Zinc Dialkyl Dithiophosphate Tribofilms Research Articles

On the mechanical and tribological performances of the tribofilm formed by zinc dialkyl dithiophosphate

The zinc dialkyl dithiophosphate (ZDDP) as multi-effect additive used in lubricating oil has always been paid more attention on the antioxidation, anticorrosion and antiwear performances. How to explain its role in tribology field has been paid more attention in the world. This studying showed a new insight into the antiwear mechanism from the perspective of tribofilm of lubricating oil containing ZDDP using SEM/EDS, Raman spectrometer, atomic force microscope (AFM) etc. It was found that ZDDP tribofilm contained a little graphitic carbon and iron oxides besides metal phosphates and sulfides. The bonding of ZDDP tribofilm to the substrate was stronger than that of base oil (poly-alpha-olefin, PAO) tribofilm. The mechanical properties of ZDDP tribofilm were more stable than those of PAO tribofilm. Statistical analysis revealed that ZDDP tribofilm was harder and less adhesive effects than PAO tribofilm, which might provide it with the ability to resist abrasive and adhesive wear. The similar elastic modulus of the two kinds of films indicated their similar plastic deformation. Moreover, the micro friction-reducing ability of ZDDP tribofilm might be the origin of ZDDP reducing the macro friction under boundary lubrication.

High Friction Mechanism of ZDDP Tribofilm Based on in situ AFM Observation of Nano-Friction and Adhesion Properties

Zinc dialkyl dithiophosphate (ZDDP) is a typical anti-wear additive that forms tribofilm on sliding surfaces and prevents adhesion wear but increases the friction force. In this study, the nano-friction and adhesion properties of ZDDP tribofilms were investigated by performing in situ observations by atomic force microscopy to explore the relationship between the formation process and physical properties of ZDDP tribofilm. The results showed that the nano-friction force increased with the formation of the ZDDP tribofilm. In addition, the adhesion force on the ZDDP tribofilm increased with the friction force. It was confirmed that the Young's modulus and nanoindentation hardness of the ZDDP tribofilms were lower than those of steel. However, the contact areas calculated using the Derjaguin–Muller–Toporov (DMT) contact theory were similar. In addition, the shear strengths calculated using the DMT model of the ZDDP tribofilm were substantially higher than those of steel.

Effect of engine oil additives reduction on the tribofilm structure of a cylinder liner model surface

PurposeThe purpose of this paper is to provide a general picture for describing the formed tribofilm, including chemical and physical aspects in the micro-scale and the nano-scale. In a previous study, the durability of zinc dialkyl dithiophosphate (ZDDP) tribofilms on cylinder liner samples has been investigated in a tribometer model system by using fresh and aged fully formulated oils and replacing them with PAO8 without additives. Analyses of the derived tribofilms by means of X-ray photoelectron spectroscopy and scanning electron microscopy could give some hints about the underlying mechanisms of the tribofilm build-up and wear performance, but a final model has not been achieved.Design/methodology/approachThus, characterisation of these tribofilms by means of focused ion beam-transmission electron microscopy (FIB-TEM) and energy dispersive X-ray spectroscopy is presented and a concluding model of the underlying mechanisms of tribofilm build-up is discussed in this paper.FindingsFor tribotests running first with fresh fully formulated engine oil, a rather homogeneous ZDDP-like tribofilm is found underneath a carbon rich tribofilm after changing to non-additivated PAO8. However, when the tests run first with aged fully formulated engine oil, no ZDDP-like tribofilm has been found after changing to non-additivated PAO8, but a wear protective carbon rich tribofilm.Originality/valueThe obtained results provide insights into the structure and durability of tribofilms. Carbon-based tribofilms are built up on the basis of non-additivated PAO8 because of the previously present ZDDP tribofilms, which suggests an alternative way to reducing the consumption of antiwear additives.

The Importance of Temperature in Generating ZDDP Tribofilms Efficient at Preventing Hydrogen Permeation in Rolling Contacts

ABSTRACTHigh-strength steels are particularly susceptible to hydrogen embrittlement and therefore rolling element bearings can experience premature failure that drastically reduces their operation life. Some lubricant additives can prevent this by generating protective tribofilms during operation. This study investigates the effect of temperature on the growth of zinc dialkyl dithiophosphate (ZDDP)-generated tribofilms and the relationship with the permeation of hydrogen in the substrate.Rolling contact tests were conducted using ZDDP at 333, 363, and 393 K and the concentration of hydrogen was measured after the tests. It was found that low temperatures inhibit the reactivity of the additive, leading to an inadequate coverage of the surface that allows dissociation and permeation of hydrogen in the sample. Higher temperatures induce uniform films in the contact, although excessive temperatures can promote unnecessary wear of the substrate. An optimum temperature can balance the two main processes occurring in the contact related to the reactivity of the additive: the growth of a protective film and the corrosive wear of the material.

A new insight into the interfacial mechanisms of the tribofilm formed by zinc dialkyl dithiophosphate

Understanding the true interfacial mechanisms involved in the growth of tribofilms generated by Zinc Dialkyl Dithiophosphate (ZDDP) is important because it is the most widely used anti-wear additive and there is legislative pressure to find efficient environmentally-friendly replacements. The main focus of this study is to investigate the durability of the ZDDP tribofilm and correlate it to the chemical and physical properties of the glassy polyphosphates. A novel experimental method has been developed to study the effect of lubricant temperature and contact load on tribofilm growth and durability. Results show that physical parameters such as temperature and pressure significantly influence the tribofilm durability. XPS analyses were carried out before suspending the test and after changing the oil to assess the difference in chemical structure of the tribofilm before and after stopping the test. The chemical analyses suggest that there are different chemical properties across the thickness of the tribofilm and these determine the durability characteristics.

Interaction Between Selected MoS2 Nanoparticles and ZDDP Tribofilms

Nanoparticles based on transition metal dichalcogenides (TMD) are considered to hold great promise as boundary lubricating additive/material for improving friction and wear of engineering functional surfaces. However, TMD nanoparticles cannot provide a comprehensive surface protection against oxidation, corrosion or sludge control. Therefore, the current lubricant developments may still have to depend on conventional additives such as zinc dialkyl dithiophosphate (ZDDP), and it is essential to understand the interaction of nanoparticles with such additives in order to explore how these nanoparticles could be commercially employed in fully formulated lubricants. This paper examines the tribological properties of three different nanoparticles: inorganic fullerene-like MoS2, rhenium-doped MoS2 and MoS2 nanotubes in steel and steel with preformed ZDDP tribofilm surfaces using a pin-on-disc-type tribometer under reciprocating sliding conditions. The resulting tribofilms have been evaluated using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, transmission electron microscopy and atomic force microscopy. The results show that although the nanoparticles are able to reduce friction in all cases, the resulting tribofilm composition and morphology, and their lubricating mechanisms are significantly different. The MoS2 nanoparticles and nanotubes show good synergism with ZDDP, and tribofilms formed from nanoparticles exhibit improved friction and wear properties compared to that typically formed from ZDDP.

An XPS Study on the Composition of Zinc Dialkyl Dithiophosphate Tribofilms and Their Effect on Camshaft Lobe Wear

In this study, various zinc dialkyl dithiophosphate (ZDDP)-derived tribofilms were analyzed with X-ray photoelectron spectroscopy (XPS) to determine the relationship between tribofilm chemical composition and wear severity. Samples were generated on camshaft lobes from engine tests conducted at different oil temperatures, and wear depths were measured with a surface profilometer. Tribofilm specimens were analyzed by XPS to assess changes in film chemistry as a function of wear severity. Tribofilms present on all camshaft lobes were found to contain polyphosphate glass of various chain lengths. Long-chain polyphosphates were observed on low wear tracks, whereas short-chain polyphosphates were found on more severe wear tracks. Variations in polyphosphate chain length are also apparent at different wear depths along a low wear track. The presence of long-chain polyphosphates on low wear tracks was indicative of their superior antiwear properties, whereas the occurrence of short-chain polyphosphates on more pronounced wear tracks was indicative of their inferior antiwear properties.

Friction response of ZDDP thermal film and tribofilm

An original dynamic tribotest was used to study the friction response of two kinds of tribological surface: zinc-dialkyl-dithio-phosphate (ZDDP) thermal film and ZDDP tribofilm. This apparatus allows friction to be determined with no need for measuring tangential force. A velocity dependent friction contribution was found in each case in addition to a velocity independent one, showing the viscous character of such films. Furthermore, during test on ZDDP thermal film, the viscous damping represents 18% of the total dissipated energy as it is 16% for ZDDP tribofilm.

Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions—Part II: Morphology and nanoscale properties of tribofilms

Tribofilms generated using lubricants containing zinc dialkyl dithiophosphate (ZDDP) and fluorinated zinc dialkyl dithiophosphate (F-ZDDP) were examined. Nanoindentation yielded reduced elastic modulus and hardness as a function of the penetration depth of the nano-indenter for different loading conditions. The mechanical strength of the tribofilm was evaluated by conducting scanning wear and nano-scratch tests. Results indicate that load used to create the tribofilms plays an important role in determining their hardness with the tribofilms formed at higher loads exhibiting higher hardness. Tribofilms formed from F-ZDDP were more resistant to scratch and wear compared to ZDDP tribofilms. FIB measurements indicate F-ZDDP tribofilms were thicker than ZDDP tribofilms.

Study of the ZDDP Antiwear Tribofilm Formed on the DLC Coating Using AFM and XPS Techniques

Abstract To meet the challenge of the increasing demand of fuel economy, in recent years low friction nonferrous coatings such as diamond-like carbon (DLC) coatings have become very popular for automotive tribo-components. The interaction of lubricant additives, which are designed for ferrous surfaces, with nonferrous coatings is an important issue for the automotive and lubricant industries. The aim of this paper is to establish a link between the evolution of antiwear zinc dialkyl dithiophosphate (ZDDP) tribofilm and the tribological performance of a DLC coating under boundary lubrication conditions. Experiments were performed in a pin-on-plate reciprocating tribotester to produce the tribofilm. Atomic force microscopy (AFM) was used to record high resolution topographical images of the ZDDP films while chemical analysis of the ZDDP tribofilms was performed using X-ray photoelectron spectroscopy (XPS). Results in this study show that the ZDDP tribofilm consists of short chain zinc pyrophosphate (Zn2P2O7) and zinc metaphosphate (ZnOP2O5), and it is formed along the raised portion of the initial marks of the DLC surface.

XPS study of the influence of temperature on ZnDTP tribofilm composition

Antiwear additives, such as zinc dialkyldithiophosphate (ZnDTP), find application in many different industrial sectors. Although it is understood that certain ZnDTP concentrations need to be used to achieve an effective antiwear performance, there has been very little work published concerning the effect of temperature on the interactions of the additive and its adsorption mechanism on steel. In this article, 100Cr6 (52100) steel ball-on-disc experiments under solutions of zinc dialkyldithiophosphate (ZnDTP) in poly-α-olefin (PAO) were performed at different temperatures, ranging from 25 to 180 °C. The discs were analysed after the experiments by means of small-area, imaging and angle-resolved X-ray photoelectron spectroscopy (XPS). The composition of the reaction film was found to change as a function of the applied temperature and also to vary within the film as a function of depth: Longer polyphosphate chains were found at higher temperatures as well as towards the outer part of the reaction film.