Studies on ZDDP Thermal Film Formation by XANES Spectroscopy, Atomic Force Microscopy, FIB/SEM and 31P NMR

Abstract

X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of thermal films on steel samples, which were generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive. These films were formed at 150°C by immersing steel coupons in ZDDP oil solutions. The phosphorus L-edge XANES spectra show that these films are composed of polyphosphates, unreacted ZDDP and other thiophosphate intermediates. Phosphorus K-edge FY XANES was used to monitor the thickness of these films, and the data are consistent with thickness derived by focussed ion beam (FIB) milling and SEM imaging. The sulphur K-edge TEY and FY XANES spectra show that these films are composed of different sulphur components, which depend upon the formation times. The surface morphology of these films was investigated using atomic force microscopy (AFM). These images show that the surface morphology of the thermal films changes with the formation time. 31P NMR spectra show that both primary and secondary ZDDP decomposes gradually at 150°C.

In-situ reflection-XANES study of ZDDP and MoDTC lubricant films formed on steel and diamond like carbon (DLC) surfaces

“…The main peak was observed at 2152.5 eV, shifted by ~2.5 eV from the peak observed for unreacted ZDDP (see Figure 3). This shift is in agreement with the previous XANES studies done on ZDDP tribofilms [47,56]. Previous XANES studies on phosphates with different cations [57] show that P K-edge appear to be the same whether the cation is Fe or Zn.…”

“…These results support the findings from the only other published research work on in-situ analysis of ZDDP thermal films obtained with ATR-FTIR analysis [46]. The nature of P species formed on the surface by heating the ZDDP-containing lubricants is shown to change only at temperatures higher than 150⁰C [46,47]. Not observing any change of P K-edge structure at 100⁰C indicates that there is no additive decomposition at this temperature, however analysis of P L-edge could provide more conclusive information on this.…”

“…This was also confirmed by the in-situ study of a similar system using the ATR-FTIR [46]. Current results shown in Figure 3 and Figure films has been done only post-test using techniques such as soft X-ray XANES [47]. Figure 4, Figure 6 and Figure 9 show the in-situ chemistry of S films formed on both steel and DLC by heating the ZDDPcontaining lubricant, not reported before.…”

“…To the knowledge of authors, this is the first chemical composition study of in-situ and real-time thermal film ZDDP-derived S species in a non-vacuum environment. Three peaks could be observed in the S K-edge spectra of ZDDP thermal films formed on steel: 2471.9 eV (unreacted ZDDP), 2473 eV (sulphoxides [47]) and 2481. 6 eV (sulphates [47]).…”

“…Three peaks could be observed in the S K-edge spectra of ZDDP thermal films formed on steel: 2471.9 eV (unreacted ZDDP), 2473 eV (sulphoxides [47]) and 2481. 6 eV (sulphates [47]). Comparing the key features of these spectra it can be observed that a peak at 2473 eV starts to appear after around 1.5 hours heating at 100˚C.…”