Film Thickness and Friction of ZDDP Tribofilms

Abstract

Tribofilm formation by several zinc dialkyl-and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet.

Cooperativity Between Zirconium Dioxide Nanoparticles and Extreme Pressure Additives in Forming Protective Tribofilms: Toward Enabling Low Viscosity Lubricants

“…Additionally, with severe enough conditions that there is primarily boundary contact, there is no substantial dependence in the tribofilm traction coefficient based on oil viscosity. This seemingly high traction coefficient range compares well to α-ZrP nanoplatelets [34] for example, and various forms of ZDDP (although a direct comparison between the measurements here and other studies cannot be made due to differences in measurement methods and conditions) [31]. When sufficient speeds are reached to move towards the EHL regime as seen in the MTM Stribeck curves (Supporting Information Section B), the traction coefficient decreases, even reaching sub-0.06 for ZrO 2 +75W-80 at a 2000 mm/s entrainment speed.…”

“…Regardless of cause, for SRRs > 0%, minimal difference in film thickness is seen between the 75W-80 and PAO samples. It is also worth noting that, while it is possible SLIM overestimates tribofilm thickness due to tribofilm surface roughness preventing full conformation to the SLIM lens [31], a systematic offset in the absolute values of reported tribofilm thicknesses would not change the observed trends or conclusions for the presented data. The full set of SLIM images, along with additional scanning electron microscopy images and elemental analysis from energy dispersive X-ray spectroscopy on representative tribofilms formed at a 50% SRR, are provided in Supporting Information Section D.…”

“…To gain a quantitative, non-destructive assessment of the degree of wear and the thickness of tribofilms for test specimens, scanning white light interferometry (WLI) of the film surface was used in combination with the SLIM images. The high refractive index of ZrO 2 necessitated reflective coatings on the tribofilms for accurate scanning WLI measurements [31]. A Cressington Sputter Coater 108 was used to coat the test specimens using a Au/Pd target.…”

Surface Reaction Films from Amine-Based Organic Friction Modifiers and Their Influence on Surface Fatigue and Friction

“…In our previous paper [2], we showed that addition of TDP to ZDDP blends generated smoother tribofilms (suggested in Fig. 3) leading to enhanced lubricant entrainment into the contact and hence reduced frictional forces [2,35]. Pad features in ZDDP-tribofilms are believed to from at asperity conjunctions which promote the film-growth (since they are subjected to higher shear stresses) [33].…”

Surface characterization of steel/steel contact lubricated by PAO6 with novel black phosphorus nanocomposites

“…Phosphorus is an active element during sliding and has excellent lubricating properties, especially incorporated in zinc dialkyldithiophosphate (ZDDP) [7][8][9], commonly incorporated in lubricating oils for mechanical equipment. Phosphorus, itself, is the eleventh most abundant element in the earth's crust.…”