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
Graphical abstract
Research Highlights
► This study is the first of its kind to examine the role of fluorinated ZDDP as an anti-wear agent in comparison to ZDDP. ► Phenomenological models explaining the formation, chemistry and structure of in-situ formed tribofilms at extreme pressure has been developed. ► XANES using multiple beamlines including P L and K edge, S L and K edge, Fe L edge, O K edge and Zn L edge have been used to develop a deeper understanding of the local coordination of these elements in the tribofilm. ► Transmission electron microscopy has shown that wear debris is essentially amorphous but contains nanocrystalline particles of Fe3O4 and the density of these particles is related to wear performance and tribofilm integrity. ► F-ZDDP has been shown to result in lower wear in comparison to ZDDP under identical tribological conditions.
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.
Section snippets
Oil sample preparation
Oil samples were prepared by adding untreated secondary zddp research sample to 100 neutral mineral base oil at loadings of 0.10 and 0.01 wt% phosphorus. The fluorinated ZDDP was produced by reacting the same ZDDP described above with FeF3. The mixture was then centrifuged to separate fluorinated ZDDP. The centrifuged fluorinated ZDDP was also added to the same neutral base oil at loadings of 0.10 and 0.01 wt% phosphorus and compared to oils with ZDDP. Details of the structure of the secondary
Friction, temperature and wear data
Wear tests were run on two oil samples each containing 0.10 wt% phosphorus as described in Section 2.2 i.e. the source of phosphorus in one sample is untreated ZDDP and in the other sample, the source of phosphorus is the fluorinated ZDDP. These wear tests are run under an applied Hertzian contact pressure of 3.56 GPa (i.e. 24 kg applied load) for test durations of 15,000 cycles. In Fig. 1 the temperature (Fig. 1(a)) and the coefficient of friction (Fig. 1(b)) data are plotted against the number
Discussion
The wear performance of fluorinated ZDDP was found to be better than that of untreated ZDDP at similar phosphorus levels. The improved wear performance is attributed to the improved, highly adherent tribofilm that was formed when fluorinated ZDDP was used. Several characterization techniques were used to further study the chemistry of tribofilms formed, leading to a phenomenological model for both tribofilms, which can explain the superior anti-wear performance of fluorinated ZDDP in comparison
Acknowledgments
The authors would like to gratefully acknowledge the Platinum Research Organization LLC for financial and technical support provided, the State of Texas for a Technology Development and Transfer Grant and Mr. Richard Nay of Hysitron Inc. for optimizing the conditions for and running the nano-indentation, scratch and scanning wear tests. Thanks to Mr. Randy Ward for making it possible to use the Nano-mechanical test facility at Hysitron in Minneapolis, MN. Help from Dr. Nasir Basit, Dr. J.C.
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