Studies on ZDDP Anti-Wear Films Formed Under Different Conditions by XANES Spectroscopy, Atomic Force Microscopy and 31P NMR

Abstract

Antiwear (AW) films, generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive, were studied as a function of formation temperature, load and rubbing time. The surface morphology of these films was investigated using atomic force microscopy (AFM), and surface roughness calculated for the observed differing surface morphologies. The morphology of the films is heterogeneous for all the tested conditions, but the surface roughness is dependent on the rubbing condition. X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of these films, and the intensity of the phosphorus K-edge was also used to monitor their thickness. The thickness of these films is in the range of 10-90 nm depending on the running conditions. Phosphorus L-edge spectra show that these films have a similar chemical nature with variable polyphosphate chain-lengths. 31P NMR was used to study the decomposition of ZDDP in the residual oils. The spectra show that the primary and secondary ZDDP react differently under the various conditions. The tribological characteristics of these AW films were probed by measuring the coefficients of friction (l) and the wear scar width (WSW) of the counter faces. l is highly related to the applied load and the results of WSW measurements show that the wear performance is related to all the tested parameters, temperature, load and rubbing time.

A Novel Method for Quantitative Determination of Ultra-low Wear Rates of Materials, Part I: On Steels

“…The calculated values of the film thickness formed in ZDDPcontaining oil and FF oil are *48 nm and *19 nm, respectively. These thicknesses are consistent with direct measurements made previously under similar conditions [26].…”

An analytical study of tribofilms generated by the interaction of ashless antiwear additives with ZDDP using XANES and nano-indentation

“…Phosphorous L edge XANES spectra provides a wealth of chemical information as the depth of sampling is close to the average thickness of the anti-wear tribofilms which is 50-100nm in most cases. The phosphorous L edge TEY and FLY spectra of the tribofilms generated from ZDDP and combinations of ZDDP with ashless PFC18 & SSC18 compounds along with the model compounds Zn 3 (PO 4 ) 2 and FePO 4 are shown in the Figure 5 2 and FePO 4 compounds respectively, and peak d, located ~146.0ev is a shape resonance peak, is characteristic of all phosphates 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 regardless of the structure, whether crystalline or glassy [50,55,56]. From the graphs, it can be deduced that for the tribofilms generated at 350N, phosphorous is primarily present in the form of zinc phosphate rather than iron phosphate as the peak position c is well aligned to the tribofilms spectra both in TEY and FLY modes.…”

“…XANES analysis has been used extensively in tribology to examine the chemical nature of the tribofilms derived using various anti-wear additives [26,29,32,39,[47][48][49][50][51][52][53]. It provides chemical information for total electron yield (TEY) and fluorescence yield (FLY) for the P L-edge of 5 and 60 nm respectively and sampling depths for TEY and FLY at the P K-edge are 50nm and >800nm, respectively [41,54].…”

“…Phosphorous L edge spectra also used for the determination of the chain length of the polyphosphates present in the tribofilms using peak intensity ratio of peaks a and c. Chain lengths of polyphosphates are characterized on the basis of peak height of a relative to peak c where ratio a/c up to 0.3-0.4 represents short-chain polyphosphates and a/c > 0.6 indicates long-chain polyphosphates [26,50,57]. Tribofilms studied in this work show very small pre-peak intensity of a in TEY mode which becomes less evident or starts to disappear in FLY mode, suggesting presence of short-chain polyphosphates and mono phosphates at the surface (≈ 5 nm) of the tribofilms whereas the bulk (≈ 30-40 nm) is largely made up of mono phosphates.…”

Friction-condition-dependent sulfide and sulfate evolution on dialkylpentasulfide tribofilm studied by XANES

“…In addition, some important facts and viewpoints have been heeded after years of research. The tribochemical reaction of ZDDP or phosphate-containing additive could be enhanced with elevated load, interval time, speed, and temperature [18,19], indicating that frictional conditions play an important role in the lubricating effectiveness of tribofilm. Therefore, a study of the interaction between frictional conditions and the evolution of sulfur-tribofilm is necessary and important for its application in oil.…”