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  • 1.
    Bair, Scott
    et al.
    G.W. Woodruff School of Mechanical Engineering, Centre for High Pressure Rheology, Georgia Institute of Technology, Atlanta.
    Vergme, Philippe
    LaMCoS UMR5259, Université de Lyon, INSA-Lyon.
    Kumar, Punit
    Department of Mechanical Engineering, National Institute of Technology, Kurukshetra.
    Poll, Gerhard
    Institut fûr Maschinenkonstruktion und Tribologie, Leibniz Universitaet Hannover.
    Krupka, Ivan
    Faculty of Mechanical Engineering, Institute of Machine and Industrial Design, Brno University of Technology.
    Hartl, Martin
    Faculty of Mechanical Engineering, Institute of Machine and Industrial Design, Brno University of Technology.
    Habchi, Wassim
    Lebanese American University, Department of Industrial and Mechanical Engineering, Byblos.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Comment on “History, Origins and Prediction of Elastohydrodynamic Friction” by Spikes and Jie2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 58, no 1, article id 16Article in journal (Refereed)
    Abstract [en]

    Progress in the classical field of EHL has for decades been paralyzed by the assumption that shear thinning should be indistinguishable from the shear dependence of the viscosity of a liquid heated by viscous dissipation and that the parameters of this simple shear dependence can be obtained from the shape of a friction curve. In the last few years, by abandoning this assumption and employing real viscosity measured with viscometers, there has been revolutionary progress in predicting film thickness and friction. Now, Spikes and Jie conclude that the previous assumption has as much merit as the use of viscosity measured in viscometers. This suggestion may be popular among those who wish to ignore viscometer measurements in favor of extracting properties from friction curves. However, within the subject article, there are numerous misstatements of fact and misrepresentations by omission, and the recent progress using real viscosity is not acknowledged. The debate has degenerated into a friction curve fitting competition which is not helpful. The great progress of the last few years would not have been possible using the concepts and methods espoused in this article

  • 2.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Habchi, Wassim
    Department of Industrial and Mechanical Engineering, Lebanese American University, Byblos.
    Bair, Scott
    Georgia Institute of Technology, Centre for High Pressure Rheology, G.W. Woodruff School of Mechanical Engineering, Atlanta.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Correction to: Friction reduction in elastohydrodynamic contacts by thin-layer thermal insulation2018In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 66, no 4, p. 1-1Article in journal (Refereed)
    Abstract [en]

    The original version of this article unfortunately contained a mistake. The correct information is given below. [ABSTRACT FROM AUTHOR]  Copyright of Tribology Letters is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

  • 3.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Habchi, Wassim
    Department of Industrial and Mechanical Engineering, Lebanese American University, Byblos.
    Bair, Scott
    Georgia Institute of Technology, Centre for High Pressure Rheology, G.W. Woodruff School of Mechanical Engineering, Atlanta.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Friction reduction in elastohydrodynamic contacts by thin-layer thermal insulation2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 53, no 2, p. 477-486Article in journal (Refereed)
    Abstract [en]

    Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatings

  • 4.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Isaksson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The influence of DLC coating on EHL friction coefficient2012In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 47, no 2, p. 285-294Article in journal (Refereed)
    Abstract [en]

    High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Björling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used

  • 5.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The effect of DLC coating thickness on elstohydrodynamic friction2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 55, no 2, p. 353-362Article in journal (Refereed)
    Abstract [en]

    The application of surface coatings has been shown to reduce friction in elastohydrodynamic lubrication (EHL), not only in the mixed and boundary regime when asperity interactions occur, but also in the full film regime. Several studies suggest that the full film friction reduction is due to a violation of the no-slip boundary condition and thus slip is taking place between the solid and the liquid. Another hypothesis proposes that the full film friction reduction is due to the low thermal conductivity of diamond-like carbon (DLC) coatings. In this work, two DLC coatings with the same composition, but different thicknesses, are investigated with uncoated steel specimens as a reference, all with the same surface roughness. Friction tests in a ball-on-disk machine show that both coatings reduce friction compared to the uncoated reference case in full film EHL. The thicker coating is significantly more effective at reducing friction than the thinner one at a maximum friction reduction of 41 % compared to 29 % for the thinner coating. Moreover, contact angle measurements, surface energy measurements, and spreading parameter calculations show no statistically significant differences between the two coatings, suggesting that the friction reduction capabilities of coatings in full film EHL cannot be described by solid-liquid interactions alone. The difference in friction reduction between the specimens in this work is mainly attributed to different thermal properties.

  • 6.
    Deng, Liang
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Mozgovoy, Sergej
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Development of a Tribological Test Programme Based on Press Hardening Simulations2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 2, article id 43Article in journal (Refereed)
    Abstract [en]

    Press hardening is widely utilized to form ultra-high-strength steels characterized by a high strength-to-weight ratio for automotive components. Press hardening processes include heating boron–manganese steels to austenite phase, forming the steels at a high temperature, and cooling the formed blanks until the martensite phase is reached . However, press hardening processes lead to severe contact conditions between the blank and the tools including contact pressure, relative sliding, and high temperatures, which result in tool wear and increased maintenance cost. The contact conditions that occur in the stamping tool are difficult to study on site. Additionally, simplified tests, such as pin on disc and ball on disc, are insufficient to reproduce press hardening conditions in laboratory environments . The aim of this study includes developing a tribological test with press hardening conditions in which tool steel pins continuously slide on fresh and hot boron–manganese steel strips. The test programme mimics press hardening conditions with respect to sliding distance, sliding velocity, contact pressure, and surface temperature that were studied based on finite element (FE) simulations of a press hardening experiment. Furthermore, a FE simulation of the tribological test is established and it provides contact temperature in the pin tip with a high accuracy. A tribological test is used to study friction and mass loss with variational pressures and velocities that represented typically variational contact conditions in the press hardening. The tribological test is also used to obtain correlations between the tribological behaviours and process parameters in press hardening including pressure and sliding velocity.

  • 7.
    Deng, Liang
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Pelcastre, Leonardo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Experimental Evaluation of Galling Under Press Hardening Conditions2018In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 66, no 3, article id 93Article in journal (Refereed)
    Abstract [en]

    Severe adhesion, also referred to as galling, is a critical problem in press hardening, especially in stamping tools used for hot forming of Al–Si-coated ultra-high strength steel. Galling is known to develop rapidly on the tool surface and it negatively affects the quality of the formed products. Earlier research on this topic has focused on the galling initiation. However, studies on the galling development during extended sliding and the corresponding quantitative measurement still lack depth. In the present study, a tribological test is established to study the galling development under press hardening conditions. The tribological test set-up aims to simulate extended sliding between the Al–Si-coated boron steels and the tool die material. The contact conditions in the interface are studied by a numerical model of the tribological test. The friction coefficients and material transfer are discussed taking into account the variation of the different test conditions. Using the results from the tribological tests, the galling simulation is performed in the numerical model. A geometry-updated sample based on the galling (transferred material build-up) height is simulated and the consequent pressure fluctuation is obtained in the numerical model. This contributes to the explanation of the severe transferred material accumulation during the test.

  • 8. Marklund, Pär
    et al.
    Berglund, Kim
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The influence on boundary friction of the permeability of sintered bronze2008In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 31, no 1, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Components made of sintered bronze are often used in tribological systems. Examples of applications are self lubricated bearings, wet clutches and brakes and synchromesh components for manual gearboxes. The bronze material in these applications is often porous and permeable. However, the required level of permeability i.e., the ability for a fluid to flow inside the material varies widely for different applications. This implies the need to investigate if the permeability can influence the properties of a tribological system. Various studies have been performed in order to investigate the permeability of different materials but a possible relationship between permeability and boundary lubrication performance has not yet been thoroughly investigated. In this article this relationship has been investigated in a pin-on-disk apparatus for test specimens with a permeability that is well-defined. Materials with three different permeabilities were investigated together with non-permeable test specimens. The results indicate that permeability has a small influence on the friction coefficient when the tribological system is operating with good lubrication of the contact. The function of the tribological system when working under starved conditions is also investigated and a very clear relationship between permeability and the ability to cope up with starved running conditions is shown. This is explained in terms of the varying ability of the materials' to store lubricant in pores. Non-porous test specimens were also tested as a comparison with the permeable test specimens in the investigation with starved running conditions . The results show that a material with high permeability works much better under starved running conditions than that with low permeability.

  • 9. Prakash, Braham
    et al.
    Hiratsuka, K.
    Chiba Institute of Technology.
    Sliding wear behaviour of some Fe, Co and Ni based metallic glasses while rubbing against bearing steel2000In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 2-3, p. 153-160Article in journal (Refereed)
    Abstract [en]

    The sliding wear behaviour of several compositions of Fe-, Co- and Ni-based metallic glasses have been studied while rubbing against AISI 52100 bearing steel under reciprocating-sliding conditions. The wear resistances of Fe-based metallic glasses and Ni-based metallic glass (MBF 50) have been found to be superior to that of the mating AISI 52100 bearing steel. The examination of worn surfaces indicates that the superior wear resistance of metallic glasses is not merely owing to their high hardness but it is determined by phenomena of material transfer vis-à-vis the mating material and the formation of protective oxide layers on the metallic-glass surface during sliding.

  • 10.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 1, article id 7Article in journal (Refereed)
    Abstract [en]

    In most applied monitoring investigations using acoustic emission, measurements are carried out relatively, even though that limits the use of the extracted information. The authors believe acoustic emission monitoring can be improved by instead using absolute measurements. However, knowledge about absolute measurement in boundary restricted systems is limited. This article evaluates a method for absolute calibration of acoustic emission transducers and evaluates its performance in a boundary restricted system. Absolute measured signals of Hertzian contact excited elastic waves in boundary restricted systems were studied with respect to contact time and excitation energy. Good agreement is shown between measured and calculated signals. For contact times short enough to avoid interaction between elastic waves and initiating forces, the signals contain both resonances and zero frequencies, whereas for longer contact times the signals exclusively contained resonances. For both cases, a Green’s function model and measured signals showed good agreement.

  • 11.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Elastic Waves of a Single Elasto-Hydrodynamically Lubricated Contact2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 1, article id 5Article in journal (Refereed)
    Abstract [en]

    Elastic waves are widely used for condition monitoring of rolling element bearings through vibration or acoustic emission measurements . While vibration signals are understood to a high degree due to many scientific investigations as well as a long history of usage in the field, acoustic emission signals of rolling element bearings are poorly understood. Therefore, this investigation presents simulation studies and measurements of a single elasto-hydrodynamically lubricated (EHL) contact. In this investigation the EHL contact is a ball bouncing on a lubricated plate. The simulation based on Green’s function and the measurements based on a accelerometer to some extent agree. A shift of zero frequencies towards higher frequencies when compared to Hertzian reference measurements was determined for an infinite plate setup. Taking boundary restrictions into account, elastic waves of a Hertzian contact and an EHL contact only differ by a damping of higher resonances which is most likely caused by the EHL film.

  • 12.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Monitoring of Running-in of an EHL contact using Contact Impedance2016In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 63, no 3, article id 35Article in journal (Refereed)
    Abstract [en]

    Running-in is an important process for elasto-hydrodynamic lubricated contacts, which affect both service life and operating performance. However, the possibilities of monitoring running-in are still poor. Therefore, the properties of electrical contact impedance as a monitoring tool were studied by using an in-house made ball on disc apparatus. The contact impedance was monitored during run-in experiments with different initial surface roughness of the discs, different slide-to-roll ratios and with pure or additive containing paraffinic oil. The relationship between surface roughness parameters, contact resistance and contact capacitance was investigated. While the contact resistance seems to be affected by the parameter Rz, the contact capacitance seems more dependent on Rq. In addition, the experiments showed that surface active additives do not necessarily need to influence the contact impedance.

  • 13.
    Shah, Faiz Ullah
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Glavatskih, Sergei
    Machine Design, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden.
    Antzutkin, Oleg
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Boron in tribology: from borates to ionic liquids2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 51, no 3, p. 281-301Article in journal (Refereed)
    Abstract [en]

    Boron compounds are widely used in a range of tribological applications such as friction modifiers, antioxidants, antiwear additives, and in many cases as environmentally friendly lubricants. The chemical nature and structure of boron compounds provide multifunctionality. They are used as (1) solid lubricants such as boric acid and hexagonal boron nitride, (2) liquid lubricants such as ionic liquids, (3) lubricant additives such as borate derivatives of various organic and inorganic compounds, and (4) coatings such as cubic boron nitride and different metal borides. Boron is also one of the most favorable elements for coatings and thin films in biotribological and biomedical applications. This review outlines the growing role of boron in lubrication over the past several decades, summarizesthe main findings, and identifies future challenges related to boron chemistry.

  • 14.
    Shah, Faiz Ullah
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Glavatskih, Sergei
    System and Component Design, KTH, Royal Institute of Technology.
    Antzutkin, Oleg
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Novel alkylborate–dithiocarbamate lubricant additives: synthesis and tribophysical characterization2012In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 45, no 1, p. 67-78Article in journal (Refereed)
    Abstract [en]

    Boron-based lubricant additives have recently received significant attention, because of their wear-reducing and frictional properties and low pollution. At the same time, dithiocarbamate complexes with different metals have a long history of being used as multifunctional additives to lubricants. In this study, novel, environmentally friendly additives containing alkylborate and dithiocarbamate groups with alkyl or methylbenzyl substitutes in one molecule were studied. Tribological tests were performed with the additives admixed in a mineral oil using steel-on-steel contacts in a four-ball tribometer. Borate derivatives of different dithiocarbamate ligands were synthesized by several step reactions to investigate tribochemical properties of boron, sulfur, and nitrogen combined in one selected compound. Viscous liquid products were characterized by multinuclear 1H, 13C, and 11B NMR spectroscopy. The surface morphology and the elemental composition of the tribofilms were investigated using an optical profiler and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). It was found that some of these novel compounds provide better antiwear performance and similar frictional properties compared with a commercially available ZnDTP package. Traces of sulfur in the tribofilms formed with both 0.2 and 1.0 wt% of alkylborate–dithiocarbamate compounds in a mineral oil were detected with EDS.

  • 15.
    Sharma, Vinay
    et al.
    University of Texas, Arlington, Department of Material Science & Engineering.
    Johansson, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Timmons, Richard B.
    University of Texas, Arlington, Department of Chemistry & Biochemistry.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Aswath, Pranesh B.
    University of Texas, Arlington, Department of Material Science & Engineering.
    Tribological Interaction of Plasma-Functionalized Polytetrafluoroethylene Nanoparticles with ZDDP and Ionic Liquids2018In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 66, no 3, article id 107Article in journal (Refereed)
    Abstract [en]

    Polytetrafluoroethylene (PTFE) nanoparticles were coated with consecutive plasma deposited siliceous and methacrylate coatings. Secondary zinc dialkyldithiophosphate (ZDDP), phosphonium cation and phosphate anion ionic liquid (IL), and IL with phosphonium cation and dithiophosphate anion were mixed with the functionalized nanoparticles. Tribological studies were carried out for seven separate formulations including base oil, oils with only additives, and oils with additives and functionalized PTFE particles. Results indicate strong synergistic interactions of ZDDP and ILs with functionalized nanoparticles providing enhanced friction and wear performance. Chemical analysis of the tribofilms using X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy indicates functionalized PTFE nanoparticles interact synergistically with ZDDP and ILs to form silicon- and fluorine-doped tribofilms resulting in superior tribological performance.

  • 16.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-corrosive and Biomaterials Protic Ionic Liquids with High Lubricating Performance2016In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 63, no 1, article id 1Article in journal (Refereed)
    Abstract [en]

    Achieving non-corrosive and green ionic liquids is a big challenge for the tribologist. A kind of biomaterials-based protic ionic liquids (PILs) was synthesized in this paper. Rapeseed oil, group 1 mineral oil and one commercially available fully formulated gear oil were as used reference to study the property of the synthesized PILs. The copper strip standard corrosion test was employed to study the anti-corrosion property. The boundary lubrication and elastohydrodynamic lubrication performance of the lubricants was studied on an Optimol SRV-III oscillating friction and wear tester, and a WAM (model 11) ball-on-disk test rig, respectively. The lubricating mechanism of the synthesized PILs was also discussed in this paper

  • 17.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mu, Liwen
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Friction and wear behavior of CF/PTFE composites lubricated by choline chloride ionic liquids2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 49, no 2, p. 413-420Article in journal (Refereed)
    Abstract [en]

    The use of ionic liquids (ILs) as lubricants has received increasing attention in recent years. The use of ILs, however, is limited by the corrosion problem and their potential toxic property. Here we present the results of our initial study on the tribological properties of carbon fiber (CF)-filled polytetrafluoroethylene (PTFE) composites, which have an excellent chemical resistance property, lubricated by choline chloride ILs. The difference between choline chloride ILs and water and hydraulic oil as lubricants was studied at the same time, as was the effect of the anion on the lubricating property of choline chloride ILs. The worn surface and transfer film of CF/PTFE composites were studied by scanning electron microscopy. Our results indicate that the lubricating property of choline chloride ILs is much better than that of water and hydraulic oil. The friction coefficient and wear rate of CF/PTFE composites lubricated with ILs were approximately 60 and 50 % lower than those under the dry friction condition. Among the three kinds of ILs tested, the best tribological properties of the CF/PTFE composites were found for those sliding in the mixture of 1,2-propanediol and choline chloride. The worn surface and transfer film of CF/PTFE composites were also much smoother than those under the dry friction, water lubrication, and hydraulic oil lubrication conditions.

  • 18.
    Simmons, Gregory F
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Synthetic lubricants in hydrodynamic journal bearings: experimental results2011In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 42, no 1, p. 109-115Article in journal (Refereed)
    Abstract [en]

    Synthetic lubricants and additives have seen many major improvements in recent years. However, very littleis known about the performance peculiarities of these new lubricants in actual machines. To fill this gap, anew full-scale hydro-dynamic journal bearing test rig has been constructed to evaluate the behavior ofconventional and new bearing designs, synthetic lubricants and variations in operating parameters. This testrig’s bearing has diameter 180 mm with measuring capabilities including continuous film thickness and filmpressure as well as temperature. The new machine was used to compare a number of synthetic lubricants tomineral based lubricants, finding that performance of the synthetic lubricants was superior to their mineralbased counterparts of much higher viscosity grade. These tests showed that high viscosity index (VI) syntheticlubricants had higher viscosity in the region of maximum pressure and lower viscosity elsewhere in the bearingthan similar mineral based lubricants. This reduction in viscosity in low pressure zones was found to producea measurable reduction in friction and power loss in the bearing system. This paper provides comparativeperformance results of several formulations of current and future turbine oils from measurements of losses,oil film thickness, and temperature under a range of operating parameters. Lubricants tested include ISOVG68 and VG32 mineral based turbine oils (VG68 and VG32), ISO VG32 synthetic ester based oil (SE32),two ISO VG22 synthetic ester based oils (SE22 and SV22), and ISO VG15 synthetic ester based oil (SE15).It was found that SE32 and VG68 provided similar performance at lower speeds while SV22 provided similarperformance to VG68 at the highest speed. Likewise, SE22 and SV22 provided similar performance toVG32 at low speeds while SE15 provided similar performance to VG32 at medium to higher speeds.Generally, the results demonstrate the potential for replacing mineral based lubricants with high performancesynthetic lubricants of significantly lower viscosity grade without sacrificing bearing safety.

  • 19.
    Tomala, Agnieszka
    et al.
    Institute of Applied Physics, Vienna University of Technology, AC²T research GmbH - Austrian Center of Competence for Tribology, Viktor-Kaplan-Straße 2 D, 2700 Wiener Neustadt.
    Vengudusamy, B.
    AC²T research GmbH - Austrian Center of Competence for Tribology, Viktor-Kaplan-Straße 2 D, 2700 Wiener Neustadt.
    Ripoll, M. Rodriguez
    AC²T research GmbH - Austrian Center of Competence for Tribology, Viktor-Kaplan-Straße 2 D, 2700 Wiener Neustadt.
    Suarez, Aldara Naveira
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Remškar, M.
    Institut “Jožef Stefan”, Ljubljana.
    Rosentsveig, M.
    Weizmann Institute of Science, Rehovot.
    Interaction Between Selected MoS2 Nanoparticles and ZDDP Tribofilms2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 59, no 1, article id 26Article in journal (Refereed)
    Abstract [en]

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

  • 20.
    Totolin, Vladimir
    et al.
    ACT research GmbH, Viktor-Kaplan-Strasse 2D, Wiener Neustadt.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Gabler, Christoph
    ACT research GmbH, Viktor-Kaplan-Strasse 2D, Wiener Neustadt.
    Brenner, Joseph
    ACT research GmbH, Viktor-Kaplan-Strasse 2D, Wiener Neustadt.
    Dörr, Nicole
    ACT research GmbH, Viktor-Kaplan-Strasse 2D, Wiener Neustadt.
    Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid Additive in Alkylborane-Imidazole Complexes2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 53, no 2, p. 421-432Article in journal (Refereed)
    Abstract [en]

    The assessment of ionic liquids (ILs) as lubricants in several tribological systems has shown their ability to provide remarkable reduced friction and protection against wear, whether they are used as additives or in the neat form. However, their corrosion and limited solubility in non-polar hydrocarbon oils represent the bottleneck-limiting factors for the use of ILs as lubricants. Therefore, in order to tackle these problems, mixtures of alkylborane-imidazole complexes with one halogen-free IL as additive were used in this study. The knowledge of the additive-surface interactions and hence the understanding of tribological properties are an important issue for lubricant formulations and were also investigated in this work. Thus, combination effects between two ionic liquid additives, a halogenated and a halogen-free one, were evaluated by a ball-on-disc-type tribometer under boundary lubrication conditions. Effective friction reduction and anti-wear properties have been demonstrated in tribological investigations when adding between 0.7 and 3.4 wt% of the halogen-free IL into base fluid composed of alkylborane-imidazole complexes. X-ray photoelectron spectroscopy analyses of the steel specimens were conducted to study the correlation between tribological properties and chemical surface composition of the boundary films formed on the rubbing surface. This work suggests potential applications for using halogen-free ILs as additives for synthetic ionic liquid lubricants

  • 21. Waara, Patric
    et al.
    Norrby, Thomas
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribochemical wear of rail steels lubricated with synthetic ester-based model lubricants2004In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 17, no 3, p. 561-568Article in journal (Refereed)
    Abstract [en]

    Headchecks are a common type of damage in heavily loaded curved freight tracks. This paper deals with synthetic ester formulations' ability to prevent damage caused by headchecks through mild tribochemical wear. An experimental study pertaining to wear and friction of two rail steels lubricated by two synthetic ester base fluids, TMP-oleate and TMP-C8-C10, has been carried out. Six different free fatty acids were used in this study to act as performance additives. Three of the fatty acids were mono-acids with different, straight, carbon chain lengths (stearic acid C18, decanoic acid C10 and octanoic acid C8), one was a mono-unsaturated straight-chain fatty acid (oleic acid C18:1) while two were dibasic acids with intermediate carbon chain length (C9 and C10). Each fatty acid was blended with either ester, one at a time. The tests were carried out by using a high frequency reciprocating friction and wear test machine. In these tests, the gage face/wheel flange contact was simulated, and all tests were conducted in the boundary lubrication regime. An initial contact pressure of 316 MPa and a maximum sliding speed of 0.11 m/s were employed during the tests. The tests showed a wide range of wear rates, as well as different surface features depending on the interactions between synthetic esters, fatty acids and steel. The use of stearic and azaleic acid in lubricating rail steels results in very smooth surfaces with significant differences in their wear rates.

  • 22.
    Westerberg, Lars-Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Farré-Lladós, Josep
    Department of Mechanical Engineering, UPC - Technical University of Catalonia.
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Casals-Terré, Jasmina
    Department of Mechanical Engineering, UPC - Technical University of Catalonia.
    Grease flow in an elbow channel2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 57, no 3, article id 30Article in journal (Refereed)
    Abstract [en]

    The flow of lubricating greases in an elbow channel has been modeled and validated with velocity profiles from flow visualizations using micro-particle image velocimetry. The elbow geometry induces a nonsymmetric distribution of shear stress throughout its cross section, as well as varying shear rates through the transition from the elbow inlet to the outlet. The flow has been modeled both for higher flow rates and for creep flow. The influence of the grease rheology and flow conditions to wall slip, shear banding and an observed stick–slip type of motion observed for low flow rates are presented. The effect on the flow of the applied pressure is also modeled showing that the flow is sensitive to the pressure in the angular ( ϕ ) direction of the elbow. For high pressures, it is shown that the flow is reversed adjacent to the elbow walls.

  • 23.
    Westerberg, Lars-Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Höglund, Erik
    Lugt, Piet
    SKF ERC, Tribology Group.
    Baart, Pieter
    SKF ERC, Tribology Group.
    Free-surface grease flow: influence of surface roughness and temperature2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 59, no 1, article id 18Article in journal (Refereed)
    Abstract [en]

    Grease flow in grease lubricated systems can often be qualified as free-surface flow. It occurs for example in rolling bearings after the churning phase or on open gears. Here only a fraction of the bearing or gearbox volume is filled with grease. Part of the grease is flowing in relatively thin layers induced by centrifugal forces caused by rotation of the various components. In this paper a model problem is investigated in the form of a free-surface flow of grease on a rotating disc. Experiments have been performed where the onset of flow and remaining grease have been studied varying the surface roughness, temperature and the centrifugal forces. The experiments have been coupled to analytical models describing the flow and temperature distribution in the grease. It was found that the impact of surface roughness could be neglected. The flow is determined by the centrifugal forces and rheology of the grease. Temperature effects the rheology but also the oil separation creating low shear strength/low viscosity layers at the surface.

  • 24.
    Westerberg, Lars-Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Baart, Pieter
    SKF Engineering & Research Center, Nieuwegein.
    Free-surface grease flow on a rotating plate2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 56, no 2, p. 317-325Article in journal (Refereed)
    Abstract [en]

    Grease lubrication is traditionally used in a great variety of mechanical systems such as rolling bearings, seals, and gears where it has been shown more advantageous than oil, mainly due to its consistency allowing the grease to stay inside the system and not leak out. Knowledge of the flow dynamics of grease is important for the understanding and prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. Free-surface effects play an important role in rolling bearings and open gears as the configuration normally is filled with about 30 % grease to avoid heavy churning. In this study, an analytical model of the stationary uniform flow on a rotating disc is developed and validated with experiments. The model results in the velocity profile for the flow in the thin fully yielded viscous layer in connection to the surface as well as an expression for the plug flow region on top of the viscous layer. Furthermore, the depth-averaged velocity is derived as is the shear stress value on the plate. From the latter, follows a condition for the grease to start moving and in turn yielding an expression for the viscous layer thickness as a function of the grease yield stress value, grease density, angular velocity, and radial position. In addition, an expression of the layer thickness containing the ratio between the flow rate and the layer width which in turn can account for effects not included in the model such as wall slip and surface adhesion and thus add another degree of freedom into the model. Experiments with two different greases having NLGI grade 1 and 2, respectively, shows it is possible to obtain a good fit with the analytically obtained thickness using the rheological parameters for actual greases.

  • 25.
    Westerberg, Lars-Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sarkar, Chiranjit
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Farré, Josep
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lubricating grease flow in a double restriction seal geometry: a Computational Fluid Dynamics approach2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 3, article id 82Article in journal (Refereed)
    Abstract [en]

    In this paper, numerical simulations of lubricating grease flow in the grease pocket of a double restriction seal geometry using computational fluid dynamics are presented. The grease is treated as a single-phase Herschel–Bulkley fluid with different rheological properties corresponding to NLGI grade 00, 1 and 2. The numerical code and rheology model have been validated with a semi-analytical solution based on flow measurements using microparticle image velocimetry. The flow has been modelled for low and high rotational speeds driving the flow, and elevated temperatures. Also, the evolution of contaminant particles in the grease pocket is investigated. It was found that the flow and velocity distribution in the pocket—and consequently the contaminant particle concentration evolution, is characterized by the shear thinning rheology of the grease. With higher shear rates in the grease and higher temperatures, the grease approaches a more Newtonian type of behaviour leading to a reduced yield and shear thinning characteristics directly affecting the grease ability to transport contaminant particles.

  • 26.
    Yin, Xiang
    et al.
    State Key Laboratory of Materials-oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University.
    Wu, Jian
    State Key Laboratory of Materials-oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University.
    Li, Chang
    State Key Laboratory of Materials-oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University.
    Lu, Xiaohua
    Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing .
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Right Way of Using Graphene Oxide Additives for Water-Lubricated PEEK: Adding in Polymer or Water?2018In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 66, no 3, article id 103Article in journal (Refereed)
    Abstract [en]

    Water-lubricated polymer is attracting more and more interest from the industry. Adding nanoparticles is considered to be an effective way to improve the tribological performance. In this work, water-lubricated Polyetheretherketone (PEEK)-steel contacts were employed as the objects of study. A careful comparative study was made by investigating the effect of adding graphene oxide (GO) into water or into PEEK. Results show that adding GO into water can significantly reduce the wear and friction coefficient of pure PEEK, which is much more effective than adding GO into PEEK. Under the lubrication of GO aqueous dispersion, the wear of PEEK remains very low even under a harsh condition where the pressure reaches 50 MPa and the linear sliding speed is 0.7 m/s. Neat PEEK and GO/PEEK composites in pure water exhibit serious wear under this harsh condition. The excellent lubricating properties of GO aqueous dispersion are attributed to GO nanosheets entering into solid contacts, which can not only form a protective layer on steel counterpart repairing the worn surface, but also strongly adhere to the PEEK matrix resulting in an in situ-formed GO coating and prevent the scratch by steel counterpart.

  • 27.
    Zhang, Chen Hui
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhao, Ying Chun
    State Key Laboratory of Tribology, Tsinghua University, Beijing.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Yan
    State Key Laboratory of Tribology, Tsinghua University, Beijing.
    Lou, Jian Bin
    State Key Laboratory of Tribology, Tsinghua University, Beijing.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    EHL properties of polyalkylene glycols and their aqueous solutions2012In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 45, no 3, p. 379-385Article in journal (Refereed)
    Abstract [en]

    Polyalkylene glycols (PAGs) are a type of synthetic lubricants widely used as compressor lubricants, gear oils, hydraulic fluids, and metal working fluids. The PAGs with typical molecular structure can dissolve in water, which makes it a candidate for the base stock of water-based lubricants. Till now, most of the investigations on the water-based lubrication have focussed on the additives. In this work, the potential of PAGs aqueous solutions to replace water as base stocks has been investigated. Four types of PAGs with different molecular weight and their aqueous solutions with different concentrations were studied to reveal their elastohydrodynamic lubrication (EHL) behavior. It has been found that the PAGs solutions can form EHL film like traditional oils. The film-forming capability depends on the viscosity, the pressure-viscosity coefficient, and the molecular weight of PAGs. The results indicate that the PAGs aqueous solution can be employed as base stocks of water-based lubricant

  • 28. Åhrström, Bert-Olof
    et al.
    Andersson, O.
    Umeå universitet.
    Holweger, W.
    NMI, Naturwissenschaftliches und Medizinisches Institut an der Universität Tübingen.
    Transient elastohydrodynamic limiting shear behaviour related to molecular interaction2005In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 20, no 3-4, p. 255-262Article in journal (Refereed)
    Abstract [en]

    Previous studies have revealed that friction behave in unexpected ways as pressure increase in a lubricating film. It has been determined that it is not a viscosity-related but rather originating directly from molecular structure. This study strives to elucidate the importance of understanding the properties that govern friction, when undertaking simulation of elastohydrodynamic events. A thorough investigation of the maximum transferable shear stress in the film for a paraffinic mineral oil has been undertaken. A previously stated theory of decreasing friction coefficient with increasing pressure is further substantiated by the current measurements. Classical thermodynamics, in the shape of transient "hot-wire" measurements, have been used to study macro-molecular changes (as function of pressure). The instance of global relaxation correlates nicely to measurements

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