Comparative tribological behavior of phosphazene derivatives with different fluorine substituents in three base oils

Abstract

In this work, three phosphazene derivatives (HCCP-CH₃, HCCP-CF₃ and HCCP-OCF₃) were synthesized and lubrication mechanisms characterized. The tribological behaviors as additives was evaluated in vegetable oil (JVBO), pentaerythritol tetraoleate (PETO-4) and coal-to-liquid (CTL) by using a four-ball friction tester under a wide range of concentrations and loads, which indicates that all three phosphazene derivatives have excellent load-carrying capacity, anti-wear, and friction-reducing properties. The characterizing results for the tribofilm show that the fluorinated phosphonitrile derivatives significantly enhanced the tribological performance, among which HCCP-OCF3 exhibits the most remarkable friction-reducing and anti-wear properties. The thermogravimetric analysis (TGA) results show that HCCP-OCF₃ owns the higher thermal stability for among these three structures. The formation mechanism was an initial iron oxide layer forms under thermal and mechanical stress, followed by a compact Fe–F/Fe–O–P layer serving as an extreme-pressure barrier, and a Fe–N layer developing near the steel surface due to nitrogen diffusion. The results demonstrate that the substitution structure of phosphazene derivatives significantly influences tribofilm formation, friction behavior, and extreme-pressure performance, revealing their feasibility as integrated lubricating and flame-retardant additives for advanced lubrication systems.