Experimental observation of zinc dialkyl dithiophosphate (ZDDP)-induced iron sulphide formation

Abstract

Zinc Dialkyl DithioPhosphate (ZDDP) as a well-known anti-wear additive enhances the performance of the lubricant beyond its wear-protection action, through its anti-oxidant and Extreme Pressure (EP) functionality. In spite of over thirty years of research attempting to reveal the mechanism of action of ZDDP, there are still some uncertainties around the exact mechanisms of its action. This is especially the case with the role of sulphide layer formed in the tribofilm and its impact on surface fatigue. Although iron sulphide on the substrate is hypothesised in literature to form as a separate layer, there has been no concrete experimental observation on the distribution of the iron sulphide as a dispersed precipitate, distinct layer at the steel substrate or both. It remains to be clarified whether the iron sulphide layer homogeneously covers the surface or locally forms at the surface. In the current study a cross section of the specimen after experiment was prepared and has been investigated with Transmission Electron Microscopy (TEM) and Energy-Dispersive X-ray (EDX) elemental analysis. A 5-10 nm iron sulphide layer is visualised on the interface as a separate layer underneath the phosphate layer with an altered distribution of tribofilm elements near the crack site. The iron sulphide interface layer is more visible near the crack site where the concentration of the sulphur is enhanced. Also, ZDDP elements were clearly detected inside the crack with a varied relative concentration from the crack-mouth to the crack-tip. Sulphur is present inside the crack to a higher extent than in the bulk of the tribofilm.

Effect of base oil polarity on the micropitting behaviour in rolling‐sliding contacts

“…The asperity contact stress was also considered to be a main factor to promote the formation of micropits, thus this contact fatigue phenomenon can be retarded by the increase of lambda ratio or reduce of surface roughness. However, recently, Soltanahmadi et al found out that more micropits formed with enhanced reaction layer thickness between the contacting surfaces.…”

Antiwear and extreme pressure properties of hazelnut oil blended with ZDDP

“…The protective films are generated due to tribochemical reactions between steel and ZDDP components at the rubbing surfaces. The earlier studies have also shown that tribochemical reactions result in the formation of P-O-P, -P = O and P-O = Zn bonds and form protective phosphate glass on the surfaces (Morina et al , 2006; Spikes, 2004; Zhang and Spikes, 2016; Soltanahmadi et al , 2017).…”

Revealing the interface nature of ZDDP tribofilm by X-ray photoelectron spectroscopy and atom probe tomography

“…Some recent works present the existence of the sulfur-enriched layer differently by various methods, Shimizu and Spikes (2016) found the enrichment of S at the initial state of sliding, but it was afterward pushed to the edge of the contact zone and transformed to be P-and Zn-enriched. Soltanahmadi et al (2017) have made the sulfide interlayer visualized by using transmission electron microscopy (TEM)/energy dispersive Xray spectroscopy (EDS). Moreover, X-ray photoelectron spectroscopy (XPS) identified a sulfur/oxygen enrichment at the bottom side of the bulk tribofilm (Heuberger et al, 2007;Dorgham et al, 2018).…”