The Formation of Tribofilms of MoS2 Nanotubes on Steel and DLC-Coated Surfaces
Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS2 nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS2 nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS2-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS2-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS2-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS2 and the steel or DLC coating was observed.
Grease lubrication of rolling element bearings — role of the grease thickener
The contribution of grease thickener to lubricant film formation was examined in this paper. Lubricant film thickness and friction were measured for different grease thickener types in a bearing simulation device. The results showed that the greases formed thick (20–80nm), low friction surface layers at low speeds, which were much greater than the corresponding base oil film. These films appeared to be formed by the physical deposition of thickener in the track during overrolling of the grease. This was confirmed by infrared reflection analysis, which showed the deposited films to have increased thickener content. The ability of grease to form renewable physically deposited solid films has significant implications for optimising lubricant formulation for certain applications, e.g. bearings operating at high temperatures and low speeds where a conventional elastohydrodynamic lubricating film would be inadequate.
Tribological properties of additives for water-based lubricants
Fully formulated water lubricating fluids have been thoroughly described in literature. However, the influence of individual additives on tribological properties of these compositions is still not fully clear. In this paper we present frictional, anti-wear and anti-corrosion properties for separate solutions of anti-corrosion, anti-foaming and anti-microbial agents – amines (ethanolamine oligomers, ethylamine oligomers), friction modifiers – glycols (monoethylene glycol, 1,4-butylene glycol) and amine derivatives with longer hydrocarbon chains (3-amino-1-propanol, 4-amino-1-butanol). As a reference, we also performed some tests with pure water. The results show that the additives used in the tests in particular concentrations significantly improve tribological properties of water. The best performance – the lowest friction and no traces of corrosion – was obtained for triethylamine. Ethylene glycol and 1,4-butylene glycol significantly reduced friction, however their anti-wear behaviour was unsatisfactory. Ethanolamines, which combine properties of amines and alcohols showed an increase in friction and corrosion, but significantly reduced wear. The conclusion is that the examined additives can enhance one property of a tribosystem while adversely affecting another.
Impact of Viscosity Modifiers on Gear Oil Efficiency and Durability
This paper is part one of a longer term comparison of viscosity modifier behavior in modern automotive gear oil (AGO) fluids and the impact of these properties on fluid efficiency and durability. This first installment will compare the rheological properties, including EHD film thickness and traction coefficients, of the fluids across broad operating temperature, shear and load regimes and correlate these findings with rig efficiency testing.The effects of traction, EHD film thickness and high shear rheology on operating temperature are well documented and it is of particular interest to determine the extent to which different viscosity modifiers can beneficially impact these properties compared to a Brightstock-based SAE 80W90 grade fluid. The efficiency improvements of a VM would be for naught if it were not sufficiently shear stable and so comparisons are made between shear stable VM technologies.The final discussion introduces new tapered roller bearing shear test conditions that correlate more closely to fluid shear in the field than the current 20 h tapered roller bearing shear test, CEC L-45-A-99.
Tribological Bench and Engine Dynamometer Tests of a Low Viscosity SAE 0W-16 Engine Oil Using a Combination of Ionic Liquid and ZDDP as Anti-Wear Additives
We have previously reported an oil-miscible phosphonium-organophosphate ionic liquid (IL) with effective anti-wear functionality when added to a base oil by itself or combined with a conventional zinc dialkyldithiophosphate (ZDDP) for a synergistic effect. In this research, we investigated whether this synergy manifests in formulated engine oils. An experimental SAE 0W-16 engine oil was generated using a combination of IL and ZDDP with equal phosphorus contribution. The prototype engine oil was first evaluated using tribological bench tests: anti-wear performance in boundary lubrication and friction behavior (Stribeck curves) in elastohydrodynamic, mixed, and boundary lubrication. The forthcoming standard Sequence VIE engine dynamometer test was then conducted to demonstrate improved fuel economy. Results were benchmarked against those of another experimental engine oil with almost the same formulation except using ZDDP only without the IL (similar total phosphorus content) and a baseline SAE 20W-30 engine oil. The IL-ZDDP formulation consistently outperformed the ZDDP-only formulation and the results from the bench and engine tests are well correlated.
Expanding the Development of More Durable Friction Modifiers with Sustained Friction-Reduction: Extended Tribological Studies and Oil-Aging
AbstractTo gain some insight into friction modifier (FM) performance retention in engine oils, we have developed a series of tribology measurements to measure and understand friction reduction performance retention by extended tribology measurements of the changes in the coefficient of friction (COF) with time. In some cases, after several days of data collection, these tests give us insight into how friction modifiers might perform in real engine operating conditions with typical long oil drain intervals. Results are presented from both a series of sequential tribology oil studies with and without FMs, as well as longer isothermal hold studies, developed using a Cameron Plint TE-77 cylinder-on-plate, and PCS-Instruments Mini-Traction-Machine ball-on-disk friction instruments. Specifically studied, were Glycerolmonooleate (GMO), Molybdenum dialkyldithiocarbamate (MoDTC), and an experimental organic friction modifier (Exp-OFM1) of a completely ashless (containing no inorganic metals) C, H, O, N surface active molecular structure. The results show the friction reduction retention can vary significantly with friction modifier type. Nitro-oxidation of the oils was also studied by tribological methods and Fourier Transform Infrared (FT-IR), to track induced changes in COF, viscosity, oxidation, and nitration, simulating oil aging.
The impact of organic friction modifiers on engine oil tribofilms
Organic friction modifiers (OFMs) are important additives in the lubrication of machines and especially of car engines where performance improvements are constantly sought-after. Together with zinc dialkyldithiophosphate (ZDDP) antiwear additives, OFMs have a predominant impact on the tribological behaviour of the lubricant. In the current study, the influence of OFMs on the generation, tribological properties and chemistry of ZDDP tribofilms has been investigated by combining tribological experiments (MTM) with in situ film thickness measurements through optical interference imaging (SLIM), Alicona profilometry and X-ray photoelectron spectroscopy. OFMs and antiwear additives have been found to competitively react/adsorb on the rubbing ferrous substrates in a tribological contact. The formation and removal (through wear) of tribofilms are dynamic processes which result from the simultaneous interaction of these two additives with the surface of the wear track. By carefully selecting the chemistry of OFMs, the formulator can achieve lubricants that generate ZDDP antiwear films of optimum thickness, morphology and friction according to the application-specific requirements.
A Micro-IRRAS Study of Lubricant Degradation Under Thin Film Conditions
This paper studies lubricant degradation in a rubbing contact under thin film conditions. Friction tests have been carried out in an MTM test device using a steel ball/steel disc configuration. The tests were run with a small amount of lubricant present. At the end of the test the disc was retained for analysis. Micro-reflection FTIR spectroscopy was used to analyse the lubricant chemistry within and close to the rubbed track. The technique offers a more direct method for studying lubricant degradation products during rubbing. The results are compared to those from more conventional experimental methods.
Application of an Organic Molybdenum Anti-Friction Additives in API SM Gasoline Engine Oil
AbstractAn anti-friction additive was studied in API SM 5W-30 oil. Its tribological performance was evaluated on a four-ball tester, SRV and MTM. This additive has been found to be an effective antifriction additive for lubricants. The investigation indicates that this additive possesses excellent energy saving and reducing friction characteristics in API SM 5W-30 lubricating oil.
AN EXPERIMENTAL APPROACH TO THE STUDY OF RAIL WHEEL/FLANGE LUBRICATION
In this study, lubricated flange contact is simulated using various experimental methods as shown in Fig. 1. A full-scale wheel-flange contact simulator consists of rotating real train wheel and static rail specimen which is loaded against wheel flange corner. Only a sliding component of velocity is simulated. When a steel rail specimen is used friction force is measured using a load cell and wear of the specimen is evaluated using an optical profilometer. The specimen can be replaced with a glass one which allows us to determine a lubricant film distribution in the contact. The lubricants used were two bio-degradable greases, one based on rapeseed oil thickened with aluminum and graphite (Grease A) and one based on high-grade esters with solid lubricants (Grease B). Detail information about lubricant film thickness distribution under starved conditions is obtained on optical ball-on-disk contact simulator using thin film colorimetric interferometry. Friction and wear under lubricating conditions is investigated using Mini Traction Machine (MTM – PCS Instruments) with ball-on-disk and pin-on-disk configuration in more detail.
PCMO Lubricant Friction Modifier Performance Durability-Extended Tribology Studies
: Designing friction modifiers (FM) resistant to performance degradation is important to meet future demands for increased fuel economy, by using low viscosity oils and longer drain intervals. Whether studying organic or organometallic FM-additives, tribological research can help increase our understanding of friction modifier surface-adsorption and performance retention, focusing on how the coefficient of friction (COF) changes with contact time. In this study we measured the performance characteristics of glycerol monooleate (GMO) industry standard organic FM, Molybdenum dithiocarbamate (MoDTC), common known FMs [1, 2], and an experimental organic friction modifier (EXP-OFM) with performance retention. For these studies passenger car motor oil (PCMO) was formulated with Group III oil base stocks and a full range of additives meeting the viscometric requirements for a 5W-30 and 5W-20 finished oil. These additives include: viscosity and pour point improvers, calcium sulfonate detergents, polyisobutylene succinimide dispersant, amine and phenolic antioxidants, zinc dialkyldithiophosphate (ZDDP) anti-wear, silicone anti-foam, and the friction modifiers. To monitor changes in the COF with time (performance retention), the Cameron Plint TE-77 tribological testing instrument and PCS Instruments Mini-TractionMachine were utilized in non-routine extended tribology test methods. Two types of friction modifier tribology studies were then carried out to measure retention of performance. The first study measures initial FM performance (COF-decrease) in oil held for an 60 minutes (after standard testing 50 o C to 160 o C) to insure tribofilm formation. Following this hold stage and after draining and refilling test cell with nonfriction modifier oil (heated to 160 o C), the COF is monitored for ninety minutes. Finally, after draining and refilling the test cell with the original friction modifier containing oil, the COF (at 160 o C), is monitored for an additional ninety minutes. An example of the results is given in the figure below in Figure 1. These tests measured the memory of the tribofilm initially formed and ability for it to be refreshed to its original state with subsequent FM containing PCMO. The results indicated the 5W-30 oil with GMO at (1 % wt.) treat rate gave an initial friction reduction of -29% COF and retained a reduction of -18% COF on switching to non-FM oil. This could then be completely refreshed to -29% COF after the second oil switch back to GMO containing oil. The same base formulated 5W-30 with Exp-OFM at (1 %wt), had an initial friction reduction of -45% COF, retained -31% COF with non-FM oil, and was then refreshed back to a – 44% COF reduction. In contrast, although MoDTC at (1 %wt) shows a stronger initial friction reduction of -70% COF, during the first hold period, before oil switches, this degrades up to -45% COF. In addition, after the non-FM oil switch, it degrades further all the way up to only -13% COF reduction. Finally, following the second MoDTC (1 %wt) oil switch, the COF again first drops to -70% COF and then degrades up to a -44% COF reduction.
Fundamental Investigations of the Tribological Properties of Biological Interfaces
The objective of the proposed program is to understand the fundamental nature of forces existing at solvated polymer-polymer and protein-terminated polymer-polymer interfaces under shearing conditions, thereby allowing the future design of biological/bioinspired tribological systems lubricated by aqueous solutions. The proposed experimental program will be aimed at measuring interfacial forces over a range of length scales, for sets of systematically prepared and well-characterized polymer and biopolymer interfaces. This program will also explore the influence of solvent conditions on the measured interfacial interactions. The proposed program will explore the role of polymer architecture (composition and structure) in determining the adsorption, load-bearing, and frictional properties of self-assembling waterborne organic coatings. It will also examine the lubricious properties of a number of different adsorbed proteins, suspected to be tribologically active in biological systems. The fundamental goal of the program is to elucidate the modes of interaction between polymer surfaces (penetration, repulsion, compression) and the role of solvent (related to polymer swelling and conformational changes) in modifying interactions between the polymer surfaces. The applied aim of the program is to provide design criteria and performance limitations of low-friction biological and/or bioinspired interfaces.
Contribution of Oil Traction to Diesel Engine Cam Galling
Heavy Duty diesel engines typically use roller followers in contact with the cam to reduce friction and accommodate high Hertzian stresses. When the rolling contact slips into sliding, cam galling can occur that may lead to major cam failures. Oil traction has been identified as a possible source to cause slipping. In this study, oil traction was first measured in a Mini Traction Machine (MTM). The results were then validated by a series of engine tests to show that the measured oil traction correlated with the occurrence of cam galling. Finally, the MTM was used to evaluate various engine oil formulations. It is concluded that some advanced base oils, if not properly compensated by the additive package, exhibit dangerously low oil traction. Oil traction needs to be part of the oil formulation considerations.
Tribological Film Formation on Hydrogenated DLC/Steel Contacts From Fully Formulated Automotive Lubricants
Tribological film formation from fully-formulated synthetic automotive lubricants on hydrogenated DLC/steel contacts was compared with that on steel/steel contacts. Four DLC coatings containing various metal dopants (W and Si) were evaluated. Lubricants formulated with organic friction modifiers performed better on DLC/steel than on steel/steel in both sliding and rolling/sliding contacts (lower friction and wear). In contrast, lubricants formulated with MoDTC friction modifiers had higher sliding contact friction on DLC/steel than on steel/steel. No evidence was observed for tribological film formation on the DLC surfaces.
Frictional Properties of PVA Hydrogel
Total hip replacement is one of the most successful surgical treatments of modern medicine. Typically, at present, hard-on-hard bearing surfaces are widely used for components of artificial hip joints. Hard-on-hard means that both components have high modulus of elasticity in range of hundreds of GPa. However, these materials suffer from relatively high friction and wear rate. This is connected especially with occurring lubrication regime. To approach conditions presented in natural joints, it is necessary to think about artificial cartilage. One of the anticipated materials for artificial cartilage is polyvinyl alcohol (PVA) hydrogel. PVA hydrogel has water content about 85 % and its elastic modulus is approximately E ≈ 1.2 MPa, which is similar to natural cartilage. The main disadvantage of PVA hydrogel is its lower strength. In this study, commercial mini traction machine (MTM) was used to determine friction coefficient for various slide-to-roll ratios (SRR). Bovine serum was used as a lubricant and the tests were carried out under ambient temperature for three various speeds u1 = 25 mm/s; u2 = 50 mm/s; u3 = 100 mm/s and two different loads F1 = 5.2 N; F2 = 9.8 N, respectively. As expected, friction coefficient was very low, less than 0.05 under some conditions. In future, optical method based on the principle of fluorescent microscopy will be used for studying lubricant film thickness and protein adsorption on bearing surfaces.
New Bench Test to Study Mild Lubricated Wear
This extended abstract describes a new bench test for measuring and studying mild, lubricated wear. The method combines MTM, which is able to produce contra-rotation so as to obtain high sliding speed while retaining low entrainment speed and thus boundary lubrication conditions, with ICP which is employed to analyze trace levels of iron in lubricants so as to monitor mild wear throughout a test.
Mini-Traction-Machine Tests to Assess the Effect of Base Oil and Additive Interactions on Surface Charge and Friction
Electrostatic condition monitoring was implemented on a MTM to examine the influence of additive adsorption from three base oils. Statistical analysis (ANOVA) indicates significant affects on measured charge and friction levels for both additive and base oil type.
Tribological studies of potential vegetable oil-based lubricants containing environmentally friendly viscosity modifiers
The amphiphilic properties that result from the fatty acid composition of vegetable oils contribute to a better lubricity and effectiveness as anti-wear compounds than mineral or synthetic lubricant oils. Despite these advantages, vegetable oils show only a limited range of viscosities and this constrains their use as suitable biolubricants in many industrial applications. For the reason, ethylene–vinyl acetate copolymer (EVA) and ethyl cellulose (EC) have been added to the vegetable oil-based lubricants studied. To address this issue, the frictional and lubricant film-forming properties of improved vegetable oil-based lubricants (high oleic sunflower (HOSO), soybean (SYO) and castor (CO) oils), blended with 4% (w/w) of EVA and 1% (w/w) of EC, have been studied. It has been found that castor oil shows the best lubricant properties, when compared to high oleic sunflower and soybean oil, with very good film-forming properties and excellent friction and wear behaviour. This can be attributed to its hydroxyl functional group that increases both the viscosity and polarity of this vegetable oil. Regarding the effect of the viscosity modifiers studied, ethylene–vinyl acetate copolymer exerts a slight effect on lubricant film-forming properties and, thus, helps to reduce friction and wear mainly in the mixed lubrication region. Ethyl cellulose, on the other hand, was much more effective, mainly with castor oil, in improving both mixed and boundary lubrication.
A new scuffing test using contra-rotation
The mode of lubricant failure known as scuffing provides a significant design constraint in high sliding gears, cams and metal cutting and forming processes. It is therefore important to have an effective test method to measure the scuffing resistance of lubricant formulations. In most existing scuffing bench tests, a moving surface is rubbed against a stationary one at a fixed sliding speed and the load at which scuffing occurs is determined. This approach has two disadvantages. One is that wear of the stationary surface can lead to a large decrease in effective contact pressure during a test. The second is that viscous lubricants often generate significant elastohydrodynamic films at the sliding speeds employed. This means that the scuffing tests measure a complex combination of the influence of the fluid and boundary film-forming properties of the lubricant on scuffing rather than reflecting solely the influence of lubricant formulation. This paper describes a new scuffing test method in which the two metal surfaces are rubbed together in mixed rolling–sliding with the two surfaces moving in opposite directions with respect to the contact, i.e. in contra-rotation. This enables the sliding speed to be decoupled from the entrainment speed so that the scuffing properties of a lubricant can be determined in boundary lubrication conditions over a wide range of sliding speeds. Also, because both surfaces move relative to the contact, wear is distributed and this minimises changes in contact pressure during a test.
Numerical Solution of Hydrodynamics Lubrications with Non‐Newtonian Fluid Flow
This paper focuses on solution of numerical model for fluid film lubrication problem related to hydrodynamics with non‐Newtonian fluid. A programming code is developed to investigate the effect of bearing design parameter such as pressure. A physical problem is modeled by a contact point of sphere on a disc with certain assumption. A finite difference method with staggered grid is used to improve the accuracy. The results show that the fluid characteristics as defined by power law fluid have led to a difference in the fluid pressure profile. Therefore a lubricant with special viscosity can reduced the pressure near the contact area of bearing.
Nanoparticle-doped lubricants : Potential of Inorganic Fullerene-like (IF-) molybdenum disulfide for automotive applications
The growing environmental concerns, along with the continuous increase in the price of fossil fuels, have highly motivated car manufacturers worldwide to improve the efficiency of their vehicles. The tribological properties of engine and gearbox lubricants have a significant impact on the global efficiency of vehicles, as they contribute to reducing friction in many contacts and allow the downsizing of various components by providing their surfaces with anti-wear protection. The recent breakthroughs in nanoparticle synthesis have opened new prospects in terms of lubricant additivation, with the discovery of the excellent friction and wear reducing properties of nanoparticles such as Inorganic Fullerene-like (IF-) molybdenum or tungsten disulfides. The tribological potential of IF-MoS2 for automobile applications was investigated in this work. The respective influences of nanoparticle size and structure were first of all studied, revealing that poorly crystalline nanoparticles were more efficient in maintaining low-friction tribofilms on steel substrates in severe boundary lubrication regimes regardless of size (for the range studied). All the nanoparticles tested however showed similar performances when proper oil recirculation was ensured, providing a continuous feeding of the contact in nanoparticles. The IF-MoS2 nanoparticles lost their lubricating abilities when added to fully-formulated lubricants. This behavior was attributed to the presence of dispersants in the oil, which dispersed the nanoparticles effectively but prevented them from forming tribofilms on the rubbing surfaces. The well-dispersed IF-MoS2 were shown to enter the contact and exfoliate, but an excessive adsorption of the dispersants on the released MoS2 platelets and/or the steel surfaces is thought to prevent tribofilm adhesion. A balance between nanoparticle dispersion and tribological performance was then found, by using very low concentrations of dispersants. The behavior of nanoparticle-doped oils in various scenarios related to automobile applications was finally explored. The IF-MoS2 provided significant friction and wear reduction at ambient temperature and in milder rolling/sliding test conditions, for smooth and rough surfaces. The risks related to the presence of nanoparticles in the oil in full-film lubrication regimes were partially lifted, with no significant influence on friction witnessed for all the test conditions considered. The ability of IF-MoS2 nanoparticles to protect steel surfaces from surface-initiated Rolling Contact Fatigue was finally shown.
Frictional Properties of Molybdenum-Based Lubricating Oil Additives Using Green Chemistry
In this study, a green process was developed to synthesize a novel molybdenum disulfide (MoS₂)-based friction modifier (FM) for improving fuel economy performance of lubricants. These new materials were synthesized using less hazardous elemental sulfur as opposed to other sulfur sources like hydrogen sulfide (H₂S) and carbon disulfide (CS₂). Using various bench and motoring friction torque tests, it was shown that friction reduction was benefited by utilizing low molecular weight organic backbone when designing molybdenum FMs. Also, it was shown that newly synthesized molybdenum-based FMs were comparable to other well-known MoS₂ precursors.
Non-varnishing and tribological characteristics of polyalkylene glycol-based synthetic turbine fluid
Varnish build-up in heavy duty gas turbines is a leading cause of costly unplanned shutdowns and resulting lost power generation capacity. The culprit is the conventional petroleum-based turbine oil, which breaks down to form varnish and sludge that cause servo valves to stick. Use of filtration to remove solid degradation by-products addresses a symptom but not the root cause of varnish formation: the petroleum-based turbine oils themselves. For the best protection against varnish-related shutdowns, a switch from petroleum-based turbine oil to non-varnishing polyalkylene glycol (PAG)-based synthetic turbine fluid was undertaken by four power plants in North America. PAG-based synthetic turbine fluid chemistry, its tribological characteristics and successful trials in GE 7FA heavy duty gas turbines (General Electric Company, Fairfield, CT, USA) are discussed in this paper. PAG-based synthetic turbine fluid has outperformed the hydrocarbon turbine oil in reduced sludge and varnish formation due to its polar nature and solvating power in plant trials as well as in a modified ASTM D 2893 test.
Elastohydrodynamic Models for Predicting Friction in Point Contacts Lubricated with Polyalphaolefins
This paper shows the results of a comparison between theoretical models for predicting friction in point contacts under elastohydrodynamic lubrication and the experimental results obtained in a Mini Traction Machine (MTM). The types of models used are two variations of the Newtonian Theory, applicable to polyalphaolefins (PAO): the Limiting Shear Stress Model and the Carreau’s model. The experimental stage includes a wide variation of the operating conditions, by using different control parameters, such as temperature, contact materials, slide-roll ratio, sliding velocity and load.
A Study of Antiwear Additive Film Build Up Using the MTM (Mini-Traction Machine)
The measurement of lubricant film thickness under elastohydrodynamic (EHL) contact conditions is well established and a variety of experimental techniques have been used, the most accurate and widely used of which is optical interferometry. This lends itself particularly well to the study of the all-important mixed and boundary regimes, since the films are of the same order of magnitude as the wavelength of light. The vast majority of these studies have been made under pure rolling conditions, since the necessary optical coatings preclude the use of high sliding speeds within the critical thin fluid film regime. These conditions are however precisely those required to activate ZDDPs and other antiwear additives, making accurate ‘in situ’ optical studies of additive film build-up difficult. A modification to the existing MTM (mini-traction machine) has allowed steel on steel contacts to be run under high sliding speed conditions, thereby allowing antiwear additive reactions to occur. By optically measuring the film thickness of these reaction films as they form, in tandem with friction measurements, a full picture of both friction and reaction film build-up can be made. Using a novel LED (light emitting diode) light source and accurate calibration procedures, the film thickness of the whole contact area can be measured down to a few nanometres. This paper presents a study of the behaviour of standard additive combinations under realistic operating conditions and describes the rig in detail.
Experimental investigations on the contact fatigue life under starved conditions
Over the last decades, severisation of the operating conditions in lubricated contact has led to the decrease of film thicknesses and to a new failure mode. For example, starvation occurs in high-speed or grease-lubricated bearings and in mechanisms operating with a limited lubricant supply. The film thickness under starved conditions depends on the amount of lubricant in the contact inlet and can be calculated or measured. But in many industrial applications, the lubricant supply is unknown. This paper presents the effect of starvation on the fatigue life of machine elements. The first part links the operating conditions, the lubricant supply and the traction coefficient. The second part presents fatigue life measurements for different lubricant flow rates.
Tribology and energy efficiency: from molecules to lubricated contacts to complete machines
The impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industThe impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industrial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.rial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.
Modification to the lubrication properties of xanthan gum fluid gels as a result of sunflower oil and triglyceride stabilised water in oil emulsion addition
A range of xanthan gum fluid gels and fluid gel emulsion mixtures have been constructed and their lubrication behaviour compared to high oleic sunflower oil. In addition, the lubrication properties have been measured after the addition of oil to the fluid gel, along with the effect of dispersing 10% (wt/wt) of a stabilised and un-stabilised oil continuous emulsion into the fluid gel postproduction. This study has highlighted a method of producing xanthan gum fluid gels as well as a fat mimetic formulation based on a xanthan gum fluid gel/oil formulation, which has lubrication properties equivalent to that of standard sunflower oil during soft tribology experiments. The final formulation was shown to have similar initial lubrication behaviour as sunflower oil with a 93% oil reduction.
WS2 nanoparticles – potential replacement for ZDDP and friction modifier additives
In high-pressure, high-temperature sliding contacts, WS2 nanoadditives react with the metal substrate to generate 100+ nm chemical tribofilms with a layered structure and excellent tribological properties. The friction, wear and micromechanical properties of WS2 tribofilms are compared with those of tribofilms formed by the zinc dialkyldithiophosphate (ZDDP) antiwear additive and ZDDP-organic friction modifier (OFM) mixture. Nanoindentation measurements showed that WS2 generates tribofilms with higher values of hardness and Young’s modulus than ZDDP and ZDDP + OFM, which explains its excellent antiwear properties. The friction performance of WS2 surpassed that of ZDDP + OFM. The striking reduction of boundary friction is credited to the layered structure of the WS2 tribofilm, with exfoliated/squashed WS2 nanoparticles that fill the gaps and cover the reacted tribofilm. In view of these results, WS2 proves to be a suitable candidate for the replacement of problematic lubricant additives currently in use.
Tribological Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)
The film-forming, friction, and antiwear properties of zinc dialkylphosphates (ZDPs) were investigated and compared with the corresponding zinc dialkyldithiophosphates (ZDDPs). The primary ZDPs generally show similar friction and antiwear performance to the primary ZDDPs, although some differences are seen between them in film-forming properties. For the secondary ZDP and ZDDP, there are some clear differences in their tribological properties. This indicates that the properties of the primary ZDPs and ZDDPs may be controlled predominantly by adsorbed films consisting the intact additives and/or their decomposition compounds, and that the properties of the secondary ones may be controlled by glassy reaction films consisting zinc/iron polyphosphates.
Lubricant film formation properties of alkyl imidazolium tetrafluoroborate and hexafluorophosphate ionic liquids
The film thickness and friction properties of four imidazolium ionic liquids (1-butyl-3-methyl tetrafluoroborate and hexafluorophosphate and 1-hexyl-3-methyl tetrafluoroborate and hexafluorophosphate) were measured for mixed rolling-sliding conditions and the results compared to an additised mineral oil. Film thickness results showed that three of the fluids demonstrated classical EHL behaviour; however, the 1-butyl-3-methyl fluids gave anomalously thick, time-dependent films at low speeds (<0.3 m/s). Post-test inspection of the specimens revealed a loosely bound brown film deposited in the track. Film formation appeared to originate in the bulk fluid where brown “fibrous” agglomerations were observed. These were flocculated by shear flow and deposited in the track after passing through the contact. Overall the RTIL friction coefficients were less than the mineral oil for all conditions investigated. In the absence of thick film formation all RTILs gave a similar friction coefficient of 0.03 in the boundary regime, which is thought to be due to electrical double layer formation. In the fluid film regime traction was determined by the nature of the anion.
Effect of Friction Material on the Relative Contribution of Thin-Film Friction to Overall Friction in Clutches
In order to prevent shudder in automatic transmissions, friction must decrease as the sliding speed between the friction plates in clutches decreases. Theoretical studies have shown that friction in wet clutches is a combination of boundary friction and the friction due to flow of fluid through the friction materials (thin-film friction). Therefore, these physical properties of oils should control the anti-shudder performance of automatic transmission fluids. Recently, we demonstrated that boundary and thin-film friction contribute to friction measured at low speeds in JASO SAE No.2 and LVFA tests. Two different friction materials are used in these tests and the relative effect of thin-film friction on low speed friction is greater in the JASO SAE No. 2 test than in the JASO LVFA test. This difference in the relative effect of thin-film friction on overall friction could be due to the different speed conditions used to measure low speed friction in the SAE and LVFA tests or could be due to the difference in the friction materials used in these tests.Therefore, we have expanded our research to determine the effect of sliding speed and friction material on the relative effect of thin-film and boundary friction on overall friction measured in friction material-steel contacts. As sliding speed increases, the relative contribution of thin-film friction to overall friction increases. The relative contribution of thin-film friction to overall friction at each speed depends upon the friction material used. The relative effect of thin-film friction to overall friction is greater for friction materials in which the rate of fluid flow into the material is slow. The relative effect of thin-film friction on overall friction is also greater for friction materials with lower surface roughnesses.
Polyalkylmethacrylates (PAMAs) are widely used as both viscosity index improvers and dispersant boosters in engine, hydraulic, and transmission oils. Since they are employed in a relatively high concentration in these roles, it is desirable that they be able to enhance other characteristics of a lubricant and, in particular, its boundary lubricating properties. A series of functionalized PAMAs have been synthesized that can adsorb from oil solution onto rubbing surfaces to produce thick boundary films. These films enhance lubricant film formation in slow speed and high temperature conditions and thus make a significant contribution to their lubricating ability. The current paper describes a systematic study of the influence of functionalized PAMAs on boundary lubrication performance. The high frequency reciprocating test rig (HFRR) was applied to investigate friction and wear under pure sliding. A new test method has been developed which allows wear to be monitored in a rolling and sliding contact based on the mini traction machine (MTM). This, in combination with other tests, is employed to investigate the influence of polymer architecture, functionality, concentration and molecular weight on friction and wear in a range of lubricant formulations. This enables the tailored design of polymers which offer low friction and wear properties.
The Influence of Surface Roughness on the Lubrication Properties of Adsorbing and Non-Adsorbing Biopolymers
The lubrication properties of a glycoprotein (pig gastric mucin or PGM) and a high-molecular-weight hydrosoluble polymer (guar gum) have been studied. Friction has been measured over a wide range of entrainment speeds and Stribeck curves have been obtained spanning the boundary, mixed and hydrodynamic lubrication regimes. The adsorption properties of the polymers have also been assessed using evanescent wave spectroscopy. The results show that the polymer that adsorbs on solid surfaces is able to reduce friction in the boundary lubrication regime (PGM). Guar, which does not adsorb on surfaces, shows high friction in boundary lubrication but still promotes the onset of mixed lubrication; thus friction starts to fall from its boundary values at low speeds. These results can be explained in classical terms of entrainment of polymer solution into the thin film conjunction and associated shear thinning in the contact inlet. With roughened surfaces, a shift of the Stribeck curves towards high speed is observed.
Rheology and tribological properties of Ca-alginate fluid gels produced by diffusion-controlled method
A new method has been developed for the production of alginate fluid gels. By using the turbulent flow field generated within a pin stirrer, fluid gel particles can be produced in a reproducible and controlled manner. Using this approach yielded an average particle diameter smaller than 10 μm for all alginate fluid gels produced. A weak gel-like behaviour is reported for the produced materials, which displayed a gradual increase of G′ upon increasing polymer concentration. Steady shear results revealed the existence of an apparent yield stress, indicating that a significant interparticle potential persisted after the production process had finished. Soft-tribology was used to assess the lubrication properties of Ca-alginate fluid gels. The entrainment of particles in the ball-on-disc contact resulted in a localised increase of friction in the mixed regime, which was found to be dependent on both particle intrinsic properties and the surface roughness of the disc. An increase of polymer concentration resulted in an overall decreased friction for systems with similar particle dimensions. These distinct but complementary functional responses of fluid gels, arising from increasing alginate concentration, offer real possibilities towards building microstructures with enhanced sensory attributes.
The Influence of Anchoring-Group Structure on the Lubricating Properties of Brush-Forming Graft Copolymers in an Aqueous Medium
We have compared the lubricating properties of two different PEG-grafted, polycationic, brush-forming copolymers to gain a deeper understanding of the role of the polyionic backbone in the lubricating behavior of such materials, when used as additives in aqueous lubricant systems. Previously, poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) has been shown to adsorb onto oxide surfaces from aqueous solution and substantially lower frictional forces. Poly(allylamine)-graft-poly(ethylene glycol) (PAAm-g-PEG), which also has a polycationic backbone, has been synthesized in several different architectures, and its performance investigated via adsorption tests, rolling- and sliding-contact tribometry, and the surface forces apparatus. These tests show a clear reduction of friction forces with PAAm-g-PEG compared to water alone. However, when compared with PLL-g-PEG, while PAAm-g-PEG copolymers did not adsorb to the same extent or exhibit as high a lubricity in sliding geometry, they showed a similar lubricating effect under rolling conditions. The difference in the chemical structure of the backbones, especially the flexibility of the anchoring groups, appears to significantly influence both the extent and kinetics of polymer adsorption, which in turn influences lubrication behavior.
Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts
Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces. In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.
Soot Wear in Diesel Engines
In response to regulatory requirements, lubricant manufacturers are seeking oils that minimize soot thickening and the accompanying soot wear. Formulation technology is being developed by additive manufacturers to satisfy these requirements. For example, such work is in progress at Chevron Oronite Company LLC, using the Cummins M-11 exhaust gas recirculation (EGR) engine test as a surrogate for the anticipated soot wear test for PC-10. Simultaneously, the authors developed bench tests to screen candidate formulations and reduce costs., A ball-on-disc sliding wear test, using a PCS Instruments MTM® tribometer, has been investigated. Sliding conditions at high pressure are required for soot polishing wear. Conditions that correlate tribometer test results with M-11 engine results at high soot concentrations (∼9 per cent) have been found. Both ball wear and Stribeck curves were determined in these tests. The high-wear oil progresses from mixed lubrication conditions to boundary lubrication at higher sliding speeds than the low-wear oil. X-ray photoelectron spectroscopy experiments were also conducted on the ball wear scars, revealing differences in the chemical constitution of the tribofilms from the two oils.
Soft-tribology: Lubrication in a compliant PDMS–PDMS contact
We investigate the influence of surface roughness and hydrophobicity on the lubrication of a soft contact, consisting of a poly(dimethylsiloxane) (PDMS) sphere and a flat PDMS disk. The full Stribeck curves, showing boundary, mixed and elasto-hydrodynamic (EHL) lubrication, are presented for varying surface roughness and hydrophobicity. It is found that neither surface roughness nor hydrophobicity influence the friction coefficient (μ) within the EHL regime. However, increasing surface roughness decreases μ in the boundary regime, while extending the limits of the boundary and mixed lubrication regimes to larger values of the product of velocity and lubricant viscosity (Uη). The transition from the mixed lubrication to EHL regime is found to take place at lower values of the film thickness parameter Λ for increasingly rough surfaces. We found Λ=0.7 in the case of a root mean square (r.m.s.) surface roughness of 3.6 μm, suggesting that the effective surface roughness in a compliant compressed tribological contact is lower than that at ambient pressures. Rendering the PDMS surface hydrophilic promotes full-film lubrication and dramatically lowers μ in the boundary regime by more than an order of magnitude. This influence of surface wetting is also displayed when examining a range of lubricants using hydrophobic tribopairs, where the boundary μ decreases with decreasing lubricant–substrate contact angle. Implications of these measurements are discussed in terms of the creation of model surfaces for biotribological applications.
The influence of soot and dispersant on ZDDP film thickness and friction
The effect of dispersed soot in engine oils is an increasingly important issue in terms of both engine durability and fuel efficiency. Using carbon black as a soot analogue, a study has been carried out to investigate the main factors that determine the impact of soot on friction and ZDDP film formation in formulated oils. It has been found that dispersed carbon black can rapidly remove ZDDP reaction films by abrasion. However, this removal can be prevented or limited by the choice of an optimal dispersant additive.
Boundary Lubrication of Oxide Surfaces by Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) in Aqueous Media
In this work, we have explored the application of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) as an additive to improve the lubricating properties of water for metal-oxide-based tribo-systems. The adsorption behavior of the polymer onto both silicon oxide and iron oxide has been characterized by optical waveguide lightmode spectroscopy (OWLS). Several tribological approaches, including ultra-thin-film interferometry, the mini traction machine (MTM), and pin-on-disk tribometry, have been employed to characterize the frictional properties of the oxide tribo-systems in various contact regimes. The polymer appears to form a protective layer on the tribological interface in aqueous buffer solution and improves both the load-carrying and boundary-layer-lubrication properties of water.
The Influence of Molecular Architecture on the Macroscopic Lubrication Properties of the Brush-Like Co-polyelectrolyte Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) Adsorbed on Oxide Surfaces
The co-polymer poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) has been investigated as a potential biomimetic boundary-lubrication additive for aqueous lubrication systems. In this work, the influence of the co-polymer’s architecture on its tribological performance has been investigated. The architectural parameters investigated comprise side-chain (PEG) length, Lys/PEG grafting ratio and backbone chain (PLL) length. The tribological approaches applied in this work include ultra-thin-film interferometry, the mini-traction machine (MTM), and pin-on-disk tribometry. Both an increase in the molecular weight of the PEG side chains and a reduction in the grafting ratio result in an improvement in the lubricating properties of aqueous PLL-g-PEG solution at low speeds. MTM measurements show that an increase in the molecular weight of the PLL backbone results in an increase of the coefficient of friction.
Macrotribological Studies of Poly(L-lysine)-graft-Poly(ethylene glycol) in Aqueous Glycerol Mixtures
We have investigated the tribological properties of surfaces with adsorbed poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) sliding in aqueous glycerol solutions under different lubrication regimes. Glycerol is a polar, biocompatible liquid with a significantly higher viscosity than that of water. Macrotribological performance was investigated by means of pin-on-disk and mini-traction-machine measurements in glycerol-PLL-g-PEG-aqueous buffer mixtures of varying compositions. Adsorption studies of PLL-g-PEG from these mixtures were conducted with the quartz-crystal-microbalance technique. The enhanced viscosity of the glycerol-containing lubricant reduces the coefficient of friction due to increased hydrodynamic forces, leading to a more effective separation of the sliding partners, while the presence of hydrated polymer brushes at the interface leads to an entropically driven repulsion, which also helps mitigate direct asperity–asperity contact between the solid surfaces under boundary-lubrication conditions. The combination of polymer layers on surfaces with aqueous phases of enhanced viscosity thus enables the friction to be reduced by several orders of magnitude, compared to the behavior of pure water, over a large range of sliding speeds. The individual contributions of the polymer and the aqueous glycerol solutions in reducing the friction have been studied across different lubrication regimes.
Influence of sanding parameters on adhesion recovery in contaminated wheel–rail contact
Adhesion in wheel/rail contact influences performance and safety of railway traffic. Low adhesion brings problems during braking and traction. Sanding is the most common way how to increase adhesion when the poor contact conditions due to a contamination occur. On the other hand, excessive sanding leads to higher wear of wheel and rail. To optimize the sanding process, description of the influence of sanding parameters on the adhesion in the contaminated contact is highly required.
In this work a new twin-disc machine in scale 1:3 was developed and addressed to the study of wheel/rail adhesion under different contact conditions. An influence of sanding parameters such as sand quantity, wheel slip and rolling speed was investigated using a real sanding system in the contact contaminated with water, leaves and wheel flange grease.
It has been shown that under wet, leaf or grease contamination, quantity of the sand applied during fixed time period has significant effect on adhesion recovery only for low wheel slip and low rolling speed. In the contaminated contact the effect of sanding on adhesion recovery increases with wheel slip and rolling speed.
Behaviour of lubricants in the mixed elastohydrodynamic regime
A range of lubricant base fluids have been chosen and their film-forming properties measured in the mixed elastohydrodynamic regime in pure rolling conditions. The Dowson-Hamrock elastohydrodynamic film thickness equation was found to remain valid over a large film thickness range for the tested base fluids. For hexadecane, the film thickness equation remains valid down to a film thickness of about 0.5 nm, a size equivalent to a molecular monolayer on each solid surface. However thicker films than predicted by elastohydrodynamic theory were found for all the other tested fluids at low speeds in the very thin film regime. This effect can be interpreted as boundary film formation by the fluids. The origins of these boundary films are still being explored but may be due to adsorption of molecules of the fluid on the solid surfaces and/or enhancement of viscosity due to the presence of a solid surface. This study has shown that full film lubrication is still possible for smooth surfaces in pure rolling in the very thin film regime where the film thickness is far smaller than the composite surface roughness.
Behaviour of colloidally-dispersed solid particles in very thin film lubricated contacts
The behaviour of colloidal solid particles dispersed in lubricants is of interest for a number of reasons. Firstly, many of the additives used in lubricants are actually in the colloidal rather than the true, solution state. Secondly, soot in engine oils and lubricant degradation products are generally colloidal. Thirdly, there is an increasing interest in the development of lubricants containing dispersed nanoparticles to limit friction and wear. For all the above it is important to understand when and how colloidal solid particles pass through lubricated contacts and also their behaviour in such contacts.
In the current study, the behaviour of dispersed particles covering a range of sizes between 20 and 200 nm diameter has been studied in lubricating contacts. Optical interferometry has been employed to explore the behaviour of such particles in very thin film contacts. Friction has also been measured. Of particular interest was the effect of slide-roll ratio on particle entrainment and the influence of the ratio of the particle diameter to the elastohydrodynamic lubricant film thickness.
Film-Forming Additives – Direct and Indirect Ways to Reduce Friction
This paper reviews the current understanding of lubricated contacts
and the origins of friction in terms of the Stribeck curve. It discusses
the various ways that lubricant additives can modify friction in thinfilm
lubricated contacts, examining both the nature and properties of
the films formed by these additives, as revealed by film thickness measurements,
and their consequent effect on the Stribeck friction curve.
A study of parched lubrication
The effect of a ZnDTP anti-wear additive on the micropitting resistance of carburised steel rollers
Improved Processing of High Alloy Steels for Wear Components in Energy Generation Systems, Transportation and Manufacturing Systems
Oak Ridge National Laboratory (ORNL), in Partnership with Avure Technologies Inc. and Carpenter
Technology Corporation, explored methods to improve durability of steels considered essential to
bearings in wind turbine applications by using hot isostatic pressing of steel alloy powders as a
The objective of the project was to evaluate the wear mechanism of selected high alloy tool steels,
440C, 440XH, and M62, fabricated by powder metallurgy (PM) techniques via the hot isostatic press
(HIP) process, and compare these results to two conventionally produced wrought alloys, 52100 and
M50. Samples from the five candidate metals were characterized microstructurally via optical and
scanning microscopy methods. A micro-pit wear testing machine was purchased and used to simulate
bearing loads in a wind turbine gear box The candidate samples were subsequently heat treated in
accordance with industrial protocols and machined into wear test specimens suitable for the micro-pit
wear testing machine. . Wear testing produced surfaces that were subsequently analyzed for wear
and damage using both optical microscopy and scanning electron microscopy methods.
Optimized rolling-sliding experiments with a relatively small slip ratio (5%) and higher load (650N)
exhibited signs of abrasive wear on all specimens, but not all specimens had the same degree of
micro-pitting damage. The conventional wrought 52100 steel had the most severe micro-pitting,
consisting of distorted craters, many of which were connected by micro-cracks. Cross-sectional
examination showed multiple micro-cracks extending into the alloy microstructure from the same pit.
The depth of the cracks was several times the depth of the micro-pit. Type 440XH had the least
amount of micro-pitting damage under similar test conditions. Micro-pits on 440HX were more
localized and the micro-cracks associated with them tended to avoid carbides during propagation.
Abrasive processes were observed on all the test specimens and were produced by debris from micopitting.
The combination of micro-pitting in conjunction with abrasive processes could work together
to worsen the surface damage, diminishing bearing service life.
The results suggest that there is a potential advantage of PM consolidated high alloy tool steels, such
as 440XH produced via hot isostatic pressing to better resist micro-pitting when compared to other
alloy types and wrought processing methods.
Experience with a Disc Rig Micropitting Test
The experimental work carried out was aimed at developing a test method that was able to consistently
produce micropitting damage and could discriminate between a good oil (i.e., one that rarely produces
micropitting in service) and a poor oil (i.e., one that does produce micropitting in service). The small–scale
3–Disc test rig that was used for this work employs 3 discs to apply the test load to a 12mm–diameter test
roller. This test geometry allows a large number of stress cycles (typically 600,000 to 800,000 cycles/h) to be
generated at the contact track on the roller.
The disc rig control system allows test parameters such as entrainment velocity, contact stress and slide/roll
ratio at the disc/roller contacts to be accurately and independently controlled. This enables the effect of key
parameters to be studied in isolation, which is something that cannot be easily achieved using conventional
gear test rigs.
The early work carried out using the disc rig was aimed at producingmicropitting damage by operating the rig
at contact conditions similar to those used in the FZG micropitting gear test method. These early tests
confirmed that the damage produced to the roller track exhibits characteristics that are typical of micropitting
damage, and showed that the severity of the micropitting produced was affected by the amount of running–in
carried out on the roller prior to applying the full test load.
A test procedure has been developed which provides a good level of repeatability and which allows
discrimination between oils which producemicropitting in service and those which do not. In addition, a study
of the effect of slide/roll ratio (SRR) has shown that the severity of micropitting damage increases as SRR
increased, whereas at 0% SRR nomicropitting occurred and, at negative SRRs,microcracking occurred but
not micropitting. This is the way that SRR seems to affect micropitting in gears.
Effect of lubricants on micropitting and wear
Micropitting was studied using a three-contact disc machine having a central roller in contact with
three harder, annular counter-discs (‘‘rings’’) of precisely controlled roughness. Roughness, running
conditions, base stock and additive concentration were varied. The response of the same lubricants in a
reciprocating sliding wear test operating in the boundary regime was also studied.
Results of experimental studies of the rolling contact behaviour of carburised steel rollers are
reported. All the tests with the additive present led to micropitting. However, severe micropitting wear
was only observed when the calculated film thickness exceeded 12% of the centre-line average
roughness of the rings.
It was found that there was an approximately inverse correlation between the micropitting damage
in the disc machine test and the mild wear in the reciprocating sliding test. This was attributed to the
tendency of anti-wear additives to prevent running-in of the rough surface.
The Influence of Engine Oils on Friction Reduction
Antiwear Performance of Low Phosphorus Engine Oils on Tappet Inserts in Motored Sliding Valvetrain Test
The overall purpose of this research is to determine the antiwear capability of low phosphorus engine oils containing 0.05 wt% phosphorus. The antiwear performance of 0.05 wt% phosphorus engine oils was evaluated using a laboratory valvetrain bench test rig coupled with an on-line wear measurement technique and a high frequency reciprocating rig (HFRR). Low phosphorus engine oils were compared with GF-3 engine oils containing 0.1 wt% phosphorus. In addition to fresh oils, long drain used oils from fleet vehicles were also analyzed and investigated. This information is important to develop engine oil formulations to meet the latest government emission and fuel economy requirements. The results indicate that by appropriately selecting and balancing supplemental antiwear and/or antioxidation additives the wear loss due to the reduction of zinc dialkyldithiophosphate (ZDDP) may be compensated or even reduced
Evaluation of EHD films by electrical capacitance
The elastohydrodynamic (EHD) lubrication regime occurs in many machine elements where a combination of hydrodynamic effect, elastic deformation of the loaded surfaces and increase in the viscosity of the lubricant with pressure ensures the formation of a very thin, but continuous film of lubricant separating the contacting surfaces. Electrical methods to determine this film’s thickness have preceded optical methods, which are widely used today. Although they generally give more qualitative thickness information, electrical methods have the main advantage that they can be applied to metallic contacts in machines, which makes them useful tools in the study of elastohydrodynamically lubricated contacts. This paper is part of a larger study on the application of electrical capacitance for the evaluation of film formation in EHD contacts. The main focus is on the quantitative measurements of film thickness using electrical capacitance. A new approach allowing the lubricant film thickness to be extracted from the measured capacitance is developed using a chromium-coated glass disc and subsequently applied to a steel-on-steel contact. The results show good agreement with optical measurements and theoretical models over a range of film thickness.
The Tribological and Chemical Analysis of Top Ring Zone Samples of Fully Formulated Oil Taken From a Four Stroke Gasoline Engine
With increasing pressure on engine oil manufacturers to extend oil drain intervals and reduce fuel consumption, whilst changing the composition of fully formulated oils to meet the new CEC, ILSAC and OEM specifications, there is an ever increasing need to understand the effect of oil degradation on the operating conditions and tribological performance of engines . This work samples oil from the rear of the top piston ring of an engine during the first 15 minutes from cold start and operating at steady state under three different loads. These samples, used 40 hour sump oil and fresh oil have been subjected to tribological tests and chemical analysis.
Pushing the Boundaries of the HFRR: Impact of Increased Test Severity on Wear
The high frequency reciprocating rig (HFRR) was developed in the early 1990s as a test method to assess diesel fuel lubricity in order to provide wear protection for fuel injection pumps. This was necessary in response to the many field failures that occurred following the introduction of ultra-low sulphur diesel in Sweden. The prevalent fuel injection equipment (FIE) technology at this time utilised rotary pumps capable of reaching maximum fuel pressures of ∼650 bar in systems for direct injection engines.
The continued drive for efficiency led to many changes in FIE technologies, materials and pressures. Modern high pressure common rail pumps reach significantly higher pressures, with 2200 bar available today and pressures up to 3000 bar discussed in the industry. Alongside these hardware changes there have been significant changes in the diesel fuel, with a continued move towards more highly refined fuels, and the introduction of highly paraffinic sources from Fischer-Tropsch processes and hydrogenated vegetable oils.
Despite these changes there have been no widespread field issues since the introduction of HFRR specifications. The objective of this work was to understand the flexibility of the HFRR test by investigating the impact of increased test severity on wear. The main variable investigated was the effect of load, which was varied from 2-10N. Other variables investigated were frequency, test duration and stroke length. Additionally, preliminary studies were conducted using phosphate coated specimens more in line with those expected in current and future FIE systems.
HFRR tests utilising a more highly loaded contact exhibit an increased mean wear scar. Trends observed are similar to those under standard conditions (2N), with differentiation between untreated fuels with poor lubricity and those that are additised. Certain fuels exhibit unusual friction coefficient behaviour, with temporary periods of significantly elevated friction. This is attributed to micro-seizure; however under additised conditions it was not observed. One interesting observation was a correlation between wear scar pattern and additive technology. Ester based lubricity additives give a striated wear scar, whereas acid technologies produce a stippled pattern, likely due to low level corrosion, a known possibility with this type of chemistry. The relatively soft nature of the phosphate coatings used meant that under the highly loaded contact conditions of the HFRR they quickly wore through; however the presence of additives minimised this wear and prolonged coating lifetimes. Further work indicated that other fuel additives such as cetane improver can impact lubricity performance, and may require different lubricity improver treat rates in order to meet specifications.
In summary, increasing contact severity within the HFRR increases the level of wear in the system, with some evidence of micro-seizure. However, the use of additives under these conditions can reduce the wear to acceptable levels and prevents micro-seizure events. The trends and discrimination between conventional and higher load conditions remains similar. This indicates that modifications to the HFRR test method are unlikely to provide a means of defining how to achieve additional protection for future more severe conditions and the current test method remains suitable.
New Method of Measuring Permanent Viscosity Loss of Polymer-Containing Lubricants
Effect of oil rheology and chemistry on journal-bearing friction and wear
Legislation and market pressures are calling for increased engine power, reduced engine size, and improved fuel consumption. The use of low-viscosity lubricants is considered as a means to enhance fuel economy by reducing viscous friction, particularly in engine bearings. Journal bearings mostly operate under hydrodynamic lubrication with a thin film of oil separating the journal and bearing shell. There are, however, certain conditions, especially under high load or low speed, when the film thickness will be low enough to allow boundary lubrication to occur. In this study, the effect of lubricants with different viscosities, different types of viscosity modifiers, different additives, different types of dispersants, and different lubricant formulations have been studied under hydrodynamic and boundary lubrication regimes. For hydrodynamic conditions, a high-temperature high-shear viscometer, meeting the requirements of ASTM D4741 was used to measure viscosity at 106 s−1. In addition, a new ultra high-shear viscometer, from PCS Instruments, was used to measure viscosity at shear rates near to 107 s−1. Bearing weight loss and load bearing capacity were measured on a rig developed in-house using a specially designed half-bearing shell loaded against a rotating journal. A PCS journal-bearing rig was used to measure the bearing friction under transient load.
Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant, contacts
A laser-induced fluorescence (LIF) technique has been used to measure fluid film thickness in a compliant, sliding contact under low-load/low-pressure conditions. The soft contact between an elastomer hemisphere and a glass disc is lubricated by a liquid containing fluorescent dye. The contact is then illuminated with 532 nm laser light through the glass disc, and viewed with a fluorescence microscope. From the intensity of emitted radiation, film thickness maps of the contact are determined. Previous calibration procedures have used a separate calibration piece and test specimen with possible errors due to differences in reflectivity between the calibration and test specimens. In the work reported in this paper a new calibration process is employed using the actual test sample, thereby avoiding such errors.
Results are reported for a sliding contact between PDMS and glass, lubricated with glycerol and water solutions under fully flooded and starved conditions. It was found that, for glycerol, the measured film thickness is somewhat lower than numerical predictions for both lubrication conditions. It is suggested that a combination of thermal effects and the hygroscopic nature of glycerol may cause the lubricant viscosity to drop resulting in thinner films than those predicted for fully flooded contacts. Starvation occurs above a critical entrainment speed and results in considerably thinner films than predicted by fully flooded I-EHL theory. A numerical study has been carried out to determine the effect of the observed starvation on film thickness. Predicted, starved film thickness values agree well with those obtained experimentally.
The Study of Very Thin Lubricant Films in High Pressure Contacts Using Spacer Layer Interferometric Methods
The key role of a liquid lubricant is to form a separating film and thereby limit direct contact between asperities on opposing, solid, rubbing surfaces. If the film thus formed has lower shear strength than the asperity conjunctions, it will reduce friction. Liquid lubricant films will also generally produce a reduction in damage accumulation and thus wear of the solid surfaces via a number of different mechanisms. These include prevention of adhesive or abrasive contact between asperities, smoothing of contact stress distribution across the contact and accommodation of third body, solid particles such as wear debris within the film.
Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry
Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.
Elastohydrodynamic Study of Blends of Bio-Based Esters with Polyalphaolefin in the Low Film Thickness Regime
The film thickness in elastohydrodynamic (EHD) conditions for soybean oil (SBO), oleic estolide ester (EST) and their binary blends with polyalphaolefins (PAO2 or PAO40) were studied at 30 and 100 °C. Changes with time, for up to 200 min, were monitored. SBO and its blends with the lower viscosity PAO2 showed initially good agreement with the Hamrock–Dowson (H–D) equation down to 1–3 nm film thickness. 60 min or more after the start of the measurements, boundary layers with thickness up to 4.7 nm were observed. The blend of SBO with the more viscous PAO40 showed initially a good agreement with H–D at 100 °C. Negative deviations in film thickness were observed 15 min after the start of the measurements. At extended periods of time, up to 200 min, they were less pronounced but still detectable. EST–PAO2 blend showed initially formation of boundary layers with thickness around 2 nm. The boundary layer at 30 °C did not change for 200 min, while at 100 °C showed a decrease in thickness and/or viscosity with time. The EST and the EST–PAO40 blends showed good agreement with the H–D equation and did not display a boundary or fractionation layer within 200 min
Assessing boundary film formation of lubricant additivised with 1-hexyl-3-methylimidazolium tetrafluoroborate using ECR as qualitative indicator
Boundary film formation of ionic liquid (IL) 1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM][BF4], as an additive of hydrocracked mineral oil is evaluated for a steel–steel contact. Accelerated wear testing was carried out using a high frequency reciprocating rig (HFRR) under these test conditions: maximum contact pressure of 1.04 GPa, two different temperatures (40 and 100 °C) and three different times (300, 1800 and 3600 s). Wear volumes were measured using a non-contact 3D profilometer while worn surfaces were characterized using XPS. Furthermore, electrical contact resistance (ECR) was used as qualitative indicator of the formation of electrically insulating films in the sliding contact.
Experiments show that the rate of boundary film formation of base oil-ionic liquid blend is faster than neat base oil. Moreover, ECR was in good agreement with film formation and friction behaviour. Ionic liquid as additive not only decreases the time of running-in but also the time of wear-in. Results of neat base oil show that wear-in was not reached during any duration of tests. The improved friction and wear results for the blend are closely related to the boundary film formation on the worn surfaces due to the reactivity of the anion with the steel surfaces.
The Effect of Wetting and Surface Energy on the Friction and Slip in Oil-Lubricated Contacts
This work shows the influence of solid–liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson’s linear correlation coefficient of 0.999). By tailoring the wetting and surface energy—achieved by varying the DLC/DLC contacts with different types of DLC coatings—the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A “Slip-inducing interaction model based on surface forces” is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid–liquid interface.
Influence of thermal effects on elastohydrodynamic (EHD) lubrication behavior at high speeds
This paper describes a study of point contact elastohydrodynamic (EHD) lubrication behavior at high speeds (up to 20 m s−1). Central film thicknesses were measured by optical interferometry device. The influence of slide-roll ratio and operating temperature on the central film thickness was determined. The influence of thermal effects on the reduction of film thickness was discussed via the analysis of numerical simulation method considering thermal effects. Subsequently, the experimental data was used to amend a set of unified parameters for the thermal corrections for different types of oil at high speeds
Gear Oil Viscosity Modifiers and Their Impact on Efficiency
The essential functions of an automotive gear lubricant viscosity modifier (VM) are to maintain fluid film protection of gears and bearings as the lubricant warms to operating temperature, to improve cold temperature flow for efficient lubrication in winter and to minimise viscosity loss in a high shear, high load environment. Although a number of different VM technologies can be considered appropriately resistant to permanent shear for automotive gear oils use, their effect on fluid efficiency can vary widely. This paper outlines the study of a series of different VM technologies assessing relationship of operating temperature, operating viscosity and axle efficiency under different load and speed regimes. The fluids presented were formulated to equal kinematic viscosity at 100 °C but vary widely in viscosity index (VI), elastohydrodynamic (EHD) traction and EHD film thickness. The differences observed during efficiency testing were qualitatively related to the rheological properties of the VM technology present and further related to the operating temperature and operating viscosity.
The Lubricant Film-Forming Properties of Modern Fire Resistant Hydraulic Fluids
Fire resistant hydraulic fluids tend to show significantly poorer tribological performance in hydraulic systems than conventional mineral oil-based fluids. There have recently been performance problems associated with increases of operating temperatures of mining hydraulics. This paper describes measurements of the elastohydrodynamic and boundary film-forming properties of a range of different hydraulic fluid types at temperatures up to 80°C. These are compared with friction and wear results obtained using the same fluids.
Elastohydrodynamic (EHD) Traction Properties of Seed Oils1
The elastohydrodynamic traction coefficient (tc) properties of nine seed oils of varying chemical structures, polyalphaolefin oil (PAO) and hexadecane, were investigated using a ball-on-disk traction apparatus. The seed oils were castor oil, a triglyceride with hydroxyl functional group; jojoba, a monoglyceride; and seven triglyceride seed oils with varying fatty acid compositions. Two types of experiments were conducted at constant temperature (40 or 100°C) and constant load (10, 20, 30, or 40 N): tc as a function of slide-to-roll ratio (srr) at 1 m/s entrainment speed (u); and tc as a function of u at 50% srr. In both types of experiments, tc increased with decreasing temperature, increasing load, and increasing srr. All u vs. tc experiments gave the familiar Stribeck-type profiles. A maximum in tc values was observed in some srr vs. tc experiments. Regression analysis showed excellent agreement between limiting tc (tc at 1 m/s u and 50% srr) values from these two types of experiments. Hexadecane and PAO displayed higher tc values than the seed oils, even though their viscosities were up to 80 and 7 times lower, respectively, than that of seed oils. This observation cannot be rationalized using molecular structure arguments. The results were attributed to differences in polarity between the two groups of oils. Unlike PAO and hexadecane, seed oils are polar, adsorb on friction surfaces, and lower boundary friction, which contributes to the lowering of tc in the EHD regime.
Lubricating oil composition
In situ viscosity measurement of confined liquids
The viscosity of liquids governs crucial physical and engineering phenomena, ranging from diffusion and transport processes of nutrients and chemicals, to the generation of friction and the physics of damping. Engineering fluids frequently experience local conditions that change their bulk rheological properties. While viscosity data can easily be acquired using conventional rheometers, the results are not always applicable to fluids under engineering conditions. This is particularly the case for fluids being sheared at high pressure under severe confinement, which experience very high shear stresses and often show extensive shear thinning. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This work describes a novel in situ viscosity measurement technique to fill this gap. It involves the quantification of the fluorescence lifetime of a fluorescent dye that is sensitive to viscosity. The capability of the developed technique is verified by taking measurements in submicron thick films of two model fluids confined in a ball on flat contact. Viscosity measurements were successfully performed at pressures up to 1.2 GPa and shear rates up to 105 s−1. Spatial heterogeneity in viscosity caused by variations in pressure within the thin fluid film could be observed using the technique. It was also possible to detect differences in the rheological responses of a Newtonian and a non-Newtonian fluid. These first in situ high pressure, high shear viscosity measurements demonstrate the versatility of the proposed technique in providing information on the viscosity in conditions where contemporary techniques are insufficient. More importantly it highlights the complexity of the rheology of engineering fluids and provides a means of verifying existing theories by performing in situ measurements. Information on local viscosity is crucial for understanding the physics of confined fluids and to facilitate improvements in engineering technology.
Reactions of zinc-free anti-wear additives in DLC/DLC and steel/steel contacts
In this work, the ability of ashless anti-wear additives to form protective tribofilms on diamond-like carbon (DLC) and steel surfaces was investigated and compared to the reactions of ZnDTP. Reciprocating sliding tests were performed under mild tribological conditions for steel/steel and DLC/DLC contacts to avoid wearing through the DLC coating. A comparison of the friction behavior of ZnDTP with two ashless additives, a butylated triphenyl phosphorothionate (b-TPPT) and an amine phosphate (AP), indicated that the latter additive behaved in a different manner to the first two. b-TPPT shows the lowest friction coefficient for DLC/DLC contacts whereas the AP give the lowest friction coefficient for steel sliding against steel. AFM and ESEM were performed to analyze the surfaces and showed the build up of tribofilms from ZnDTP and b-TPPT onto both the steel and DLC surfaces. No evidence of such a film formation on DLC was observed when the AP were used.
Tribofilm formation from ZnDTP on diamond-like carbon
Increasing use of DLC coatings in oil-lubricated, mechanical systems has led to a great deal of interest in optimising the interactions between lubricant additives and coatings. We have looked at the formation of a tribofilm from ZnDTP on diamond-like carbon (DLC) when sliding in oil against DLC on a polished steel substrate under very low wear conditions. It was found that the tribofilm formed in the absence of iron adhered very weakly to the surface and could be removed by washing in an ultrasonic bath. The wear was too low to be detected with laser profilometry. AFM imaging of the surfaces of steel and DLC after sliding of steel/steel and DLC/DLC showed that the tribofilm formed on DLC had a very different morphology to that formed on steel. EDX was used to demonstrate that the tribofilm contained zinc, sulphur and phosphorus. ToF-SIMS measurements showed that iron was neither present at the DLC surface nor in the tribofilm.
Ionic liquids as a neat lubricant applied to steel–steel contacts
This paper studies the use of 3 ionic liquids ([(NEMM)MOE][FAP], [BMP][FAP] and [BMP][NTf2]) as neat lubricant within steel–steel contact conditions. Tribological tests (at 40 and 100 °C) were conducted in a HFRR tribometer and hence a complementary study was developed using a MTM tribometer. The wear surface on the discs was measured after the HFRR tests by confocal microscopy and also analyzed by SEM and XPS. The [BMP][NTf2] showed the lowest friction coefficient in the MTM and HFRR tests at 40 °C but at 100 °C its tribological behavior worsened due to its lowest viscosity. Similar results were found for wear behavior. Both antifriction and antiwear results were related to the tribofilms formation from the ECR and XPS measurements.
Performance of Friction Modifiers on ZDDP-Generated Surfaces
The efficiency of a hypoid axle—a thermally coupled lubrication model
The final drive unit in road vehicles, such as medium and heavy trucks, and four-wheel-drive and rear-wheel-drive passenger cars, usually consists of a hypoid or spiral bevel geared transmission and differential, housed in a self-contained, dip-lubricated axle. Such units are subjected to very variable duty—including extreme combinations of speed, gradient, applied torque and external temperature—and are typically cooled by natural and forced convection on the exterior surface. On the other hand, there are appreciable internal power losses due to gear friction and churning and to bearing and seal losses. These losses are highly dependent upon the lubrication regime of the internal components and hence to the thermal behaviour of the entire axle.
In the present paper, we describe a thermally coupled model of axle lubrication. The torque and speed demand is first found from a specified duty (“drive cycle”) which includes terrain as well as speed-versus-time and external temperature data. The evolution of sump oil and component temperatures is followed, and increments of energy loss evaluated in each time-step. Elastohydrodynamic film thickness is determined for the hypoid gear set, using a development of Buckingham’s method, and friction losses calculated using a simple oil rheological model based on tribometer (MTM) testing. Churning, seal and bearing (speed-dependent) losses are found using empirical algorithms. Energy losses over complete drive cycles for different lubricants are derived, enabling the relative fuel economy for different oils to be evaluated.
Results show that (i) the bulk temperature rise of the axle is highly dependent on the specified vehicle duty and (ii) the efficiency can be strongly influenced by choices available to the lubricant formulator. Taken together, these findings suggest that specialist axle lubricant formulations for particular vehicle types and applications will be attractive as a route to optimum fuel economy.
Gear oil additive
Gear oil formulations comprising a gear oil and a film forming agent are disclosed. The film forming agent comprises a polymeric ester which is the reaction product of at least one polyfunctional alcohol, a dimer fatty acid, an optional aliphatic dicarboxylic acid having 5 to 18 carbon atoms and one or more ingredients to reduce the acid value of the polymeric ester to below 5 mgKOH/g with the resultant polymeric ester having a kinematic viscosity at 100° C. ranging from 400 to 5000 mm2/s and a weight average molecular weight ranging from 5000 to 20000. When used as an automotive gear oil formulation the specifications for API GL-4 gear oils are at least satisfied. Use of the gear oil formulation in manual transmissions, transfer cases and differentials and use of the gear oil formulation in an industrial gear suitable for lubricating spur, helical, bevel, worm and hypoid gears are disclosed. Methods of lubrication are also disclosed.
Effects of Gear Oil Properties on Pitting Life in Rolling Four-Ball Test Configuration
There is a connection between the efficiency of oils and their wear and/or surface damage protective properties, an area not so well described in the literature. One such damage mode is macroscale contact fatigue on gear tooth flank surfaces, also called pitting. The present study is aimed at investigating the correlation between gear oils’ physical properties, important in terms of gear transmission losses, and pitting life. Eight gear oils were formulated giving different combinations of base oil, viscosity, and concentration of friction modifiers. All eight oils also contained an additive package designed to meet GL-5 specifications. This study consists of three parts. In the first, the oils’ physical properties were measured using a set of bench tests. In the second, the pitting lives of the oils were evaluated using rolling four-ball tests. The third part deals with the correlation between the measured physical properties of the oils and their pitting lives. This is achieved through multiple linear regression, with a view to finding the salient properties that have a significant influence on pitting life. The results show that gear oils’ physical properties do have a large influence on the pitting lives. Oil properties that lower interfacial tangential stresses are beneficial in enhancing pitting life.
The Rolling Contact Fatigue Behaviour of Motorcycle Lubricants
Rolling contact fatigue is a particular type of fatigue that occurs in heavily loaded, non-conformal contacts, such as gears and rolling element bearings. It is primarily a failure mode associated with repeated cyclic loading that generates high local Hertzian pressures, leading to local plastic deformation and substantial surface or sub surface stress. This in turn leads to crack formation and propagation. In some instances this results in sudden and often critical mechanical failure of contacting parts. This failure mode can, to a certain degree, be controlled by the appropriate choice of lubricant; in terms of both the physical and chemical properties of the films formed at the surface.
A three contact disc machine has been used to examine the rolling contact fatigue of motorcycle lubricants in such heavily loaded contacts. Three counterface test rings of equal diameter (54mm) are mounted 120° apart with a smaller (12mm diameter) test roller in the centre. Using this configuration, a large number of contact cycles are possible in a short period of time (up to one million per hour), which greatly accelerates the testing test.
Using appropriate test conditions, the rig is able to replicate the tribological conditions typically found in both gears and bearings, giving the macropitting failure mode prevalent in such components. The magnitude of the applied stress may well be below the yield strength of the material, but it is the repetitive application of load which is sufficient to cause fatigue failure. By its very nature, any form of fatigue is a statistically random process and any finite collection of similar specimens, run under a fixed set of conditions (load, speed and lubricant) will belong to a population defined by the manner in which the fatigue lives of individual members are distributed. The results are presented in the form of a single mode Weibull distribution.
In this paper it is shown that lubricants within the same viscosity grade can give statistically significant differences in rolling contact fatigue resistance. This is shown to depend primarily on lubricant composition (additive/base oil combination), which give quite different friction performance, particularly in the mixed lubrication regime. The paper will conclude with a discussion of the statistical significance of the fatigue results and its applicability to real engines.
Comparison of frictional properties of gear oils in boundary and mixed lubricated rolling–sliding and pure sliding contacts
The friction responses of five fully formulated gear oils including mineral and synthetic oils were studied. This article examines the impact of contact motion types (rolling–sliding and pure sliding) and contact pressure on boundary and mixed friction properties of the selected gear oils in MTM (minitraction machine) and SRV (Schwing-Reib-Verschleiss tribometer). Mineral oils are found to be less affected by contact pressure compared to synthetic oils. Gear oils that show adsorption appear to be less sensitive to contact motion type in mixed lubrication while behave much more sensitive in boundary lubrication regimes. The ranking of gear oils for mixed friction was similar regardless of contact motion types at low contact pressures while differ at high contact pressures
Rolling contact fatigue of bearing components coated with carbon nitride thin films
Bearing rollers were coated with CNx films using high power impulse magnetron sputtering deposition in order to reduce their rolling-contact fatigue as investigated using a Micro-Pitting Rig tribometer under poly-alpha-olefin lubricated conditions. Coated rollers with a ~15 nm thick W adhesion layer to the substrate, exhibit the best performance, presenting mild wear and no fatigue after 700 kcycles. The steady-state friction coefficient was ~0.05 for both uncoated and coated rollers. Uncoated rollers show run-in friction in the first 50 kcycles, because of steel-to-steel contact, which is absent for coated rollers. Analytical transmission electron microscopy and X-ray photoelectron spectroscopy show that the presence of a CNx coating prevents steel-to-steel contact of the counterparts, prior to the elastohydrodynamic lubrication, reducing their wear and increasing the lifetime expectancy.
In this study, the capabilities of a WC/aC:H coating to affect the onset of micropitting in SAE 52100 bearing steel is examined
The growing demands for renewable energy production have recently resulted in a significant increase in wind plant installation. Field data from these plants show that wind turbines suffer from costly repair, maintenance and high failure rates. Often times the reliability issues are linked with tribological components used in wind turbine drivetrains. The primary failure modes in bearings and gears are associated with micropitting, wear, brinelling, scuffing, smearing and macropitting all of which occur at or near the surface. Accordingly, a variety of surface engineering approaches are currently being considered to alter the near surface properties of such bearings and gears to prevent these tribological failures. In the present work, we have evaluated the tribological performance of compliant highly hydrogenated diamond like carbon coating developed at Argonne National Laboratory, under mixed rolling/sliding contact conditions for wind turbine drivetrain components. The coating was deposited on AISI 52100 steel specimens using a magnetron sputter deposition system. The experiments were performed on a PCS Micro-Pitting-Rig (MPR) with four material pairs at 1.79 GPa contact stress, 40% slide to roll ratio and in polyalphaolefin (PAO4) basestock oil (to ensure extreme boundary conditions). The post-test analysis was performed using optical microscopy, surface profilometry, and Raman spectroscopy. The results obtained show a potential for these coatings in sliding/rolling contact applications as no failures were observed with coated specimens even after 100 million cycles compared to uncoated pair in which they failed after 32 million cycles, under the given test conditions.
Influence of a WC/aC: H Coating on Micropitting Wear of Bearing Steel
In this study, the capabilities of a WC/aC:H coating to affect the onset of micropitting in SAE 52100 bearing steel is examined. Experiments are conducted in which two pairs of contacts (steel on steel, and steel on WC/a-C:H) and three sliding-to-rolling ratios of -10%, 0% and +10% are tested at high contact stress and in a low h regime. Results reveal that the WC/a-C:H coating can dramatically increase the micropitting resistance of the coated part, achieving a much higher number of contact cycles and higher levels of contact stress than the steel-on-steel contact
INVESTIGATION AND CHARACTERIZATION OF MICROPITTING IN BEARING STEEL
Investigating the Process of White Etching Crack Initiation in Bearing Steel
White etching cracks (WECs) have been identified as a dominant mode of premature failure within wind turbine gearbox bearings. Though WECs have been reported in the field for over a decade, the conditions leading to WECs and the process by which this failure culminates are both highly debated. In previously published work, the generation of WECs on a benchtop scale was linked to sliding at the surface of the test sample, and it was also postulated that the generation of WECs was dependent on the cumulative energy that had been applied to the sample over the entirety of the test. In this paper, a three-ring-on-roller benchtop test rig is used to systematically alter the cumulative energy that a sample experiences through changes in normal load, sliding, and run-time, in an attempt to correlate cumulative energy with the formation of WECs. It was determined that, in the current test setup, the presence of WECs can be predicted by this energy criterion. The authors then used this information to study the process by which WECs initiate. It was found that, under the current testing conditions, the formation of a dark etching microstructure precedes the formation of a crack, and a crack precedes the formation of white etching microstructure
Effects of Black Oxide and a WC/a-C:H Coating on the Micropitting of SAE 52100 Bearing Steel
In this study, the capabilities of a WC/a-C:H coating and a black oxide surface treatment to affect the onset of micropitting in SAE 52100 bearing steel are examined. Experiments are conducted in which three pairs of contacts (steel on steel, black oxide on black oxide and steel on WC/a-C:H), and three sliding-to-rolling ratios of −10, 0 and +10 % are tested at high contact stress and in a low Λ regime. Results reveal that the WC/a-C:H coating can dramatically increase the micropitting resistance of the coated part, achieving a much higher number of contact cycles and higher levels of contact stress than the steel-on-steel contact. On the other hand, it is found that black oxide surface treatments provide no improvement in micropitting resistance over untreated steel–steel contacts. Specimens tested at the higher slide-to-roll ratios have lower rates of wear with the black oxide surface treatment than with the untreated steel–steel contacts.
An experimental study of micropitting, using a new miniature test-rig
A miniature three-contact disc machine was employed to reproduce micropitting under controlled condition and study the effect of lubricant composition on the phenomenon. In this preliminary work, a PAO base stock was tested on its own and with a ZDDP anti-wear additive. Micropitting only occurred when the ZDDP was present. During the tests, the roughness of the counterface was found to decline gradually. When the ZDDP was present, the roughness declined more slowly. The results suggest that certain anti-wear additives may be detrimental to micropitting resistance simply because they prevent wear rather than by any direct influence on micropitting.
Film-Forming Additives – Direct and Indirect Ways to Reduce Friction
The main role of a lubricant is to form a protective, low shear strength film between rubbing surfaces and thereby reduce friction and surface damage. The science, or art, of both the lubricant and the mechanical designer is to develop combinations of lubricant and mechanical system best able to form such films. This task is not straightforward since modern technology is continually demanding lower friction and better protection over an ever-widening range of operating conditions. Furthermore, environmental concerns are also producing both design constraints and the need for rapid change.
The aim of this paper is to show how progress is being made by experimental research which looks inside rubbing contacts to see how lubricants behave therein. The paper focuses on concentrated contacts, as found in gears, cams and rolling element bearings, and describes a number of techniques for probing such contacts to observe just how a range of lubricant types, from greases to emulsions, behave in such contacts to reduce friction and form films.
The Design of Boundary Film-Forming PMA Viscosity Modifiers
Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces to produce thick, viscous boundary films. These films enhance lubricant film formation in slow-speed and high temperature conditions and thus produce a significant reduction in friction. This article describes a systematic study of this phenomenon, which makes use of the versatile nature of polymethacrylate (PMA) chemistry. Dispersant polymethacrylates with a range of different functionalities, molecular weights, and architectures have been synthesized using controlled radical polymerization techniques. The influence of each of these features on boundary film formation and friction has been explored using optical interferometry and friction versusspeed measurement. From the results, guidelines have been developed for designing PMAs having optimal boundary lubricating and, thus, friction-reducing properties.
Friction Reduction and Antiwear Capacity of Engine Oil Blends Containing Zinc Dialkyl Dithiophosphate and Molybdenum-Complex Additives
The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of boundary-lubricated steel surfaces was investigated experimentally. The performance of oil blends containing three different types of Mo-complex additives of varying Mo and S contents with or without primary/secondary ZnDTP additions were investigated at 100°C. The formation of antiwear tribofilms was detected in situ by observing the friction force and contact voltage responses. Wear volume and surface topography measurements obtained from surface profilometry and scanning electron microscopy studies were used to quantify the antiwear capacity of the formed tribofilms. The tribological properties are interpreted in terms of the tribofilm chemical composition studied by X-ray photoelectron spectroscopy. The results demonstrate that blending the base oil only with the Mo-compound additives did not improve the friction characteristics. However, an optimum mixture of Mo complexes and ZnDTP additive provided sufficient amounts of S and Mo for the formation of antiwear tribofilms containing low-shear strength MoS 2 that reduces sliding friction. In addition, the formation of a glassy phosphate phase due to the synergistic effect of the ZnDTP additive enhances the wear resistance of the tribofilm. This study shows that ZnDTP- and Mo-containing additives incorporated in oil blends at optimum proportions improve significantly the tribological properties of boundary-lubricated steel surfaces sliding at elevated temperatures
Study of Zinc Dialkyldithiophosphate Antiwear Film Formation and Removal Processes, Part II: Kinetic Model
Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part paper (Fujita, et al. ), have been used to develop and test kinetic models of antiwear film formation and removal. The main component of ZDDP film formation involves the gradual coverage of the surfaces by thick, discrete islands of film material. This process can be modeled by combining a simple coverage model in which the rate of film formation is proportional to the fraction of surface not yet covered, with an induction period. The process of film removal can be modeled by assuming that the rate of film loss is proportional to the fourth power of the coverage or film thickness. The combination of these film formation and removal rate equations is able to predict the complex, transient maximum film-forming behavior of secondary ZDDP as well as the process of film formation by primary ZDDP and the removal of antiwear film by dispersant additive.
Synergistic Effects of Boron, Sulfur, and Phosphorus-Containing Lubricants in Boundary Lubrication of Steel Surfaces K
The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated experimentally. The formation of protective tribofilms and their tribochemical activity in the temperature range of 32–100°C were analyzed in terms of coefficient of friction and contact voltage results. The antiwear performance obtained with each blend was evaluated by wear volume calculations based on surface profilometry measurements. Scanning electron microscopy studies provided insight into the dominant wear mechanisms at various temperatures. The tribological performance of the tribofilms and associated wear mechanisms were found to depend strongly on the type of additive(s), tribofilm composition, and temperature. This study has revealed that sulfide-dominated tribofilms produce lower friction, whereas borate- and phosphate-dominated tribofilms are more effective in increasing the wear resistance of the steel surfaces
Time-Dependent Film Formation from ZnDTPs and Nonphosphorus Antiwear Agents
Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films formed after only 10 min of the ECR experiment showed that both primary and secondary ZnDTPs form a thin film (∼70 Å) very rapidly. Those films are rich in Zn, P, and S. Auger and XANES analyses of the same specimens were not as revealing, most likely due to the small wear scar on the balls and the unfortunately relatively large beam cross section. ECR, XPS, and AES were then performed on oils containing nonphosphorus antiwear agents in American Petroleum Institute Group II base oil. Several nonphosphorus supplemental antiwear inhibitors were evaluated. These experiments showed separation in apparent performance among the various componentstra
Film-Forming Properties of Zinc-Based and Ashless Antiwear Additives
A progressive reduction in the permitted level of phosphorus in lubricating oils, coupled with concern to maintain engine and transmission durability, means that it is becoming increasingly important to understand the detailed mechanism of antiwear additive behavior.
This paper describes a new experimental technique, which is able to measure both the thickness and distribution of antiwear additive films in rolling/sliding contacts. This enables the kinetics of antiwear film build-up to be investigated and the influence of the reaction film on friction and wear to be monitored. In the current paper, this technique is used to compare the film-forming behavior of ash-containing and ashless antiwear additives.
Friction and Wear Behaviour of Zinc Dialkyldithiophosphate Additive
In situ observation of phosperous and non-phospherous antiwear films using a Mini Traction Machine with Spacer Layer Image Mapping
Since 1940s the principal source of an antiwear additive in crankcase applications has been due to a family of additives known as zincdialkyithiophosphate (ZDDP). In this study, we have applied a novel technique, teh mini traction with spacer layer image mapping (MTM SLIM) to study film formation characteristics of monoblend oils containing only basestock and ZDDP and also fully formulated oils containing a boron antiwear additive. The purpose of this study was initially to understand the build up of ZDDP film and then to establish whether boron compounds alone could you provide significant antiwear films under mixed rolling and sliding contact.
The Elastohydrodynamic Friction and Film Forming Properties of Lubricant Base Oils
The stringent and often competing requirements of high fuel economy and low emissions are placing increasing emphasis on the selection of appropriate base oils for modem engine lubricants. Two properties now recognized as important in engine oil design are the elastohydrodynamic (EHD) traction coefficient and the pressure-viscosity coefficient. The former determines the level of friction in high pressure contacts such as cams, while the latter plays a major role in determining the EHD film thickness. Unfortunately, for many fluids there is a broad correlation between the two properties so that a low traction coefficient implies a low pressure viscosity coefficient and thus film thickness. This paper measures the traction and film-forming properties of a wide range of base oil types at realistic engine oil temperatures in order to both explore the extent of this correlation and to determine the dependence of EHD lubricant properties on base oil compositions.