Study of the interfacial mechanism of ZDDP tribofilm in humid environment and its effect on tribochemical wear; Part I: Experimental (original) (raw)
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Tribology Letters, 2003
One of the biggest challenges in engine tribology is to formulate appropriate lubricants, which will increase fuel efficiency by reducing friction, yet still provide good wear resistance. The lubricant should also be formulated to limit particulate and gaseous exhaust emissions to the levels allowed by current regulations. In real lubricant formulations there can be 10-15 additives and the interactions between additives must be taken into account. The effects of eliminating the friction modifier and friction modifier plus anti-wear additive zinc dialkyl dithiophosphate (ZDDP) from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics have been examined. Tests have been conducted under lubricated wear conditions at bulk oil temperatures of 20, 50, and 100 8C using a reciprocating pin-on-plate tribometer. Boundary lubrication conditions were varied according to the value of starting lambda ratio. The wear film has been examined by Energy Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS). In order to investigate the morphology of the reaction films formed by the additive packages of these lubricants, Atomic Force Microscopy (AFM) was used. In this paper it has been shown that tribofilms, derived from ZDDP/surface interactions, affect friction, the extent of which is determined by tribological conditions. Detergent interactions with ZDDP enhance the complexity of the tribofilm and enrich the level of C in the film whilst affecting the friction and wear response. Through integration of tribological measurements and surface analysis, progress towards improving the nature of interactions is made and forms the focus of the paper.
A Semi-deterministic Wear Model Considering the Effect of Zinc Dialkyl Dithiophosphate Tribofilm
Tribology Letters, 2015
Tribochemistry plays a very important role in the behaviour of systems in tribologically loaded contacts under boundary lubrication conditions. Previous works have mainly reported contact mechanics simulations for capturing the boundary lubrication regime, but the real mechanism in which tribofilms reduce wear is still unclear. In this paper, the wear prediction capabilities of a recently published mechanochemical simulation approach (Ghanbarzadeh et al. in Tribol Int, 2014) are tested. The wear model, which involves a time-and spatially dependent coefficient of wear, was tested for two additive concentrations and three temperatures at different times, and the predictions are validated against experimental results. The experiments were conducted using a mini-traction machine in a sliding/rolling condition, and the spacer layer interferometry method was used to measure the tribofilm thickness. Wear measurements have been taken using a white-light interferometry. Good agreement is seen between simulation and experiment in terms of tribofilm thickness and wear depth predictions.
The mutual interaction between tribochemistry and lubrication: Interfacial mechanics of tribofilm
Tribology International
A new mechanism for the action of antiwear tribofilms is proposed. The antiwear action of ZDDP additive is believed to be mainly due to the formation of tribofilms that reduce wear by chemical action. In this study, a mixed lubrication model is developed and tribofilm growth integrated into this model to simulate the effects of tribofilms on lubrication. The dynamic evolution of the contacting surfaces due to plastic deformation, wear and tribofilm growth continuously change the lubrication characteristics inside the contact. It is observed that the growth of tribofilm roughens the contact and increase contact severity. It was found that this roughness increase also helps to entrain more lubricant, resulting in thicker lubricant films. Therefore, the plot of the evolution of film thickness ratio (h central (t)/R q (t)) shows that the lubrication regime is improved by the presence of tribofilm. Therefore, not only the chemical presence but the physical presence of the tribofilm on the surfaces also helps to improve contact performance by retaining more lubricant and improving the lubrication regime.
Lubricants, 2014
The role of surface protective additives becomes vital when operating conditions become severe and moving components operate in a boundary lubrication regime. After protecting film is slowly removed by rubbing, it can regenerate through the tribochemical reaction of the additives at the contact. However, there are limitations about the regeneration of the protecting film when additives are totally consumed. On the other hand, there are a lot of hard coatings to protect the steel surface from wear. These can enable the functioning of tribological systems, even in adverse lubrication conditions. However, hard coatings usually make the friction coefficient higher, because of their high interfacial shear strength. Amongst hard coatings, diamond-like carbon (DLC) is widely used, because of its relatively low friction and superior wear resistance. In practice, conventional lubricants that are essentially formulated for a steel/steel surface are still used for lubricating machine component surfaces provided with protective coatings, such as DLCs, despite the fact that the surface properties of coatings are quite different from those of steel. It is therefore important that the design of additive molecules and their interaction with coatings should be reconsidered. The main aim of this paper is to discuss the DLC and the additive combination that enable tribofilm formation and effective lubrication of tribological systems.
Tribology International, 2019
Lubrication conditions have significant influences on the formation of tribofilms and then affect tribological behavior. In this work, the influence of tribofilm formation on the tribological behavior of textured surfaces with oval shapes was measured using a pin-on-plate tribometer. The results show that, under full lubrication, the adsorbed oil film controlled the friction and wear behavior of steel/steel tribopairs but under starved lubrication, the formation of a tribofilm significantly influenced the tribological behavior. The appropriate textured surfaces with oval-shaped dimples contributes to obtaining excellent antifriction and antiwear behavior. However, excessively high ratios of the major to the minor axis of the oval can result in high contact stresses which can destroy the tribofilm.
Real time durability of tribofilms in the piston ring – cylinder liner contact
Tribology International, 2017
In boundary lubrication, the ability to form protective tribofilms determines the effectiveness of lubricants regarding friction and wear. In order to be able to control tribofilm formation, even if additives have to be reduced due to environmental regulations, the dynamics of the tribofilm protection of surfaces are investigated. This study investigates the durability of the tribofilm formed on the cylinder linerpiston ring contact and the corresponding wear protection capability in real time. To accomplish this, additivated engine oil was replaced by a non-additivated oil during test runs, after the steady-state wear rate had been reached. For the analysis of the tribofilm durability, the chemical composition of worn surfaces is compared to the dynamic wear and friction behavior.
ZDDP and MoDTC interactions in boundary lubrication--The effect of temperature and ZDDP/MoDTC ratio
Tribology international, 2006
Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 1C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 1C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high-and low-friction species in the tribofilm. r address: A.Morina@leeds.ac.uk (A. Morina).
Tribology Transactions, 2019
A tribochemical modelling framework that considers the growth of tribofilm on the contacting surfaces has been used in this work. The model couples a fast contact mechanics model with the thermodynamics of interfaces and captures the growth of the tribofilm on the asperities. The model was shown to be able to capture the dynamics of a tribosystem and the evolution of surface topography. The model considers the effect of plastic deformation and wear in modifying the surface geometries. In a recent work of the authors, (Ghanbarzadeh et al. in Wear 2016) the same numerical model was validated against experiments of the Micropitting Rig (MPR) and the wear, topography and tribofilm thickness results were compared. In this work, while the validation of the model is presented, the effect of tribofilm kinetics and its hardness have been numerically studied to assess the evolution of surface roughness in a rolling sliding contact. Results suggest that the kinetics of the tribofilm growth significantly influences the roughness evolution with higher kinetics resulting in a rougher interface. Similarly the tribofilm hardness affect the roughness evolution and are more influential in the later stages of roughness evolution.