissociation of phosphite/phosphate under friction, leading to the formation of Fe 2 P [39,40], the AIMD simulations in Figure 4 revealed that the cross-linking with the Fe-O-P bond between phosphate and the iron substrate was the main chemical structure in phosphate glass [31,32]. The Fe-P bond only occurred in a combination of an elevated temperature of 1500 K, a high exposure of Fe, and an appearance of under-coordinated P [33].…”

“…It is important to investigate the chemical mechanism of how a glassy lubricant works when it is applied in practice at high temperature and pressure in metal forming. We have carried out some theoretical investigations of the adsorption and thin film lubrication of Na4P2O7 and Na5P3O10 [31][32][33][34], sodium tetraborate (Na2B4O7), and boron oxide (B2O3) molecules [25,35,36], as well as sodium…”

“…The pure ionic Na-O bond, the moderate covalent P-O interaction, and the mixed ionic/covalent Fe-O bond exist inside the tribological system [32]. Among these bonds, the P-O b (bridging O) bond has the highest chance to dissociate because it is the weakest, which results in depolymerization when the phosphate lubricant contacts the iron and iron oxide surfaces [31,32].…”

Understanding the tribological impacts of alkali element on lubrication of binary borate melt

“…The depolymerization reaction actively accommodates abrasive particles upon which the anti-wear capacity thrives. [13][14][15] On steel pair lubricated by borate, the boron oxides highly concentrates in the form of a compact and robust boundary lm to withstand direct asperity contacts. 7 Although the tribological behaviors of network-forming constituents have been understood in some extent, there remain unanswered questions surrounding the roles of alkali elements in melt lubrication.…”