Durability of ZDDP Tribofilms Formed in DLC/DLC Contacts

The tribological properties of diamond-like carbon (DLC) coatings have drawn much attention of OEMs and lubricant manufacturers in recent years. It is important to know whether conventional friction modifier and antiwear additives can form durable films and work as effectively with DLCs as they normally do on steel surfaces. In this study, the film-forming and friction properties of the antiwear additive ZDDP and the strength of tribofilms formed by this additive on five widely used DLC types, namely a-C:H, a-C:H:W, a-C:H:WC, Si-DLC and ta-C, have been investigated. It is found that ZDDP-derived tribofilms form on all the DLCs but exhibit different friction characteristics based on DLC type. With all DLCs, the amount of tribofilm elements measured after durability tests was less than that measured initially. Over 90 % of thiophosphate and 70 % of sulphide/sulphate were lost during durability tests. ZDDP tribofilms were found to be strongly adhered on Si-DLC and a-C:H compared with the other DLCs. The ZDDP tribofilms formed in DLC/DLC contact appear to be similar in structure to those formed in steel/steel contact but not to exhibit the antiwear performance seen in steel/steel contacts.     

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 2, chemistry

The chemical constitution of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and ashless dialkyldithiophosphate (DDP), has been examined by X-ray Absorption Near Edge Structure (XANES) spectroscopy. The identification of spectral features and interpretation of the results for P, O, Fe, and S species are given, allowing an overall mechanism to be deduced. The role of Fe in these films was investigated in some detail using P L-edge, O K-edge and Fe L-edge XANES spectra. From the P L-edge XANES spectra, the DDP films are uniformly very short chain iron polyphosphates. In contrast, the ZDDP films are formed initially as short chain polyphosphates; but after more rubbing, a bilayer phosphate film is formed with long chain Zn polyphosphates on the surface and shorter chain in the bulk of the film. The O K-edge XANES spectra show that there is, as expected, more Fe in the DDP phosphate films than in the ZDDP phosphate films. The S K-edge spectra of ZDDP films show the presence not only of ZnS as previously observed, but also the presence of FeS for the first time in the early stages of film formation. The predominant S species in the DDP films is FeS.     

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 1, growth,wear and morphology

The growth and morphology of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and an ashless dialkyldithiophosphate (DDP) over a wide range of rubbing times (10 s to 10 h) and concentrations (0.1–5 wt% ZDDP), have been examined using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy at the O, P and S K-edges and the P, S, and Fe L-edges. The physical aspects of the growth and morphology of the tribofilms will be presented in Part I and the chemistry of the films will be discussed in Part II. The major components of all films on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 Å thick) forms after 10 s, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600–800 Å after 30 min of rubbing, before leveling off or decreasing. The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations have also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed. The AFM results show that ZDDP forms larger and better developed “pads” than DDP at short rubbing times. At longer rubbing times, both films become more uniform. For the 1 h ZDDP films, the film thickness is surprisingly independent of the ZDDP concentration from 0.1 to 5 wt% ZDDP. The film thickness is also independent of the ratio of ZDDP/DDP concentrations.     

Resolving the Chemical Variation of Phosphates in Thin ZDDP Tribofilms by X-ray Photoelectron Spectroscopy Using Synchrotron Radiation: Evidence for Ultraphosphates and Organic Phosphates

X-ray photoelectron spectroscopy using a synchrotron source (SR-XPS) with variable photon energy has been used to non-destructively elucidate the variations in surface chemistry from ~5 nm to ~10 nm into the tribofilm derived from zinc dialkyldithiophosphate (ZDDP) in a mineral oil under boundary lubrication conditions. The elemental ratio of P/Zn and “bridging” oxygen (BO)/“non-bridging” oxygen (NBO) decrease as a function of distance from the top surface of the film, suggesting a decrease of the polyphosphate chain-length into the film, as shown in many recent XPS and XANES studies. More importantly, the measured P/Zn ratio of ~3, the BO/NBO ratios of >0.5, the P 2p spectra, and the absence of other balancing cations such as iron, show the first strong evidence for an ultrapolyphosphate (such as ZnP₄O₁₁), organophosphates along with other Zn polyphosphates. The existence of ultraphosphates and/or organophosphates in this film appears to be the long-awaited answer to the apparent deficiency of cations in these antiwear films.     

Studies on ZDDP Thermal Film Formation by XANES Spectroscopy, Atomic Force Microscopy, FIB/SEM and 31P NMR

X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of thermal films on steel samples, which were generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive. These films were formed at 150 °C by immersing steel coupons in ZDDP oil solutions. The phosphorus L-edge XANES spectra show that these films are composed of polyphosphates, unreacted ZDDP and other thiophosphate intermediates. Phosphorus K-edge FY XANES was used to monitor the thickness of these films, and the data are consistent with thickness derived by focussed ion beam (FIB) milling and SEM imaging. The sulphur K-edge TEY and FY XANES spectra show that these films are composed of different sulphur components, which depend upon the formation times. The surface morphology of these films was investigated using atomic force microscopy (AFM). These images show that the surface morphology of the thermal films changes with the formation time. 31P NMR spectra show that both primary and secondary ZDDP decomposes gradually at 150 °C.     

Use of permanent marker to deposit a protection layer against FIB damage in TEM specimen preparation

Permanent marker deposition (PMD), which creates permanent writing on an object with a permanent marker, was investigated as a method to deposit a protection layer against focused ion beam damage. PMD is a simple, fast and cheap process. Further, PMD is excellent in filling in narrow and deep trenches, enabling damage‐free observation of high aspect ratio structures with atomic resolution in transmission electron microscopy (TEM). The microstructure, composition, gap filling ability and planarization of the PMD layer were studied using dual beam focused ion beam, transmission electron microscopy, energy dispersive X‐ray spectroscopy and electron energy loss spectroscopy. It was found that a PMD layer is basically an amorphous carbon structure, and that such a layer should be at least 65 nm thick to protect a surface against 30 keV focused ion beam damage. We suggest that such a PMD layer can be an excellent protection layer to maintain a pristine sample structure against focused ion beam damage during transmission electron microscopy specimen preparation.     

Using the FIB to characterize nanoparticle materials

In the 1–100‐nm size regime, the properties of materials can differ significantly from those of their bulk counterparts. The present study applies the focused ion beam (FIB) tool to the characterization of nanoscale structures for scanning and transmission electron microscopy. The strength of this method is its ability to manufacture samples that cannot be produced using traditional means. The films of nanoparticles examined here are examples of such systems; the films are found to be not fully dense, composed of chemically heterogeneous areas and mechanically different from the substrate. Distinct advantages of the application of the FIB for characterization of nanoscale structures are highlighted for several nanoparticle structures. This successful application of FIB techniques provides a pathway to integrate the study of nanoscale production techniques and their resulting structure–property relationships.     

Synthesis of Fluorinated ZDDP Compounds

Volatile degradation products of zinc dialkyl dithio phosphate (ZDDP) composed of phosphorus and sulfur compounds reduce the efficiency of catalytic converters resulting in harmful emissions. A unique way of reducing ZDDP level while maintaining good antiwear performance has been achieved by reacting a novel additive FeF₃ with ZDDP. The objective of this research is to examine the chemical interactions between ZDDP and FeF₃ that yield the new chemical species responsible for the improved wear performance. The approach adopted involves studies of thermal degradation products of ZDDP that are formed in presence and absence of FeF₃. These intermediate products are responsible for the formation of protective tribofilms on the surface. Nuclear Magnetic Resonance Spectroscopy (³¹P and ¹⁹F) has been principally used to identify these products. Results have indicated the formation of new fluorinated phosphorus compounds formed as ‘early intermediate’ products in the reaction between ZDDP and FeF₃. The chemical differences observed were used to justify the improved tribological behavior of the new fluorinated ZDDP compounds in comparison to ZDDP. Paper was presented at the STLE/ASME International Joint Tribology Conference, October 23–25, 2006, San Antonio, TX. Paper Number IJTC2006-12053.     

Antiwear Behavior of ZDDP and Fluorinated ZDDP in the Presence of Alkylated Diphenyl Amine Antioxidants

Environmental regulations have called for a reduction of phosphorus content in engine oils in recent years. The anti-wear additive zinc dialkyl dithiophosphate (ZDDP), which is also an antioxidant, is one of the most important components of engine oil additives. ZDDP is a major source of phosphorus. One way to reduce phosphorus levels is to replace ZDDP with new environmentally friendly antiwear agents that have similar or superior wear performance compared to ZDDP. Another way to address the environmental issue is to reduce the amount of ZDDP in engine oils. At the same time, it is necessary to increase the efficiency of ZDDP by finding optimum conditions that would result in improved antiwear performance. The antiwear mechanism of ZDDP involves its degradation thermally and tribologically, leading to the formation of an antiwear film that consists of polyphosphates and sulphides. The structure of the antiwear film is almost similar in both types of degradation. But the breakdown efficiency of ZDDP is diminished by the parallel reaction of ZDDP with other additives, as well as the antagonistic effects of these additives. The new fluorinated ZDDP complex developed has shown better wear performance compared to ZDDP. This would allow the possibility of further reduction of phosphorus in engine oils compared to current levels. In this paper we study the interaction of ZDDP and fluorinated ZDDP with alkylated diphenylamine. The impact of antioxidant on wear performance was examined using a ball-on-cylinder tribometer. The interactions between ZDDP and the fluorinated ZDDP with the antioxidant were studied using NMR and the surface of the tribofilm was examined using SEM, TEM, and Auger spectroscopy.     

苯并三氮唑与ZDDP的复合抗磨效应及其应用研究

本文对二烷基二硫代磷酸锌(简称ZDDP)的抗磨性及其与苯并三氮唑的复合效应作了试验研究,简单探讨了其作用机理。结果表明,少量的苯并三氮唑便能使ZDDP的抗磨性能得到显著提高。并将二者的复合剂用在内燃机油配方的研究中,取得了较好的效果。     

Formation of Low-Friction Particle/Polymer Composite Tribofilms by Tribopolymerization

Inorganic fullerene-like (IF) solid lubricant particles based on MoS₂ or WS₂ have recently gained attention in various tribological applications, for instance incorporated in coatings, porous materials, greases and dispersed in oils. However, their effect in oil can be limited often due to inadequate penetration into the contact zone. Meanwhile, tribopolymerization of monomer additives in oil have proven to be efficient to reduce wear, but without significantly reducing the friction. This investigation combines these two lubrication techniques in order to form particle/polymer composite tribofilms, aimed to give low friction and wear in high-pressure sliding contacts. A cyclic amine, caprolactam, was used as the monomer together with IF as well as normal 2H WS₂ particles. Zn-DTP was used as a reference antiwear additive and poly-alfa-olefin (PAO) was used as the base oil. Reciprocal ball-on-flat sliding test results found that monomer plus particles reduced the friction by 10–50%, depending on sliding speed and type of particles, compared with particles or monomer alone. And the scattering between different tests was also considerably lower. The wear rate was also substantially reduced to a level similar to that of Zn-DTP. The particle/polymer composite tribofilms were generally much larger than the actual contact area and its thickness varied from below a few nanometres in the contact centre to several micrometers in the outer parts. Consequently, the reduced friction is believed to come from two effects: one being a trapping of particles in the contact zone, which reduces the boundary friction level; and the other a shift in lubrication regime towards EHD-lubrication due to reduced contact pressures accompanied with the formation of the composite tribofilm.     

Chemistry of ZDDP Tribofilm by ToF-SIMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (static and dynamic modes) has been applied to chemical characterization of zinc dithiophosphate additives tribofilm. Main result concerns insights in the spatial distribution of phosphate and sulphide species in the whole tribofilm thickness, at a sub-micrometer scale. The disappearance of oxide layer at the interface between steel and the tribofilm is also noticed.     

ZDDP对淡水鱼的急性毒性研究

对ZDDP在水中的急性毒性作了研究。试验以金鱼作为试验生物,对ZDDP的生物毒性作TN定。采用了概率图解法对实验数据进行了处理。结果表明,ZDDP对环境具有轻微的毒性作用,     

The History and Mechanisms of ZDDP

This paper reviews research into the mechanisms of action of the lubricating oil additive, zinc dialkyldithophosphate (ZDDP). The development of the use and research into ZDDP is first charted historically, starting with the additive's first introduction in engine oils in the late 1930s. Then our current state of knowledge of each of the main facets of ZDDP behaviour both in solution and at metal surfaces is identified and discussed. It is concluded that we now know a great deal about the properties and morphology of ZDDP antiwear films but still relatively little about the reaction pathways that lead to ZDDP film formation or about the kinetics of ZDDP film generation and removal.     

Tribochemistry of ZDDP in molecular orbital calculations

The molecular orbital parameters of zinc dialkyldithiophosphate (ZDDP) and several metal-atom-cluster models were calculated. The nature and the strength of the interactions between the ZDDP molecules and different metal surfaces are analysed and discussed with the use of frontier orbital theory. By comparing the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) of the ZDDP and the atoms cluster models of Al₆, Cu₆, and Fe₅, it is concluded that ZDDP behaves as an excellent boundary lubricant additive at the interface with iron. The derived molecular orbital parameters illustrate the advantages for tribochemistry studies.     

Effects of Temperature and Pressure on ZDDP

A recent theoretical study proposed that the anti-wear property of zinc dialkyl dithio phosphate (ZDDP) is due to the formation of chemically connected networks as a result of pressure-induced cross-linkage of phosphate groups of thermally decomposed ZDDP. To investigate the initial decomposition processes and the possibility of linking of phosphate groups in the decomposed product, in-situ high-pressure and high-temperature infrared (IR) spectroscopy using synchrotron radiation were performed on the original ZDDP. At room temperature no substantial structural change was observed up to 21.2 GPa, a pressure far exceeding the predicted onset of a structural transformation for the model zinc phosphate at 7 GPa. The observed Pressure induced broadening of the IR peaks is most likely associated with structural disorder or amorphization of ZDDP which is completely reversible upon decompression. When ZDDP is heated under pressure, an irreversible transformation was observed around 225 °C and 18.4 GPa. The experimental results show that ZDDP undergoes substantial decomposition at high pressures and high temperatures but no hint of cross-linkage of phosphate groups was found.     

Tribochemistry of ZDDP and MoDDP chemisorbed films

By means of a UHV analytical AES/XPS pin-on-flat tribotester, we have investigated tribochemical change as well as transfer mechanisms in static reaction films on steel from ZDDP (anti-wear) and MoDDP (friction modifier) lubricant additives. The results show that solid ZDDP reaction films are able to strongly reduce friction to a value of 0.1–0.3 (in the absence of a film, steel-on-steel gives a friction coefficient value of up to 2). MoDDP reaction films produce ultra-low friction (below 0.05) after an induction period. In situ AES analysis and energy-filtered XPM gave definitive evidence of frictioninduced chemical changes and selective material transfer from the flat to the pin. In the case of MoDDP, the thiophosphate film is decomposed and a MoS₂ residual film is found in the wear scar on the pin.A lecture based on this paper was presented at the Satellite Forum on Tribochemistry, International Tribology Conference, Yokohama 1995, organized by the Tribochemistry Research Committee, Japanese Society of Tribologists.     

Evaluation of Local Mechanical Properties in Depth in MoDTC/ZDDP and ZDDP Tribochemical Reacted Films Using Nanoindentation

Local mechanical properties in depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using a nanoindentation technique combined with in-situ atomic force microscopy (AFM) observation. Tapping-mode AFM observation revealed that the MoDTC/ZDDP film was much rougher than the ZDDP film. Nanoindentation measurement revealed that the MoDTC/ZDDP and ZDDP tribofilms possessed different elasto-plasticities around a depth of several nanometers from the surface, although both films showed the same hardness and modulus depth distributions except in the surface area. The same mechanical depth distributions indicated that both kinds of tribofilm were functionally graded materials; that is, they consisted of a layer near the surface with lower hardness and modulus and providing lubrication and a base layer with higher hardness and modulus and serving to modify property differences at the interface. Most importantly, the different elasto-plasticities near the tribofilm surfaces revealed that the MoDTC/ZDDP tribofilm possessed lower shearing yield stress than the ZDDP tribofilm. The results of this study suggest that the presence of some solid lubricants such as MoS₂ just below the MoDTC/ZDDP film surface reduced the boundary friction coefficient.     

Application of the dual-beam FIB/SEM to metals research

The dual‐beam microscope is a combination of a focused ion beam with an electron beam. The instrument used in this work is also equipped with an energy‐dispersive X‐ray system for local elemental analysis. This powerful tool gives access to specific features inside a material. Two different applications are presented in this paper: (1) cross‐sections and transmission electron microscope specimens cut in order to investigate the interface between an aluminium substrate and its epoxy coating; and (2) a grain boundary in a Cu₃Au alloy. In both cases, the dual beam succeeded where other methods failed.