In situ synchrotron XAS study of the decomposition kinetics of ZDDP triboreactive interfaces

Abstract

One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.

Tribochemistry: A Review of Reactive Molecular Dynamics Simulations

“…For insight at the atomic and molecular level, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, time-of-flight secondary ion mass spectroscopy, and transmission electron microscopy, among others, are common practice (see, for example, [27][28][29][30]). Such ex situ studies have recently been complemented by in situ observations of tribofilm growth at the macroscale by integrating a tribometer in a synchrotron X-ray absorption spectrometer [31,32] and at the nanoscale through use of an atomic force microscope to both mechanically drive film growth and image that growth [33]. Despite the progress made in surface characterization, direct observation of tribochemical processes remains a major experimental challenge since a realistic representation of a tribocontact and the required in situ analysis are mutually exclusive.…”

Single-asperity study of the reaction kinetics of P-based triboreactive films

“…Initially, these additives adsorb to the rubbing surfaces with a maximum coverage when the ZDDP or DDP molecules are flat and their sulfur atoms lay near the surface. The reaction of the adsorbed molecules with the oxide layer on bare steel rubbing surfaces produces initially a mixture of zinc and iron sulfate in the case of ZDDP ( Figure 1b) [2] and iron sulfate in the case of DDP (Figure 1c) [3,4]. The sulfate species are quickly reduced into sulfides at the asperity contacts under the influence of high temperature and contact pressure [2,5].…”

“…The reaction of the adsorbed molecules with the oxide layer on bare steel rubbing surfaces produces initially a mixture of zinc and iron sulfate in the case of ZDDP ( Figure 1b) [2] and iron sulfate in the case of DDP (Figure 1c) [3,4]. The sulfate species are quickly reduced into sulfides at the asperity contacts under the influence of high temperature and contact pressure [2,5]. Subsequently, the P-additive molecules decompose completely to form layers of zinc phosphate of increasing chain length away from the substrate in the case of ZDDP [5], whereas in the case of DDP layers of short chain iron phosphate are mainly formed [6].…”

“…The sulfate species are quickly reduced into sulfides at the asperity contacts under the influence of high temperature and contact pressure [2,5]. Subsequently, the P-additive molecules decompose completely to form layers of zinc phosphate of increasing chain length away from the substrate in the case of ZDDP [5], whereas in the case of DDP layers of short chain iron phosphate are mainly formed [6].…”

Reactivity of oil‐soluble IL with silicon surface at elevated temperature

“…It is crucial in the understanding of lubrication mechanisms to understand the thermal adsorption behaviour or thermally activated chemical reactions in the absence of any tribological effect. In contrast to ILs, a large number of previous papers have focused on the lubrication mechanism of ZDDP . Some have focused on the reactivity of ZDDP with steel or hard coatings at elevated temperatures to obtain a full understanding of its lubrication mechanism.…”