Antiwear Films Research Articles

Molecular simulations of amine-based organic additives at a steel surface: Effect of the internal molecular structure on adsorption

Lubricating oils mitigate wear in gears and rolling bearings lengthening the lifespan of drivetrain units for electric vehicles. Amine-based organic additives from the lubricating oil contribute to alleviating wear by adsorbing on rubbing steel surfaces of mechanical contacts and forming anti-wear films. Cross-pin wear experiments suggest that additives with branches in polar heads improve wear protection, while remaining environmentally friendly. Experiments show a reasonable correlation with the energy of adsorption calculated from molecular dynamics simulations at large surface coverages. Whereas adsorption strengths and surface coverages are barely affected by the conformation of alkyl chains (cis/trans), they are affected by the length of branches in polar heads of additives. Results suggest better performances for branch lengths containing 3–4 C atoms.

Correlating wear with the lubricant properties of heavy-duty diesel engine oils

Wear volumes were correlated with the lubricant properties of 11 used, heavy duty engine oil samples. The most important oil property in predicting wear volume is Total Acid Number, TAN. Here, the TAN value may be indicating ZDDP in its oxidised form and unable to participate in corrosive-abrasive wear. Low wear also correlates with mean soot particle size/circularity, which further suggests the abrasive aspect of this mechanism. Finally, low wear correlates with high calcium concentration in the fresh oil. This suggests a new wear reduction mechanism in which calcium from the detergent replenishes the iron within the ZDDP antiwear film.

Recent Advancements in Understanding of Growth and Properties of Antiwear Tribofilms Derived from Zinc Dialkyl Dithiophosphate Additives under Nanoscale Sliding Contacts.

Zinc dialkyl dithiophosphate (ZDDP) is a key antiwear additive in lubricants that forms robust phosphate glass-based tribofilms to mitigate wear on rubbing surfaces. The quest to unravel the enigma of these antiwear film formations on sliding surfaces has persisted as an enduring mystery, despite nearly a century of fervent research. This paper presents a comprehensive review of nanotribological investigations, centering on the tribochemical decomposition of ZDDP antiwear additives. The core of the Review explores investigations conducted through the in situ AFM-based technique, which has been used to unveil the underlying stress-assisted thermal activation (SATA) mechanism behind the formation of antiwear tribofilms on diverse surfaces. A thorough analysis is presented, encompassing governing factors, such as compression, shear, and temperature, that wield influence over the intricate process of tribofilm formation. This is substantiated by a spectrum of structural and chemical characterization-based inferences. Furthermore, atomic-scale computer simulation studies are discussed that provide profound insights into tribochemical reaction mechanisms and elucidate the details of chemical processes at atomic level.

Soot wear mechanisms in heavy-duty diesel engine contacts

In this study, the wear mechanisms of a used, soot-containing oil drained from an engine test are investigated. As reviewed at the start of this paper, many wear mechanisms have been proposed for soot-containing oils. Often these studies were conducted using fresh oils with added carbon black as a soot mimic which do not always accurately represent the oxidation pathways in engines. Hence, we apply a range of tribometer tests and surface analyses techniques to assess the wear performance of engine-drained oil on AISI 52100 steel. Results suggest that this used oil gives rise to the corrosive-abrasive wear mechanism involving repeated cycles of anti-wear film formation and subsequent removal by soot abrasion. The role of different additives in mitigating/exacerbating this mechanism are studied by testing lubricant samples with key additives omitted. This reveals that detergent helps to form a strong, soot-resistant tribofilm, while ZDDP prevents wear provided it can form a strong film but can also increase wear in a used, abrasive, soot-containing oil.

Characterising the effects of simultaneous water and gasoline dilution on lubricant performance

The introduction of hybrid electric vehicles presents new operating conditions which can increase water and fuel dilution of the engine oil. The effects of this type of simultaneous water and fuel dilution are poorly understood. Therefore, here we have characterised its effects on 1) boundary film performance 2) Lubricant rheology and film thickness. This is achieved by testing four samples: 0W‐20, 0W‐20+gasoline blend, 0W‐20+H20 water-in-oil emulsion and 0W‐20+H20+gasoline water-in-oil emulsions. A blended gasoline surrogate was used with a composition matching that found in real used engine oils.Gasoline dilution reduces viscosity under all conditions in a predictable fashion but has negligible chemical effects. Meanwhile, water affects only low-shear/low-pressure viscosity and is surface active and thus affects antiwear film formation by an apparent competition for the surface. This results in thinner and smoother anti-wear films giving both lower friction and higher wear. These individual behaviours are largely maintained even when water and gasoline dilution are present in the same blend, suggesting negligible interaction between the two components.

Rapid fabrication of ultra-wear-resistant graphene nanocrystallite film by direct laser writing

Developing protective films with high wear resistance under heavy load is an urgent challenge. Meanwhile, the conventional fabrication methods for anti-wear film usually require time-consuming and energy consumption processes with vacuum equipment. Here, we report an ultra-wear-resistant graphene nanocrystallite film that can be fabricated by direct laser writing on polyimide in a single step process. The graphene nanocrystallite film had a loose exploded structure but could surprisingly sustain more than 100,000 cycles of friction under 20 N with a stable friction coefficient around 0.17. The ultra-wear-resistant mechanism of the film was revealed that the loose exploded graphene nanocrystallites were restructured into a tough cross-linked nanocrystalline transfer film on the counterpart surface by friction. The cross-linked nano-structure could effectively alleviate the slip consumption between graphene sheets, which resulted in slow wear processes including lattice damage and amorphization of graphene sheets in transfer film. With a good practical significance, the graphene nanocrystallite film is expected to have broad application prospects with the merits of ultra-wear-resistance and easy rapid fabrication feature.

Direct Multimodal Nanoscale Visualization of Early Phosphorus-Based Antiwear Tribofilm Formation.

Understanding the mechanism of antiwear (AW) tribofilm formation and how to tune surface chemistry to control functionality is essential for the development of the next generation of oil lubricants. In particular, understanding and optimizing early AW tribofilm formation can increase the energy efficiency of mechanical systems. However, the mechanism for how these films form is not well understood. The majority of prior work has focused on analyzing only end-of-test surfaces long after the film has formed. In this work, we develop an in situ multimodal chemical imaging methodology to directly visualize the early formation of AW films on steel surfaces. We investigate an oil formulation containing a phosphorus-based additive commonly used to protect surfaces from wear and fatigue processes in machine elements, such as gears, bearings, and sliding contacts. Using nanoscale multimodal chemical imaging on combined platforms of atomic force microscopy (AFM) coupled directly with in situ nano-infrared (nano-IR) spectroscopy, and further combined ex situ with time-of-flight secondary ion mass spectrometry (ToF-SIMS), we demonstrate a direct correlation between changes in friction and local surface chemistry. In these experiments, the AFM probe acts as a single asperity contact to generate the tribofilm as well as a tool to analyze it in situ as it is forming. To verify our in situ measurements, we compare these results to the ex situ ToF-SIMS of macroscale block-on-ring tribometer-formed samples. The understanding gained here on how AW films form and how film properties can be modified by tuning the chemistry of the additives will facilitate developing transmission fluids to meet increasing demands for vehicle performance and efficiency.

ПОВЫШЕНИЕ ЭКСПЛУАТАЦИОННЫХ СВОЙСТВ ПЛАСТИЧНЫХ СМАЗОК ГРАФЕНОВЫМИ ДОБАВКАМИ

It is known that commercial lubricants do not always meet high performance requirements in terms of their tribological characteristics. In particular, it was found that at high loads in worn mechanisms, Litol-24 grease does not always provide high anti-wear properties that help extend the service life of plain bearings. In order to improve the performance properties, studies were carried out on the modification of the lubricant with numerous graphenes and comparative tests were carried out on the MI-1M friction machine. The structure of the carbon film was determined by Raman scattering on a DXR Raman Microscope Thermo Scientific spectrometer. As a result of the research, it was found that the modification of the lubricant with graphenes makes it possible to form a film on the friction surface. At the same time, the wear of parts is reduced from 0.0138 mg on the Litol-24 commercial lubricant to 0.0079 on the graphene-modified lubricant. As a result of spectral analysis, it was determined that the spectrum of the carbon film contains approximately the same intensity G-line - 1565 cm-1 and D-line - 1340 cm-1 and a low-intensity 2D line - 2700 cm-1. This spectrum is identical to the Raman spectrum of amorphous carbon. In the spectrum from the metal surface of the roller, the listed lines of the Raman spectrum are absent. In the process of testing on a friction machine of Litol 24 plastic lubricant with multilayer graphene filler, the destruction of the graphene structure and its transformation into amorphous carbon is observed. As a result of determining the roughness of the roller and the thickness of the carbon film with a profilometer, it was found that the surface roughness with a carbon film is less compared to the clean metal surface of the roller: (0.176 - 0.168 = 0.008 μm) and (0.566 - 0.485 = 0.071 μm), the depth of the profile depressions on the surface with carbon film decreased by 0.116 µm. The height of the protrusions of the profile on the surface with the carbon film increased by 0.161 µm. At the same time, it was determined by calculation that the approximate thickness of the carbon film is 0.138 μm. On the basis of the conducted studies, it can be argued that the modification of Litol-24 commercial lubricant with graphenes contributes to the formation of a protective anti-wear film on the friction surface, thereby reducing wear and increasing the service life of machine parts.

Effect of MoP additive on wear properties of cemented carbide

This paper describes the effect of MoP lubricant additive on the wear properties of cemented carbide. In the wear tests, the reciprocating sliding tests were conducted under about 7 GPa of maximum Hertzian contact pressure, and MoP, MoA, MoDTC and ZnDTP were used as additives. The results showed that in tests using Mo type additives the amount of wear in MoP was smaller than that in MoA and MoDTC, and that the film was formed on the contact surface of cemented carbide after the tests. In addition, the wear in ZnDTP was greater than that in MoP, implying that Mo and P elements in the additive affect the formation of anti-wear film on the cemented carbide surface. Moreover, it was also shown by EDX analysis that in the rubbed surface of cemented carbide in the test using MoP, Mo, P, O was detected respectively, and that the concentration of Mo was greater than that in the test using MoA. These results suggest that in the test using MoP the molybdenum trioxide and the phosphorus film were formed o the cemented carbide, resulting that the film formed by MoP should prevent the decohesion of WC grains.

MQL-Assisted Hard Turning of AISI D2 Steel with Corn Oil: Analysis of Surface Roughness, Tool Wear, and Manufacturing Costs

Precision hard turning (HT) gained more and more attention in the cutting industry in the last years due to continuous pressure of the global market for reducing costs, minimizing the environmental and health issues, and achieving a cleaner production. Therefore, dry cutting and minimal quantity lubrication (MQL) became widely used in manufacturing to meet the environmental issues with respect to harmful cutting fluids (CFs). Vegetable oils, in MQL machining, are a promising solutions to petroleum-based CFs; however, the effects and performance on surface roughness and tool wear in HT with ceramic inserts remain unclear. To address this limitation, hardened AIDI D2 steel and pure corn oil, rich in saturated and monounsaturated fatty acids, cheap and widely available, have been used to conduct dry and MQL experiments at different cutting speed and feeds. Results show that corn oil is suitable as cutting lubricant in HT, creating a strong anti-wear and anti-friction lubricating film which improves the roughness with 10–15% and tool life with 15–20%, therefore reducing costs. Best surface roughness values (Ra = 0.151 μm, Rz = 0.887 μm, Rpk = 0.261 μm) were obtained at 180 m/min and 0.1 mm/rev. The analysis of variance shows that corn oil has statistical significance on roughness, validating the results.

Correlating GDI and Diesel Soot with Carbon Black Surrogates: An MTM-SLIM Study

<div class="section abstract"><div class="htmlview paragraph">Legislations aimed at reducing CO<sub>2</sub> emissions are driving significant changes in passenger car engine hardware and lubricants. Gasoline direct injection (GDI) engines generate combustion soot which can drive wear which is characteristically different to that observed in diesel engines. The increasing market share of GDI engines has encouraged the auto OEMs and the oil suppliers to study this challenge in more depth and seek improvements which do not compromise the innate efficiency benefits of the GDI platform. This study compares soot abrasiveness by measuring the abrasive removal of Zinc dialkyldithiophosphate (ZDDP) antiwear films and resultant wear by GDI sooted oils and traditional Diesel sooted oils in the MTM-SLIM equipment. A three-way correlation has been developed between a carbon black soot surrogate, GDI sooted oils and Diesel sooted oils. Diesel sooted oils were found to be more abrasive and resulted in more severe wear than gasoline sooted oils at the same level of % soot loading. The impact of oil viscosity grade on soot induced wear has been observed. A thinner oil with lower soot concentration led to harsher wear conditions as compared to a thicker oil with higher soot loading. Both, thinner oils and increased soot loading led to increased wear levels, therefore both parameters need to be considered when wear data is being interpreted.</div></div>

Improved tribology performances of copper graphite (Cu/C) using a phosphorized oxidization surface obtained with tricresyl phosphate (TCP)

PurposeCopper-graphite (Cu/C) is a type of seal material that has been widely used in liquid rocket engines and with normal metal pairs, has a high wear rate under liquid oxygen lubrication. In this study, bearing steel disks were subjected to a high-temperature oxidization and phosphating progress to create an oxidized tricresyl phosphating film (OTCP) film, using tricresyl phosphate (TCP) as the phosphating solvent. It is hoped that the surface wear rate and friction coefficient can be reduced by this method.Design/methodology/approachThis study aim to form an anti-wear film on the surface of bearing steel, which can significantly improve the lubrication performance of Cu/C and bearing steel. The surfaces of bearing steel disks were dried to remove surface water and then put on a heating plate with a magnetic stirrer and a blue glazed oxide film obtained by heating the disks in the air at 200°C for 1 h. To create the OTCP film, bearing steel disks with a blue glazed oxide film were cleaned ultrasonically three times in ethanol and then placed in baths of TCP. After heating for 2 h at 120°C, excess TCP liquid was removed from the disks using ethanol, leaving an OTCP film on the disk surface.FindingsIt was found that steel coated with an OTCP film demonstrated better tribological performance (lower coefficient of fiction and wear rate) when pairing with Cu/C than either oxidized or untreated bearing steel. Additional chemical reactions occur when the oxide disk is immersed in TCP and the FePO4 film is formed after heating. Additionally, the OTCP film coated steel displayed good corrosion resistance, as confirmed by electrochemical corrosion tests. This finding demonstrates the potential for this process in the aerospace industry.Originality/valueThe preparation of OTCP films via high-temperature oxidization and phosphating of bearing steel was demonstrated, with the tribological properties of the OTCP film being investigated alongside those of the original surface and an oxidized film surface. The fabrication of OTCP films is easily scaled up and exhibits significant advantages as a new technology for applications in mechanical contact seal lubrication.

The Influence of Steel Composition on the Formation and Effectiveness of Anti-wear Films in Tribological Contacts

The effectiveness of antiwear additives in laboratory tests is commonly evaluated using specimens made of AISI 52100 through-hardened bearing steel. However, many lubricated machine components are made of steels with significantly different material compositions, which raises an important practical question of whether the performance of antiwear additives with these other steel types is different from that established with AISI 52100. To help answer this question, this paper investigates the influence of steel composition on the formation and effectiveness of antiwear films. Four steels that are commonly used in tribological applications, namely AISI 52100 through-hardened bearing steel, 16MnCr5 case-carburised gear steel, M2 high speed steel and 440C stainless steel are tested in rolling-sliding, ball-on-disc contacts lubricated with three custom-made oils, one containing ZDDP and two containing different types of ashless antiwear additives. The relative effectiveness of their boundary films was assessed by measuring their thickness and associated wear and friction over 12 h of rubbing at two specimen roughness levels. For ZDDP it was found that the formation of antiwear film was not significantly influenced by steel composition or specimen surface roughness. A similar tribofilm thickness, final tribofilm roughness and friction was observed with all four steels. No measurable wear was observed. By contrast, for the ashless antiwear additives the thickness and effectiveness of their tribofilms was strongly influenced by steel composition, particularly at higher roughness levels. The exact trends in film thickness vs steel relationship depended on the specific chemistry of the ashless additive (ester-based or acid-based) but in general, relative to AISI 52100 steel, M2 steel promoted ashless tribofilm formation whilst 440C retarded ashless tribofilm formation. This behaviour is attributed to the presence of different alloying elements and the ability of the additives to extract metal cations from the rubbing surfaces to support the growth of a tribofilm. In all cases ZDDP films were thicker and rougher, and produced higher friction than those formed by the ashless additives. However, unlike ZDDP, ashless blends generally produced significant wear, particularly with 16MnCr5 and M2 steels. The results indicate that to ensure reliable performance of a given machine component, the chemistry of an ashless antiwear additive should be matched with the types of steel present in the lubricated machine.

Novel Insight into Tribology of Carbon Black Soot Particles in Engine Oil

Abstract An investigation into the interaction between soot and steel surfaces can improve the development of soot wear–resistant lubricants. In this study, we have tested the wear response of carbon black (CB; a surrogate for engine soot) on different steel surfaces using the high-frequency reciprocating rig and scanning electron microscopy – energy-dispersive X-ray spectroscopy. Results show that CB in 0W20 reacts to the preconditioned steel surface differently than the nonpreconditioned steel surface. Chemisorption of zinc dialkyldithiophosphates (ZDDP) on the steel surface prior to the wear test resulted in severe wear, but the shear-induced chemisorption of ZDDP on the steel surface showed the least wear. Changes in wear were related to the concentration of zinc and phosphorus, which represent antiwear phosphate film on the surface. Chemisorption of ZDDP prior to the test triggered an aggressive response by CB, as there was a complete removal of antiwear phosphate film. Thus, unprotected steel was exposed to sulfur, a corrosion enhancer in the lubricant that resulted into micropitting and grooves on the disk surface, a representation of the corrosion–abrasion wear mechanism. Shear-induced chemisorption of ZDDP on steel surface allowed retention of antiwear phosphate films. As a result, there were no grooves or severe micropitting on the steel surface, indicating a protection against CB. This study shows that preconditioning the steel can have a profound effect on the soot wear mechanism.

Study of pressure dependence on sinterable zirconia nanoparticle tribofilm growth

Tribological sintering of antiwear films as a function of applied load has been investigated using a novel zirconia nanoparticle antiwear additive. Spherical five nanometer diameter zirconium oxide (ZrO2) nanoparticles are dispersed in polyalphaolefin (PAO) synthetic base oil and tested between AISI 52100 steel counterfaces in a ball-on-disk tribometer with a slide-to-roll ratio of 50%. The apparatus allows tribofilm thickness data to be tracked in-situ at set intervals, and tribofilms reaching a maximum thickness of 160 nm have been measured. A tribofilm formation dependence on load has been found, as higher loads assist initial growth and final thickness. Contrary to the chemical tribofilm formation processes of traditional antiwear additives such as zinc dialkyldithiophosphate (ZDDP), testing suggests the primary mechanism of tribofilm growth for zirconium oxide is nanoparticle adsorption followed by particle accumulation and sintering. A strong correlation between friction force, tribofilm width and Hertzian contact theory has been found. Initial tribofilm growth rate grew exponentially with applied load, whereas isolation of the linear growth phase yielded a linear relationship. Mechanical stylus, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) are employed to investigate the roughness, morphology and chemical nature of the sintered tribofilms, respectively.

The Indirect Tribological Role of Carbon Nanotubes Stimulating Zinc Dithiophosphate Anti-Wear Film Formation.

Experimental studies reveal that the simultaneous addition of zinc dialkyl dithiophosphates (ZDDPs) and multi-wall carbon nanotubes (MWCNTs) to a poly-alpha-olefin base oil strongly reduces wear. In this paper, it is shown that MWCNTs promote the formation of an anti-wear (AW) layer on the metal surface that is much thicker than what ZDDPs can create as a sole additive. More importantly, the nanotubes’ action is indirect, i.e., MWCNTs neither mechanically nor structurally strengthen the AW film. A new mechanism for this effect is also proposed, which is supported by detailed tribometer results, friction track 3D-topography measurements, electron diffraction spectroscopy (EDS), and Raman spectroscopy. In this mechanism, MWCNTs mediate the transfer of both thermal and electric energy released on the metal surface in the friction process. As a result, this energy penetrates more deeply into the oil volume, thus extending the spatial range of tribochemical reactions involving ZDDPs.

Flash temperature and anti-wear tribofilm growth mechanisms by asperity contact in top-ring/liner conjunction of IC engines

Understanding the growth mechanism of tribofilms at the top of the liner in IC engines is of great importance to protecting the sealing function of the top ring. In present study, the transient heat transfer is investigated between the ring and the liner in the top dead center (TDC) area with considerations of asperity contact pressure, boundary friction coefficient and reciprocating piston motion. A 3D transient model is developed to predict the evolutions of effective heat partition and conduction between the contact surfaces. It is revealed that the flash temperature in the asperity contact pairs of the top-ring/liner conjunction is remarkably higher than the steady-state liner temperature, therefore significantly enhancing the growth rate of anti-wear film formation based on a stress-activated Arrhenius model. The results show that the asperity contact model and the flash temperature model can be employed as an engineering tool for anti-wear tribofilm prediction in top-ring/liner conjunction of IC engines.

UV/Ozone-Assisted Rapid Formation of High-Quality Tribological Self-Assembled Monolayer

UV/ozone (UVO)-assisted formation of self-assembled monolayer (SAM) of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) was prepared on a glass surface. The effect of UVO exposure time on surface roughness and hydrophilicity was investigated through goniometer and atomic force microscope (AFM), and deposition time-dependent SAM quality was detected by AFM and X-ray photoelectron spectroscopy (XPS). The glass surface became smooth with UVO radiation after 10 min, and the hydrophilicity was also improved after the treatment. Confirmed by surface topography detection and chemical composition analysis, a high-quality SAM can be formed rapidly on glass with 10 min UVO treatment followed by 2 h deposition in PFDS solution. Excellent tribological performances of SAM coated with UVO treatment glass were demonstrated by friction and wear tests on AFM compared to film-deposited glass without UVO treatment and original glass. The study sheds a light on preparing high-quality lubrication and antiwear self-assembled films on the surface of engineering materials.

In-situ observations of the effect of the ZDDP tribofilm growth on micropitting

Abstract The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical systems, means that machine components are required to operate for longer periods under thin film, mixed lubrication conditions where the risk of surface damage is increased. For this reason, the role of zinc dialkyldithiophosphate (ZDDP) antiwear lubricant additive has become increasingly important in order to provide adequate surface protection. It is known that due to its exceptional effectiveness in reducing surface wear, ZDDP may promote micropitting by preventing adequate running-in of the contacting surfaces. However, the relationship between ZDDP tribofilm growth rate and the evolution of micropitting has not been directly demonstrated. To address this, we report the development of a novel technique using MTM-SLIM to obtain micropitting and observe ZDDP tribofilm growth in parallel throughout a test. This is then applied to investigate the effect of ZDDP concentration and type on micropitting. It is found that oils with higher ZDDP concentrations produce more micropitting but less surface wear and that, at a given concentration, a mixed primary-secondary ZDDP results in more severe micropitting than a primary ZDDP. Too rapid formation of a thick antiwear tribofilm early in the test serves to prevent adequate running-in of sliding parts, which subsequently leads to higher asperity stresses and more asperity stress cycles and consequently more micropitting. Therefore, any adverse effects of ZDDP on micropitting and surface fatigue in general are mechanical in nature and can be accounted for through ZDDP's influence on running-in and resulting asperity stress history. The observed correlation between antiwear film formation rate and micropitting should help in the design of oil formulations that extend component lifetime by controlling both wear and micropitting damage.

Variation in zinc dialkyldithiophosphate yield strength measured by nanopillar compression

Abstract In situ transmission electron microscopy based nanopillar compression is utilized to investigate the mechanical properties of zinc dialkyldithiophosphate (ZDDP) tribological films. Small scale testing provides localized insights into the properties of ZDDP films that spatially vary in composition at the nanoscale. Large variations in yield strength, between 0.82 and 4.8 GPa, are measured and correlated with local chemistry changes. Lower density regions of the film that tend to be carbon-rich have lower yield stresses, than higher density regions that tend to be zinc-, iron-, and sulfur-rich. It is hypothesized that the strong compositional dependence in mechanical properties at the nanoscale is an important factor contributing to the efficacy of ZDDP as an antiwear tribological film.