Tribological properties of bridged cyclotriphosphazene as lubricating oil additive

The bridged Cyclotriphosphazene (denoted L-2P) was synthesized. The chemical structure of L-2P was characterized by ³¹P-NMR and FT-IR. The effect of tetrakis (3-trifluoromethylphenoxy)-bis(4-fluorophenoxy)-Cyclotriphosphazene (X-1P) and a novel synthetic bridged Cyclotriphosphazene as additives in pentaerythritol ester (PET) on the friction and wear behavior of a steel-on-steel system was investigated. The friction-reducing and antiwear behavior of PET containing L-2P to a steel-on-steel frictional pair was comparatively evaluated with that of PET containing X-1P to the same pair, using an Optimol SRV oscillating friction and wear tester in a ball-on-disc configuration. The action mechanism of the lubricants was explored based on the observation of the morphologies and elemental distributions of the worn steel surfaces with scanning electron microscopy equipped with energy dispersive spectroscopy attachment and the analysis of the chemical states of the worn surfaces by X-ray photoelectron spectroscopy. The results indicate that both additives are effective in improving the tribological performance of PET. In particular, L-2P as an additive in PET shows the best anti-wear ability and the highest load-carrying. Such a superiority of the L-2P in PET was more prominent at increased temperature and load. Tribochemical reaction was involved in the steel-against sliding lubricated with the bridged Cyclotriphosphazene, with the generation of a surface-protective layer composed of inorganic fluoride FeF₂, which together with the adsorbed film of the additive contributed to improve the friction-reducing and antiwear behavior of the bridged Cyclotriphosphazene as additive.