Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions—Part 1: Structure and chemistry of tribofilms

Abstract

Tribofilms generated under boundary conditions at extreme pressure using lubricants containing fluorinated zinc dialkyl dithiophosphate (F-ZDDP) and zinc dialkyl dithiophosphates (ZDDP) were examined. F-ZDDP demonstrated superior wear performance compared to ZDDP even at lower phosphorus levels. The tribofilms generated by ZDDP and fluorinated ZDDP were analyzed using scanning electron microscopy, Auger electron spectroscopy and XPS. Transmission electron microscopy was conducted on wear debris. XANES spectroscopy was used to study the chemical and bonding environment of sulfur and phosphorus in these tribofilms. A phenomenological model of the tribofilms generated by ZDDP and fluorinated ZDDP was developed based on the experimental results.

Introduction

Zinc dialkyl dithiophosphates (ZDDP) are still the primary anti-wear agent, as well as an important anti-oxidant, in conventional engine oils and are the principal source of phosphorus. The phosphorus in engine oils is known to cause poisoning [1] and failure of catalytic converters as part of the car exhaust system, hence environmental regulations around the world in recent years have pushed for reductions in the phosphorus level and consequently the amount of ZDDP in engine oils.

Under shearing forces and high temperatures due to tribological conditions in internal combustion engines, the ZDDP present in oils breaks down and forms a glassy anti-wear film of polyphosphates and sulfides of iron (from engine surfaces) and zinc (from ZDDP) by a thermo-oxidative process [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].

The presence of over-based additives such as detergents, anti-oxidants, dispersants, and friction modifiers and their interactions with ZDDP have been shown to be detrimental to anti-wear properties of ZDDP [9], [12], [13], [14]. Limitations have also been observed in the anti-wear performance of ZDDP when present in base oils with no other additives [15] thus making it difficult to reduce the amount of ZDDP in engine oils.

One way to reduce the amount of phosphorus in engine oils without compromising the anti-wear performance of these oils is to replace ZDDP with a more effective anti-wear agent. This can be achieved by chemical modification of ZDDP molecules through chemical reactions with other compounds. ZDDP was observed to yield improved anti-wear performance in the presence of iron (III) fluoride (FeF₃) present as dispersed powder in oil solution [16]. Further investigations using Nuclear Magnetic Resonance Spectroscopy (³¹P and ¹⁹F) revealed that interactions between ZDDP and FeF₃ resulted in fluorination of ZDDP [17]. Preliminary wear tests using the fluorinated ZDDP showed improved anti-wear performance by the fluorinated ZDDP over untreated ZDDP. Thermal degradation studies and characterization by phosphorus and fluorine NMR spectroscopy revealed the formation of fluorinated phosphorus compounds (FPC) that contain direct P–F bonding as a result of direct interaction between ZDDP and iron (III) fluoride [17], [18], [19].

The presence of fluorinated and fluorine-containing oil additives has been shown to improve anti-wear performance of lubricants under different tribological conditions. For instance Wang et al. [20] have shown that synthetic ionic liquids of alkylimidazolium hexafluorophosphate show superior tribological behavior (e.g. lower friction) in comparison to paraffin-based oils containing ZDDP. Cutler et al. [21] have also shown that adding fluorine-containing tris-[p-(perfluoroalkylether)phenyl] phosphine to perfluoropolyalkylethers (as high-temperature lubricants) inhibits corrosive wear under conditions with low humidity associated with these types of lubricants.

Tribological tests using oil samples containing both fluorinated ZDDP and untreated ZDDP were conducted to evaluate the wear performance of the two chemistries. Chemical analysis of tribofilms generated on steel surfaces was carried out using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometry (EDS). The crystallinity and chemistry of the wear debris was examined by generating diffraction patterns using the selected area diffraction in a transmission electron microscope coupled with energy dispersive spectroscopy. X-ray absorption near edge spectroscopy (XANES) was used to determine the local structure and bonding environment of sulfur, phosphorus, iron, oxygen and zinc atoms present in the tribofilm in order to further reveal the chemical structure of the tribofilms generated from each formulation.