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  • 1.
    Gebretsadik, Daniel
    et al.
    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.
    Friction and wear characteristics of PA 66 polymer composite/316L stainless steel tribopair in aqueous solution with different salt levels2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 141, article id 105917Article in journal (Refereed)
    Abstract [en]

    Friction and wear behaviour of a PA 66 composite with 25% glass fibre reinforcement sliding against a 316L stainless steel have been investigated using different aqueous solutions under mixed/boundary lubrication. The aqueous solutions used are deionized water, seawater, seawater without group II metal salts, solution of dissolved Group II metal salts and solution of group II metal salts and NaHCO3. Lower friction and wear was obtained when lubricated with seawater. However, when deionized water or salt solutions without group II metal salts is used, increased wear was observed. The lower friction and wear in seawater solution is due to group II metal ions and bicarbonate ions that facilitate formation of the sparingly soluble carbonates that act as a lubricating tribofilm.

  • 2.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Modelling Flows in Lubrication2020In: Modeling and Simulation of Tribological Problems in Technology / [ed] Marco Paggi & David Hills, Springer, 2020, 1, p. 229-278Chapter in book (Other academic)
  • 3.
    Hultqvist, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Vrček, Aleks
    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.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Department of Mechanical Engineering, Tsinghua University, Beijing, China.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Transient analysis of surface roughness features in thermal elastohydrodynamic contacts2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 141, article id 105915Article in journal (Refereed)
    Abstract [en]

    Understanding the influence of surface roughness in elastohydrodynamically lubricated (EHL) contacts is essential to improve durability and friction performance of machine elements employing non-conformal contacting surfaces. In this work, the transient event of a surface feature passing through a thermal EHL line contact operating under different sliding conditions is investigated with the purpose of providing a deeper understanding of surface roughness influence. This is achieved by solving the EHL problem in space and time. It was seen that sliding influences the temperature rise in the contact significantly, especially in the vicinity of the asperity. However, due to the characteristic behaviour of EHL contacts, the local temperature rise mainly influence the film thickness during exiting of inlet perturbations and the asperity.

  • 4.
    Shao, Jiajia
    et al.
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.
    Willatzen, Morten
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Zhong Lin
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States.
    3D mathematical model of contact-separation and single-electrode mode triboelectric nanogenerators2019In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 60, p. 630-640Article in journal (Refereed)
    Abstract [en]

    Based on a set of finite-sized charged planes (FSCP), a simple time-dependent three-dimensional spatial model for the electric potential and electric field in an inhomogeneous medium composed of dielectric materials and metal contacts is proposed and used to assert triboelectric nanogenerator operation. Solving the problem of FSCP makes the three-dimensional spatial model relevant for practical TENG applications and allow for accurate and reliable results. Connecting the metal contacts to an electric resistance, Kirchhoff's law is used to derive a first-order time-dependent differential equation for the mobile charges on the metal contacts and the displacement current. Specially, the displacement current (Maxwell's displacement current) in a TENG equals to the conduction current in the external circuit is obtained. We then consider two important types of triboelectric nanogenerators: the contact-separation (CS) mode and the single-electrode (SEC) mode. A forced movement of the dielectric materials and/or the metal contacts leads to currents flowing in the system and a time-varying electrical potential, and therefore the generation of electrical power. Then, new and more accurate capacitance equations for CS and SEC modes of TENGs are extracted. Several examples of energy harvesting scenarios are finally analyzed.

  • 5.
    Tosic, Marko
    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.
    Jovanović, Janko
    Faculty of Mechanical Engineering, University of Montenegro, Montenegro.
    Lohner, Thomas
    Gear Research Centre (FZG), Technical University of Munich (TUM), Germany.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Stahl, Karsten
    Gear Research Centre (FZG), Technical University of Munich (TUM), Germany.
    A Computational Fluid Dynamics Study on Shearing Mechanisms in Thermal Elastohydrodynamic Line Contacts2019In: Lubricants, E-ISSN 2075-4442, Vol. 7, no 8, article id 69Article in journal (Refereed)
    Abstract [en]

    A computational fluid dynamics (CFD) model of the thermal elastohydrodynamically lubricated (EHL) line contact problem has been developed for the purpose of exploring the physical processes that occur inside a thin EHL film subjected to shearing motion. The Navier–Stokes equations are solved by using the finite volume method (FVM) in a commercial CFD software, ANSYS Fluent. A set of user-defined functions (UDF) are used for computing viscosity, density, heat source, temperature of moving surfaces and elastic deformation of the top roller according to well-established equations commonly used in the EHL theory. The cavitation problem is solved by taking into account multiphase mixture flow. The model combinations of Houpert and Ree–Eyring and of Tait and Carreau were used for modeling the non-Newtonian behavior of Squalane and the results were compared. Both rheological models suggest the existence of shear-band and plug-flow at high fluid pressure. Due to the differences in viscosity at GPa-level pressure, the chosen model has substantial influence on the computed shear stress and temperature distributions in the high-pressure region. This shows the importance of using correct rheology information in the whole range of pressure, temperature, and shear strain rate.

  • 6.
    Chen, Jun
    et al.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Li, Fanzhu
    Key Lab Beijing City Preparat & Proc Novel Polyme, State Key Lab Organ Inorgan Composites, Beijing University of Chemical Technology, Beijing, P. R. China.
    Luo, Yanlong
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ma, Xiaofeng
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Zhang, Meng
    Institute of Chemical Industry of Forestry Products, CAF, Nanjing, P. R. China.
    Boukhvalov, D. W.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Luo, Zhenyang
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    A self-healing elastomer based on an intrinsic non-covalent cross-linking mechanism2019In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 7, no 25, p. 15207-15214Article in journal (Refereed)
    Abstract [en]

    Synthesis and comprehensive examination of a polyurethane (urea) elastomer that self-heals based on intrinsic dynamic non-covalent bonds (van der Waals and hydrogen) are reported. The dynamic non-covalent bonds include hydrogen bonds and van der Waals forces. The difference in the previous approach in which hydrogen bond self-healing materials introduced dense quadruple hydrogen bonds at the ends or branched chains poly(propylene carbonate) (PPC) diol was used as the soft segment of the polyurethane (urea) material, and strong van der Waals forces were provided by the large number of carbonyl groups in its main chain; hydrogen bonds were formed by urethane bonds, urea bonds, and the carbonyl groups on PPC. The mechanical properties and healing efficiency of the self-healing polyurethane (urea) elastomer were studied. In situtemperature-dependent infrared and low-field nuclear magnetic resonance (LNMR) measurements were combined with molecular dynamics simulations to investigate the self-healing mechanisms. The results of the studies on the self-healing polyurethane demonstrate that the dynamic cross-linking between hydrogen bonds and van der Waals forces is the basic driving force for the self-healing ability of the material, and temperature is the key factor that affects hydrogen bonding and van der Waals forces. The effect of crystallization on the self-healing ability of the material was also studied. The molecular dynamics simulation results also demonstrate interplay between van der Waals forces and hydrogen bonds at different temperatures.

  • 7.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A smart friction control strategy enabled by CO2 absorption and desorption2019In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, no 1, article id 13262Article in journal (Refereed)
    Abstract [en]

    Intelligent control of friction is an attractive but challenging topic and it has rarely been investigated for full size engineering applications. In this work, it is instigated if it would be possible to adjust friction by controlling viscosity in a lubricated contact. By exploiting the ability to adjust the viscosity of the switchable ionic liquids, 1,8-Diazabicyclo (5.4.0) undec-7-ene (DBU)/ glycerol mixture via the addition of CO2, the friction could be controlled in the elastohydrodynamic lubrication (EHL) regime. The friction decreased with increasing the amount of CO2 to the lubricant and increased after partial releasing CO2. As CO2 was absorbed by the liquid, the viscosity of the liquid increased which resulted in that the film thickness increased. At the same time the pressure-viscosity coefficient decreased with the addition of CO2. When CO2 was released again the friction increased and it was thus possible to control friction by adding or removing CO2.

  • 8.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Active friction control by using CO2 and moisture2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Nowadays the demand for intelligent control of tribological interactions is strongly increasing in various applications. We often strive to minimize friction but there are also many situations where high friction is desirable. In some cases, something in between, i.e. optimum friction, is attractive. Driven by the broad application prospects, many controllable friction systems regulated with external stimuli such as solvent, pH, temperature, electric potential, and magnetic field have been designed and fabricated. When external stimuli are imposed on the smart materials, the macroscopic physicochemical properties of the materials are dramatically changed, making controllable friction behavior to become possible. However, most of these exploratory works are in nano/micro size and it’s difficult to use these incredible methods in macroscale directly due to that macroscopic laws of friction do not generally apply to nanoscale contacts. This thesis attempts to find more versatile methods of friction control and try to find the possibility to achieve friction control at macro-size.

    Firstly, since viscosity plays an important role in elastohydrodynamic lubrication (EHL) at macro-size, it is instigated if it would be possible to adjust friction by controlling viscosity in a lubricated contact. By exploiting the ability to adjust the viscosity of the switchable ionic liquids, 1,8-Diazabicyclo (5.4.0) undec-7-ene (DBU)/ glycerol mixture via the addition of CO2, the friction could be controlled in the EHL regime (Paper Ⅰ). In order to understand more about the lubricating mechanism of DBU/glycerol/CO2 mixture, the central film thickness of the lubricants as a function of the entrainment speed was investigated.

    Secondly, due to that adhesion could have influence on boundary lubrication (BL) friction at macro-size, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity, which can alter the H-bond types, leading the change of adhesion. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior lubricated by Choline L-Proline ([Cho][Pro]) could be modulated in a wide range of relative humidities (RH) (Paper Ⅱ).

  • 9.
    Dittes, Nicholas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Pettersson, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lang, Defeng
    SKF Research & Technology Development Nieuwegein, the Netherlands.
    Automated Dielectric Thermoscopy Characterization of Water Contaminated Grease2019In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 62, no 5, p. 859-867Article in journal (Refereed)
    Abstract [en]

    Machine maintenance is important for improving machine uptime, reliability, and reducing costs. Grease is used in most rolling element bearings, and one common failure criterion is water contamination, so developing a sensor which can detect water content automatically without human input could be a useful endeavor. The temperature dependence on the dielectric properties of water-contaminated grease is investigated in this paper with computer logged instrumentation. This method has been termed Dielectric Thermoscopy (DT). Several off the shelf (two lithium, one lithium complex, and two calcium sulphonate complex) and one unadditivized lithium grease are tested with varying amounts of water contamination from 0% to approximately 5%. Another grease is tested with small increments of added water from 0% to 0.97% to test the resolution of the measurement. The purpose is to use the capacitance temperature slope (termed dielectric thermoscopy) to show correlations to the water content of the grease sample and investigate if any grease types will pose problems in the measurement. A small, custom made fringe field capacitance sensor with an integrated temperature sensor has been used for this characterization and data is logged automatically with laboratory equipment and a PC. A useable and positive correlation to water content and the DT measurement of roughly 0.5 pF per 10 °C and percentage of water is found, although it was found that some greases have different behavior than others.

  • 10.
    Saari, Juhamatti
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics. Luleå University of Technology, SKF-LTU University Technology Centre.
    Strömbergsson, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Luleå University of Technology, SKF-LTU University Technology Centre.
    Lundberg, Jan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Thomson, A.
    SKF (U.K), Livingston, Scotland, United Kingdom.
    Detection and identification of windmill bearing faults using a one-class support vector machine (SVM)2019In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 137, p. 287-301Article in journal (Refereed)
    Abstract [en]

    The maintenance cost of wind turbines needs to be minimized in order to keep their competitiveness and, therefore, effective maintenance strategies are important. The remote location of wind farms has led to an opportunistic maintenance strategy where maintenance actions are postponed until they can be handled simultaneously, once the optimal opportunity has arrived. For this reason, early fault detection and identification are important, but should not lead to a situation where false alarms occur on a regular basis. The goal of the study presented in this paper was to detect and identify wind turbine bearing faults by using fault-specific features extracted from vibration signals. Automatic identification was achieved by training models by using these features as an input for a one-class support vector machine. Detection models with different sensitivity were trained in parallel by changing the model tuning parameters. Efforts were also made to find a procedure for selecting the model tuning parameters by first defining the criticality of the system and using it when estimating how accurate the detection model should be. Method was able to detect the fault earlier than using traditional methods without any false alarms. Optimal combination of features and model tuning parameters was not achieved, which could identify the fault location without using any additional techniques.

  • 11.
    Martin del Campo Barraza, Sergio
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Schnabel, Stephan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sandin, Fredrik
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Detection of particle contaminants in rolling element bearings with unsupervised acoustic emission feature learning2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 132, p. 30-38Article in journal (Refereed)
    Abstract [en]

    The detection of contaminants in the lubricant of rolling element bearings using acoustic emission signals is a challenging problem, in particular at high rotational speeds. This problem calls for new analysis methods beyond the conventional amplitude- and frequency-based methods. Feature learning is successfully used in the machine learning field to characterize complex signals. Here we use an unsupervised feature learning approach to distinguish acoustic emission signals. We investigate the repetition rates of features identified with shift-invariant dictionary learning and find that the signature of contaminated lubricant is significantly stronger than the effect on conventional condition indicators like the RMS and the enveloped RMS at rotational speeds above 300 rpm and up to 3000 rpm.

  • 12.
    Jain, Ayush
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Somberg, Julian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Development and Characterisation of Multi-Scale Carbon Reinforced PPS Composites for Tribological Applications2019In: Lubricants, E-ISSN 2075-4442, Vol. 7, no 4, article id 34Article in journal (Refereed)
    Abstract [en]

     Polymer-based materials show to be of increasing interest in replacing metal based

    materials in tribological applications due to their low weight, cost and easy manufacturability.

    To further reduce the environmental impact of these bearing materials recyclability is becoming

    more crucial, stimulating the need for high performing thermoplastic materials. In this study,

    polyphenylene sulfide (PPS) composites were prepared in an effort to enhance the polymers’

    tribological properties. Short carbon fibres (SCFs), graphene oxide (GO) and nano diamonds (NDs)

    as well as polytetrafluoroethylene (PTFE) were used as micro and nano reinforcements. The addition

    of SCFs especially decreased the linear coefficient of thermal expansions while enhancing the

    micro hardness and wettability of the polymer. Under water lubricated conditions, a decrease

    in friction up to 56% and a reduction of wear rate in the order of 103  was observed by the addition

    of SCF. The reduction in friction and wear was further enhanced by the addition of NDs, providing

    a synergistic effect of the reinforcements in micro and nano scale. By testing the individual

    reinforcements under dry conditions, PTFE and SCFs were especially effective in reducing friction

    while the release and consequent abrasion of NDs and SCFs increased the wear under a higher

    contact pressure.

  • 13.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    DLC and Glycerol: Superlubricity in Rolling/Sliding Elastohydrodynamic Lubrication2019In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 67, no 1, article id 23Article in journal (Refereed)
    Abstract [en]

    Low friction is one of the most important parameters for the development of machine components and machines with high efficiency. Many of the common machine components of today such as gears, rolling element bearings and cam-followers are defined by their non-conformal contacts leading to high-contact pressures, typically 1–4 GPa. The lubrication of such contacts is usually called elastohydrodynamic lubrication (EHL). Diamond-like carbon (DLC) coatings and glycerol have individually been shown to produce low friction in boundary, mixed and full film lubrication. A few studies have been conducted using both glycerol and DLC-coated surfaces to achieve even lower friction in pure sliding boundary-lubricated contacts. However, the literature is lacking studies of how the combination of glycerol and DLC performs in non-conformal rolling/sliding contacts where many common machine components operate. Such a study is presented in this article where a ball-on-disc test rig is used to investigate the performance of the combination of DLC and glycerol at pressures up to 1.95 GPa at various entrainment speeds and slide-to-roll ratios. The investigation shows that the DLC-glycerol combination provides very low friction values, in some cases, below the superlubricity threshold of 0.01, possibly shown for the first time at such high pressure in a non-conformal rolling/sliding contact. The low friction mechanism in full film lubrication is a combination of the low pressure-viscosity and high temperature-viscosity sensitivity of glycerol in combination with thermal insulation of the DLC coating and is presented as thermally assisted liquid superlubricity.

  • 14.
    Dai, B.
    et al.
    China Rural Technology Development Center, Beijing, China.
    Zhu, W.
    Department of Pharmaceutical Engineering, China Pharmaceutical University, Nanjing, Jiangsu, China.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Guo, X.
    Ministry of Education Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, Shanghai, China .
    Qian, H.
    Department of Pharmaceutical Engineering, China Pharmaceutical University, Nanjing, Jiangsu, China.
    Liang, X.
    Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kongens Lyngby, Denmark.
    Kontogeorgis, G.M.
    Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kongens Lyngby, Denmark.
    Effect of the composition of biomass on the quality of syngas produced from thermochemical conversion based on thermochemical data prediction2019In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 33, no 6, p. 5253-5262Article in journal (Refereed)
    Abstract [en]

    Syngas produced from thermochemical conversion of biomass has been given more attention because it can be converted to a variety of fuels and chemicals as substitutes for petroleum-based chemicals via the Fischer–Tropsch process. In this study, one wheat straw and its element content fluctuation in the feasible range are selected as samples first to study the effect of the biomass composition on the quality of syngas produced. Then, the thermochemical data (standard molar enthalpy of formation, standard molar entropy, and heat capacity) of samples are predicted by highly accurate prediction models. Thermochemical conversions of the samples are simulated by the Gibbs energy minimization method based on the results of thermochemical data prediction. At last, the effect of the biomass composition on the resource index (amounts of CO and H2 and ratio of H2/CO) and energy index (lower heat value) of syngas is calculated and analyzed. This study provides a method to obtain the relationship between the composition of biomass and the quality of syngas produced.

  • 15.
    Zhang, Yumeng
    et al.
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Zhu, Wei
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Li, Jiahui
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Zhu, Yudan
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Wang, Anran
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Lu, Xiaohua
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Li, Wei
    European Bioenergy Research Institute (EBRI), Aston Institute of Materials Research (AIMR), Aston University, Birmingham, England, UK.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effects of ionic hydration and hydrogen bonding on flow resistance of ionic aqueous solutions confined in molybdenum disulfide nanoslits: Insights from molecular dynamics simulations2019In: Fluid Phase Equilibria, ISSN 0378-3812, E-ISSN 1879-0224, Vol. 489, p. 23-29Article in journal (Refereed)
    Abstract [en]

    Single-layer molybdenum disulfide (MoS2) is a novel two-dimensional material that has attracted considerable attention because of its excellent properties. In this work, molecular dynamics simulations were performed to investigate the effect of different kinds of alkali metal ions (Li+, Na+, and K+) on the flow resistance of ionic aqueous solutions confined in MoS2 nanoslits under shearing. Three slit widths (i.e. 1.2, 1.6, and 2.0 nm) were investigated. Simulation results showed that the friction coefficient followed the order of K+ < Na+ < Li+. The friction coefficient decreased with the increasing of slit width. Unique confined spatial distributions of different types of ionic aqueous solutions led to different confined ionic hydrations for different cations. These differences lead to different orientations of surrounding water molecules and then form different hydrogen bond (HB) networks. The friction coefficient was greatly dependent on the number of HBs per water; i.e., the larger the number of HBs formed, the lower was the flow resistance.

  • 16.
    Gebretsadik, Daniel
    et al.
    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.
    Embeddability behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine oil2019In: Tribology - Materials, Surfaces & Interfaces, ISSN 1751-5831, E-ISSN 1751-584X, Vol. 13, no 1, p. 39-49Article in journal (Refereed)
    Abstract [en]

    One of the tribological requirements on engine bearing material is its ability to safely embed contaminant particles onto its surface and minimise damage to both the bearing and crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load has been employed to investigate the embeddability behaviour of selected multi-layered Pb-free engine bearing materials at three different rotational speeds using engine oil contaminated with SiC particles. Experimental results have shown that third-body abrasive wear is influenced by the lubricant film thickness. There was also difference in embeddability of the different materials. Bismuth-based overlay and MoS2 containing polyamide-imide-based overlay-coated materials show higher wear compared to tin-based overlay and a polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding against bismuth-based overlay and MoS2 containing polyamide-imide-based overlay exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-based composite overlay. 

  • 17.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Habchi, W.
    Lebanese American University, Department of Industrial and Mechanical Engineering, Byblos.
    Bair, S.
    G.W. Woodruff School of Mechanical Engineering, Centre for High Pressure Rheology, Georgia Institute of Technology, 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.
    Erratum: Towards the true prediction of EHL friction2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 133, p. 297-297Article in journal (Refereed)
  • 18.
    Ràfols, Francesc Pérez
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Generating randomly rough surfaces with given height probability distribution and power spectrum2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 131, p. 591-604Article in journal (Refereed)
    Abstract [en]

    In this work we present a simple method to generate surface topography. The main advantage of the presented method as compared with those available in the literature is that the power spectrum and the height probability distribution can be specified independently. In this article we present the method and show its versatility by generating surface topographies with three different height probability distributions: the Weibull distribution, a bimodal distribution and a distribution containing a delta function that represents worn surfaces. The MATLAB-code we used to generate the numerical examples are also provided to the reader.

  • 19.
    Mozgovoy, Sergej
    et al.
    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.
    High Temperature Friction and Wear Performance of PVD Coatings under Press Hardening Contact Conditions2019In: Advances in Tribology, ISSN 1687-5915, E-ISSN 1687-5923, Vol. 2019, article id 4981246Article in journal (Refereed)
    Abstract [en]

    Press hardening is widely employed to produce automotive structural and safety components from advanced high-strength steels. This process depends on friction between the forming tools and the work piece. Wear of the forming tools affects the dimensional accuracy of produced components and reduces their service life. It is therefore desirable to reduce wear of forming tools for press hardening applications. One way to achieve this is by applying hard physical vapour deposited (PVD) coatings on the tool. In this work, the tribological behaviour of PVD coated tool-work piece material pairs has been studied at elevated temperatures in an experimental set-up simulating the tribological conditions in press hardening. Four different PVD coatings deposited on tool steel and uncoated tools as reference were studied during sliding against uncoated and Al-Si coated 22MnB5 steel. Results show that uncoated tools exhibited the lowest coefficient of friction when sliding against uncoated 22MnB5 steel. A CrWN coating initially showed low coefficient of friction but it increased with increasing sliding distance. A TiAlN coating and one of two AlCrN coatings showed similar frictional behaviour when sliding against uncoated 22MnB5 steel. During sliding against uncoated 22MnB5 steel, adhesive wear has been found to be the dominant wear mechanism. Adhesive wear was considerably reduced in the case of hard PVD coated tools in comparison to that of uncoated tools. During sliding against Al-Si coated 22MnB5 steel, no clear advantage in terms of friction behaviour of uncoated or PVD coated tools was observed. However, the transfer of Al-Si coating material from the work piece to the tools was significantly reduced for PVD coated tools. Frictional instabilities in all cases involving Al-Si coated work piece material further confirmed the occurrence of adhesive material transfer.

  • 20.
    Johansson, Jens
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Devlin, Mark T.
    Fundamental Research, Afton Chemical Corporation, Richmond, VA, USA.
    Guevremont, Jeffrey M.
    Fundamental Research, Afton Chemical Corporation, Richmond, VA, USA.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Improving Hypoid Gear Oil Pitting Performance through Friction Reduction2019In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397XArticle in journal (Refereed)
    Abstract [en]

    Earlier studies have shown that the load-dependent friction behavior of various gear oils can affect their pitting performance; that is, low friction resulted in a long pitting life. These studies were limited, however, to test methods and running conditions quite different from those occurring in actual gear transmissions. In the present study, a more gear-like twin-disc machine with test specimens and running conditions relevant for gear contacts was used to investigate whether the same trends could be found. To analyze this possible correlation, the first step was to prepare a set of hypoid gear oils and to test their friction performance to compare various ways of improving friction behavior but also to form an understanding of why their friction performance varied. The second step was to test the pitting performance of the oils. The pitting results could then be compared to the friction properties of the oils to analyze the correlation. Other possible mechanisms behind the formation of pits are also discussed. The results show that for the oils included, the antiwear and extreme-pressure additive package and the base oil type affect friction. The results further show that additive combinations and/or base oils that result in low friction lead to enhanced pitting performance.

  • 21.
    Wu, Jian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Improving the Lubricating Performance of Ionic Liquids for Different Contacts2019Doctoral thesis, comprehensive summary (Other academic)
  • 22.
    Hultqvist, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Vrček, Aleks
    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.
    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.
    Influence of lubricant pressure response on sub-surface stress in elastohydrodynamically lubricated finite line contacts2019In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 141, no 3, article id 031502Article in journal (Refereed)
    Abstract [en]

    In order to adapt to increasingly stringent CO2 regulations, the automotive industry must develop and evaluate low cost, low emission solutions in the powertrain technology. This often implies increased power density and the use of low viscosity oils, leading to additional challenges related to the durability of various machine elements. Therefore, an increased understanding of lubricated contacts becomes important where oil viscosity-pressure and compressibility-pressure behaviour have been shown to influence the film thickness and pressure distribution in EHL contacts, further influencing the durability. In this work, a finite line EHL contact is analysed with focus on the oil compressibility- and viscositypressure response, comparing two oils with relatively different behaviour and its influence on subsurface stress concentrations in the contacting bodies. Results indicate that increased pressure gradients and pressure spikes, and therefore increased localized stress concentrations, can be expected for stiffer, less compressible oils, which under transient loading conditions not only affect the outlet but also the edges of the roller

  • 23.
    Rodiouchkina, Maria
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Berglund, Kim
    Larsson, Roland
    Influence of operational and surface conditionson the tribological performance of self-lubricating polymer composite bearingmaterials used in hydropower applications2019In: Influence of operational and surface conditionson the tribological performance of self-lubricating polymer composite bearingmaterials used in hydropower applications, 2019Conference paper (Refereed)
    Abstract [en]

    In hydropower applications, self-lubricating polymer composite bearings has proven to be a good and environmentally friendly replacement for the traditionally used grease lubricated bronze bearings. However, in recent years, end users have experienced several bearing failures due to more demanding operating conditions due to integration of fluctuating renewable energy sources, e.g. wind and solar power, into the electric power systems.

    The aim of this work is to summarize and highlight important findings regarding the influence of various parameters on the tribological behaviour of these bearing materials using a linear reciprocating pin-on-disc configuration.

    Results indicates that low sliding speed and high nominal pressure offer the best performance for these bearing materials, with a reduction in frictional loses with up to 45% and almost three times lower wear. Furthermore, friction and wear can be reduced even more by optimizing the surface topography and hardness of the shaft.

  • 24.
    Hansen, Jonny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Gear Technology, Transmission Development, Scania CV AB, Södertälje.
    Björling, Marcus
    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.
    Mapping of the lubrication regimes in rough surface EHL contacts2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 131, p. 637-651Article in journal (Refereed)
    Abstract [en]

    Understanding film formation in rough surface elastohydrodynamically lubricated (EHL) contacts have been an ongoing pursuit in lubrication science for more than half a century. This study furthers that quest by establishing a single combined friction and electrical contact resistance map that forms a clear and comprehensive overview of the lubrication performance. A ball-on-disc machine was operated under a wide variety of heavily loaded rolling/sliding contact conditions. Results show that while sweeping the contact over the SRR- and entrainment speed-domain, the primary sweep direction significantly affects running-in and consequently the transition from full-film to the mixed lubrication regime. Such knowledge sheds new light into the mechanisms that governs EHL film formation and the concurrent interplay with the mixed lubricated friction coefficient.

  • 25.
    Mozgovoy, Sergej
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Alik, Lotfi
    Luleå tekniska universitet.
    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.
    Material transfer during high temperature sliding of Al-Si coated 22MnB5 steel against PVD coatings with and without aluminium2019In: Wear of Materials, Elsevier, 2019, Vol. 426-427, p. 401-411Conference paper (Refereed)
    Abstract [en]

    Press hardening of Al-Si coated 22MnB5 steel is the dominant technology to enable light weight design in automotive applications. Transfer of the Al-Si coating onto the tool surface occurs during hot forming. This affects process economy and quality of produced components. The reported galling mechanisms are adhesion and compaction of wear debris. Surface engineering of forming tools has been proposed to minimise the transfer of Al-Si coating. Plasma nitriding of tool steel surfaces reduces adhesion but has poor abrasive wear resistance. PVD coatings have generally been found to promote galling due to higher chemical affinity but improve abrasive wear resistance. Most studied PVD coatings are transition metal nitrides containing aluminium. The aim of this study is to investigate the role of aluminium in PVD coatings and its effect on transfer of Al-Si coating material during sliding against coated tool steel at high temperatures. This work has focussed on PVD coatings (AlCrN and CrWN) deposited on plasma nitrided tool steel. Their tribological behaviour was studied using a hot strip-drawing tribometer capable of simulating the conditions prevalent in press hardening. The results showed that PVD coatings containing aluminium induce more material transfer. The material transfer is mainly related to chemical affinity since all coatings were polished to a low surface roughness (Sa =~120 nm) to minimise transfer initiated by surface defects. The hardness of the PVD coatings does not seem to influence the material transfer since the softer coating (CrWN, HV0.05 = ~1850) showed less transfer compared to AlCrN (HV0.05 = ~2100). The CrWN coating showed longer running-in compared to AlCrN due to reduced initial material transfer. Formation of thicker transfer layers governs the steady state friction mechanisms. Material transfer of Fe-Al intermetallic compounds occurs at the initial stages of sliding through direct adhesion to the PVD coating. The layers grow to > 5 µm thickness within a few decimetres of sliding.

  • 26.
    Vrček, Aleks
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hultqvist, Tobias
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Baubet, Yannick
    SKF, Nieuwegein.
    Björling, Marcus
    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.
    Micro-pitting and wear assessment of engine oils operating under boundary lubrication conditions2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 129, p. 338-346Article in journal (Refereed)
    Abstract [en]

    Current state-of-the-art engine oils tend to enhance micro-pitting damage in rolling contacts under certain operating conditions. ZDDP anti-wear additive was shown to promote such behavior. However, in order to optimize an engine oil formulation for rolling contacts, further studies are needed to assess engine oils in terms of micro-pitting and wear damage. This investigation studies the micro-pitting and wear performance of a number of engine oils for rolling contacts in a ball-on-disc configuration under conditions prevalent in crankshaft roller bearing applications. Based on the results it was concluded that an engine oil containing higher blend of PAO base oil compared to the oil mixture of Group III and PAO has a lower tendency towards micro-pitting and wear.

  • 27.
    Vrček, Aleks
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hultqvist, Tobias
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Baubet, Yannick
    SKF Engineering & Research Centre, Nieuwegein, The Netherlands.
    Björling, Marcus
    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.
    Micro-Pitting and Wear Assessment of PAO vs Mineral-Based Engine Oil Operating under Mixed Lubrication Conditions: Effects of Lambda, Roughness Lay and Sliding Direction2019In: Lubricants, E-ISSN 2075-4442, Vol. 7, no 5, article id 42Article in journal (Refereed)
    Abstract [en]

    Under certain operating conditions, rolling contacts have been shown to experience some challenges when lubricated with engine oils containing zinc dialkyldithophosphate (ZDDP) anti-wear additive. In order to better understand the main damage mechanisms during various operating conditions, further studies are needed. This article studies micro-pitting and wear damages of bearing steel surfaces under mixed lubrication conditions in a ball-on-disc setup, lubricated with different engine oils. Based on the results, micro-pitting and wear damage is shown to be highly case-dependent. In general, PAO-based engine oil tends to eliminate micro-pitting damage compared to mineral-based engine oil at less severe lubricating conditions. Moreover, a critical lambda was found for both oils, where the highest micro-pitting damage was observed. 

  • 28.
    Vrček, Aleks
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hultqvist, Tobias
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Baubet, Yannick
    SKF Research and Technology Development, Nieuwegein, Netherlands.
    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.
    Micro-pitting Damage of Bearing Steel Surfaces under Mixed Lubrication Conditions: Effects of Roughness, Hardness and ZDDP Additive2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 138, p. 239-249Article in journal (Refereed)
    Abstract [en]

    Micro-pitting presents a failure of the rolling/sliding contact metal asperities operating under boundary/mixed lubrication conditions. The studies have shown that micro-pitting failure competes with mild wear and that lubricant additives can have either detrimental or beneficial effects on micro-pitting evolution. This article describes a methodology to investigate micro-pitting damage on bearing steels using a twin-disc machine to better represent mechanical components, i.e. bearings, crankshafts, etc. In addition, effects of roughness, hardness and the ZDDP additive are presented and discussed. A sufficient hardness difference can completely eliminate micro-pitting damage mode. Furthermore, the presence of ZDDP anti-wear additive in fully formulated engine oil was shown to protect rougher surfaces and promote wear on smoother surfaces, thus completely eliminating the micro-pitting damage mode.

  • 29.
    Zhu, Wei
    et al.
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Zhang, Cheng
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Zhu, Yu-dan
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    An, Rong
    Herbert Gleiter Institute of Nanoscience, Nanjing University of Science & Technology, Nanjing, China.
    Lu, Xiao-hua
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Shi, Yi-jun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Jiang, Sheng-yu
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China.
    Molecular insights on the microstructures of nanoconfined glycerol and its aqueous solutions: The effects of interfacial properties, temperature, and glycerol concentration2019In: Journal of Molecular Liquids, ISSN 0167-7322, E-ISSN 1873-3166, Vol. 291, article id 111238Article in journal (Refereed)
    Abstract [en]

    In this work, we conducted molecular dynamics simulations to investigate the effects of rutile–liquid and graphene–liquid interfacial properties, glycerol concentrations (i.e., 10%, 40%, 70%, and 100% molar contents), and temperature (i.e., 180, 273, and 320 K) on the microstructures of nanoconfined glycerol and its aqueous solutions. Results indicated that the effect of interfacial properties on the spatial and orientation distributions of nanoconfined glycerol and water molecules was more prominent than that of temperature. In glycerol aqueous solutions, water and glycerol molecules localized into two distinct layers on the hydrophilic rutile surface but partially mixed with each other near the hydrophobic graphene surface, because water molecules near the hydrophobic surface exhibited more random orientations and formed more hydrogen bonds with glycerol molecules than those near the hydrophilic surface. Moreover, interface introduction and increased temperature drastically reduced the hydrogen bonding ability of water molecules in glycerol aqueous solutions. The addition of glycerol molecules can break hydrogen bonds between water molecules and inhibit water crystallization in glycerol aqueous solutions. Temperature and glycerol concentration exerted limited effects on the hydrogen bonding ability of water molecules on hydrophilic surfaces in glycerol aqueous solutions. Meanwhile, the presence of a hydrophilic surface can effectively decelerate hydrogen bond breakage induced by increasing temperature or by decreasing glycerol contents

  • 30.
    Strömbergsson, Daniel
    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.
    Berglund, Kim
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Saari, Juhamatti
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Thomson, Allan
    Industrial Digitalisation & Solutions, Livingston, Scotland.
    Mother wavelet selection in the discrete wavelet transform for condition monitoring of wind turbine drivetrain bearings2019In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 22, no 11, p. 1581-1592Article in journal (Refereed)
    Abstract [en]

    Although the discrete wavelet transform has been used for diagnosing bearing faults for two decades, most work in this field has been done with test rig data. Since field data starts to be made more available, there is a need to shift into application studies.

    The choice of mother wavelet, ie, the predefined shape used to analyse the signal, has previously been investigated with simulated and test rig data without consensus of optimal choice in literature. Common between these investigations is the use of the wavelet coefficients' Shannon entropy to find which mother wavelet can yield the most useful features for condition monitoring.

    This study attempts to find the optimal mother wavelet selection using the discrete wavelet transform. Datasets from wind turbine gearbox accelerometers, consisting of enveloped vibration measurements monitoring both healthy and faulty bearings, have been analysed. The bearing fault frequencies' excitation level has been analysed with 130 different mother wavelets, yielding a definitive measure on their performance. Also, the applicability of Shannon entropy as a ranking method of mother wavelets has been investigated.

    The results show the discrete wavelet transforms ability to identify faults regardless of mother wavelet used, with the excitation level varying no more than 4%. By analysing the Shannon entropy, broad predictions to the excitation level could be drawn within the mother wavelet families but no direct correlation to the main results. Also, the high computational effort of high order Symlet wavelets, without increased performance, makes them unsuitable.

  • 31.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Burtseva, Evgeniya
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    New insights on lubrication theory for compressible fluids2019In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 145, article id 103170Article in journal (Refereed)
    Abstract [en]

    The fact that the film is thin is in lubrication theory utilised to simplify the full Navier–Stokes system of equations. For incompressible and iso-viscous fluids, it turns out that the inertial terms are small enough to be neglected. However, for a compressible fluid, we show that the influence of inertia depends on the (constitutive) density-pressure relationship and may not always be neglected. We consider a class of iso-viscous fluids obeying a power-law type of compressibility, which in particular includes both incompressible fluids and ideal gases. We show by scaling and asymptotic analysis, that the degree of compressibility determines whether the terms governing inertia may or may not be neglected. For instance, for an ideal gas, the inertial terms remain regardless of the film height-to-length ratio. However, by means of a specific modified Reynolds number that we define we show that the magnitudes of the inertial terms rarely are large enough to be influential. In addition, we consider fluids obeying the well-known Dowson and Higginson density-pressure relationship and show that the inertial terms can be neglected, which allows for obtaining a Reynolds type of equation. Finally, some numerical examples are presented in order to illustrate our theoretical results.

  • 32.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-corrosive Green Lubricant With Dissolved Lignin in Ionic Liquids Behave as Ideal Lubricants for Steel-DLC Applications2019In: Frontiers in Chemistry, E-ISSN 2296-2646, E-ISSN 2296-2646, Vol. 7, article id 857Article in journal (Refereed)
    Abstract [en]

    Diamond-like carbon (DLC)–steel contacts become more and more popular in the

    industry now. Since the surface chemical properties of DLC are quite different from

    those of iron, traditional formulated lubricants have problems to form tribo-chemical

    films, which are effective to improve the tribological performance for steel-steel contacts,

    on the surface of DLC. Thus, new lubricants formulation strategies are needed to be

    considered for steel-DLC applications. A kind of green lubricant (lignin-[Choline][L-Proline]

    (L-[CH][Pro])) without any traditional tribo-chemical active element, i.e., free of P, S, B,

    etc., was studied in this paper for the steel-DLC contact. To find the difference between

    this new ILs and the traditional lubricants, a commercially available fully formulated

    lubricant was used as a reference. An Optimol SRV-III oscillating friction and wear tester

    was used to evaluate the tribological performance. Three different kinds of commercially

    available DLC coatings (Tribobond 40(Cr + a-C:H:W), Tribobond 43 [(Cr+) a-C:H),

    and Tribobond 44(a-C:Cr)] were investigated. The results show that the ILs exhibit an

    obviously lower friction coefficient than that of the traditional commercially available fully

    formulated lubricant. Among those three DLC coatings, the (Cr+) a-C:H DLC coating

    exhibits the biggest improvement of wear resistance lubricated with the new ILs than

    that of the commercially available fully formulated lubricant. It’s expected that its excellent

    tribological properties are attributed to the affinity of the ILs to the metal surface and

    the strength of the ionic liquids interactions by hydrogen bonding. Thus, forming strong

    physical adsorption strategy, instead of forming chemical tribo-films, is recommended to

    enhance the lubricating performance of lubricants for DLC.

  • 33.
    Yu, Qiangliang
    et al.
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China.
    Zhang, Chaoyang
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China.
    Dong, Rui
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China; University of Chinese Academy of Sciences, Beijing, China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Yurong
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China; University of Chinese Academy of Sciences, Beijing, China.
    Bai, Yanyan
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China; University of Chinese Academy of Sciences, Beijing, China.
    Zhang, Jiaying
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China; School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China.
    Cai, Meirong
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China.
    Zhou, Feng
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China.
    Novel N, P-containing oil-soluble ionic liquids with excellent tribological and anti-corrosion performance2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 132, p. 118-129Article in journal (Refereed)
    Abstract [en]

    Two oil-soluble ionic liquids N, N-trimethyl-N-hexadecyl ammonium bis(2-ethylhexyl) phosphate (NP-16))and 1,2-bis-NN-dimethyl-N-cetylammonium bis (2-ethylhexyl) phosphate (NP-16-2-16) have been synthesized and investigated as a potential friction reduction and anti-wear lubricant additive with anti-corrosion properties. The results indicate that the addition of 1.0 wt % NP-16 into PAO10 can significantly reduce friction coefficient and improve its anti-wear performance under RT and 100 °C. The performance is obviously better than that of traditional additives ZDDP and tricresyl phosphate. PAO10 + 1.0 wt % NP-16 even outperformed the PAO10 with about 40% lower friction and 2 orders of magnitude less wear. The analysis results of SEM, EDS and XPS show that an FePO4 or nitrogen-containing tribofilm is formed on the friction interface when NP-16 and NP-16-2-16 are selected as additives, which play a key role in reducing friction and wear.

  • 34.
    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.
    Numerical investigation of galling in a press hardening experiment with AlSi-coated workpieces2019In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 99, p. 85-96Article in journal (Refereed)
    Abstract [en]

    Press hardened steels are commonly used as a lightweight choice for manufacturing car components because of the high ratio of strength to weight. The use of ultra-high-strength steels for the design of lightweight vehicles contributes to the reduction of emissions of carbon dioxide while maintaining passenger safety. Stamping tools used in press hardening processes suffer harsh contact conditionsin terms of dramatic temperature changes, cyclic loadings, and complex interactions between coatings and oxidation. In mass production, tool wear is an inevitable problem that increases maintenance costs. Severe adhesive wear, also called galling, substantially occurs in the stamping tool used against Al—Si-coated workpieces. The galling that takes place during press hardening not only degrades the production quality but also shortens the service life of the tool. In order to properly arrange tool maintenance and minimize galling through adjusting process parameters, engineers need to know when and where galling occurs, based on modelling of the galling in press hardening simulations. In order to implement a galling simulation for press hardening, a modified Archard wear model is employed in the present study, which is a contact-mechanics-based model. The specific wear rate in the model is calibrated by the quantitative galling measurements of a high-temperature tribometer test. The tribological test is designed to mimic the press hardening conditions, where the correlations between galling and process parameters such as temperature, pressure, and sliding distance are outlined. The galling simulation is implemented in a full-scale press hardening experiment, and the predicted galling is validated in terms of severe galling positions and galling profiles. The galling profile evolution is correlated to variations in the contact conditions. Uncertainties in the numerical model, such as the choice of penalty scaling factor and friction coefficient, are analysed with a parameter study and discussed. This study demonstrates finite element (FE) simulations involving galling prediction in press hardening so as to improve product development and production efficiency.

  • 35.
    Ernens, Dennis
    et al.
    Shell Global Solutions International. University of Twente.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Van Hoecke, Dennis
    Ocas N. V..
    Roijmans, Roel F. H.
    Shell Global Solutions International.
    van Rie, Egbert J.
    Shell Global Solutions International.
    Vande Voorde, John B. E.
    Ocas N.V..
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bas de Rooij, Matthias
    University of Twente.
    Roggeband, Serge Mathieu
    Shell Global Solutions International.
    van Haaften, Willem Maarten
    Shell Global Solutions International.
    Vanderschueren, Marc
    Ocas N.V..
    Thibaux, Phillipe
    Ocas N.V..
    Pasaribu, Henry Rihard
    Shell Global Solutions International.
    On the Sealability of Metal-to-Metal Seals With Application to Premium Casing and Tubing Connections2019In: SPE Drilling & Completion, ISSN 1064-6671, E-ISSN 1930-0204Article in journal (Refereed)
    Abstract [en]

    Metal-to-metal seals are used in connections of casing and tubing in oil and gas wells. This paper describes the mechanisms of sealing metal-to-metal seals as investigated using an experimental setup and a stochastic numerical sealing model. Experiments were conducted for a variety of thread compounds and applied pin/box surface coatings. The results were used to validate a stochastic numerical sealing model for sealability. The model couples a contact-mechanics model with a flow model and takes into account the influence of all the surface-topography features by introducing the concept of seal permeability. Once validated, the model was used together with the experimental results to better understand the sealing mechanisms of metal-to-metal seals.

    The sealing configuration is a face seal with an 80-mm roundoff radius on one face pressing against a flat on the other face. The face-seal specimens were manufactured from P110 tubing to ensure material properties that are representative for casing or tubing. The test setup used is designed for investigating only the metal-to-metal seal of the connection. The setup can perform rotary sliding under constant load to simulate surface changes during makeup and subsequently perform a leakage test. The sealing limit is determined by applying 700-bar fluid pressure and then gradually reducing the normal force until leakage is observed. The data are subsequently used to validate the previously published stochastic numerical sealing model.

    The results indicate a strong dependence on the type of thread compound used for the onset of leakage. The thread compound affects the amount of wear and thus changes the surface topography of the interacting surfaces. It is shown that the stochastic numerical sealing model is capable of predicting the onset of leakage within the experimental accuracy. The model shows further that certain surface topographical features improve the sealing performance. In particular, a surface manufactured by turning on a lathe that is in contact with, for instance, a smooth shot-blasted surface topography leads to highly localized contact areas, which in turn yield the best sealing performance.

  • 36.
    Ernens, Dennis
    et al.
    Shell Global Solutions International BV. University of Twente.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Van Hoecke, Dennis
    OCAS NV.
    Roijmans, Roel F. H.
    Shell Global Solutions International BV.
    van Riet, Egbert J.
    Shell Global Solutions International BV.
    Voorde, John Vande
    OCAS NV.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bas de Rooij, Matthijn
    University of Twente.
    Roggeband, Serge Mathieu
    Shell Global Solutions International BV.
    Van Haaften, Willem Maarten
    Shell Global Solutions International BV.
    Vanderschueren, Marc
    OCAS NV.
    Thibaux, Phillipe
    OCAS NV.
    Pasaribu, Henry Rihard
    Shell Global Solutions International BV.
    On the Sealability of Metal-to-Metal Seals with Application to Premium Casing Connections2019Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Metal-to-metal seals are used in connections of casing in oil and gas wells. This paper describes the mechanisms of sealing of metal-to-metal seals as investigated using an experimental set-up and a sealability model. Experiments were conducted for a variety of thread compounds and applied pin/box surface coatings. The results were used to validate a numerical model for sealability. The stochastic model couples a contact mechanics model with a flow model and takes the influence of all the surface topography features into account. Once validated, the model was used together with the experimental results to explain the sealing mechanisms of metal-to-metal seals. The sealing configuration is a face seal with an R=80 mm round-off radius pressing against a flat. The face seal specimens were manufactured from P110 tubing. The used test set-up is designed for investigating only the metal-to-metal seal of the connection. The set-up can carry out rotary sliding under constant load to simulate surface evolution during make-up and subsequently perform a leakage test. The sealing limit is determined by applying 700 bar fluid pressure and then gradually reducing the normal force until leakage is observed. The data is subsequently used to validate a previously published model. The results indicate a strong dependence of the type of thread compound used on the onset of leakage. The thread compound affects the amount of wear and thus changes the surface topography of the interacting surfaces. It is shown that the sealability model is capable to predict the onset of leakage within the experimental accuracy. The model shows further that certain surface topographical features improve the sealing performance. Namely, a turned against a flat surface topography leads to highly localized contact areas, which in turn yields the best sealing performance. The combination of experimental data with the validated model leads to much deeper insights for the sealing mechanisms than what could be obtained using either on their own.

  • 37.
    Hansen, Jonny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    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.
    On the surface lift-off transition in rough surface EHL contacts2019Conference paper (Refereed)
    Abstract [en]

    The pumping and churning losses in transmission assemblies are minimized when gears are operated in low viscosity lubricants. Therefore, in order to improve gear efficiency, it is of crucial importance to gain knowledge about the underlying mechanism that governs elastohydrodynamic (EHL) contacts ability to form a separating oil film. This study was set out to explore the necessary requirements for EHL contacts to achieve a state of full film separation. A ball on disc device, arranged for electrical contact resistance (ECR) measurement, was operated under a wide variety of heavily loaded rolling/sliding conditions. Friction and ECR-signal were simultaneously monitored to capture the contact performance until surfaces achieved lift-off by adequate surface modification due to running-in. Special emphasis was set on post-test surface analysis to reveal whether any surface roughness parameter could provide insights in the pre-requisite for surface lift to take place.

  • 38.
    Wu, Jian
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Nanjing Tech University, Nanjing, China.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Feng, Xin
    Nanjing Tech University, Nanjing, China.
    Lu, Xiaohua
    Nanjing Tech University, Nanjing, China.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Poly(alkylimidazolium bis(trifluoromethylsulfonyl) imide)-Based Polymerized Ionic Liquids: A Potential  High-Performance Lubricating Grease2019In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 6, no 5, article id 1801796Article in journal (Refereed)
    Abstract [en]

    Polymers prepared from ionic liquids are widely called polymerized ionic liquids (PILs). Compared to monocationic and dicationic ILs, PILs have higher molecular weights, charge, and greater intermolecular interactions, which make PILs have a higher possibility to generate better lubricity. PILs of poly‐alkylimidazolium bis(trifluoromethylsulfonyl)imide (PImC6NTf2) is studied herein. Dicationic ILs of 1,1′‐(pentane‐1,5‐diyl)‐bis(3‐butylimidazolium) bis(trifluoromethylsulfonyl)imide (BIm5‐(NTf2)2) is used as additive to decrease the crystallization temperature of PImC6NTf2. Lubricity of PImC6NTf2 and PImC6NTf2+BIm5‐(NTf2)2, as well as BIm5‐(NTf2)2 for comparison is evaluated under severe conditions, i.e., 3.0 to 3.5 GPa and 200 °C. The rheological study suggests that PImC6NTf2 can be classified into grease. Tribological test results show that PImC6NTf2 has much better antiwear property than BIm5‐(NTf2)2, especially at 3.5 GPa. Adding 4% BIm5‐(NTf2)2 to PImC6NTf2 is able to reduce friction under high pressure. At 200 °C, PImC6NTf2 exhibits excellent lubricity. The mixture of 96%PImC6NTf2+4%BIm5‐(NTf2)2 shows even better antiwear property than neat PImC6NTf2 and exhibits the highest friction reducing property among the ILs at 200 °C. It is speculated that the robust strength of PILs and strong adhesion between PILs and solids are key factors in achieving the excellent antiwear property.

  • 39.
    Shetty, Pramod
    et al.
    Luleå University of Technology.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Polyelectrolyte Cellulose Gel with PEG/Water: Toward Fully Green Lubricating Grease2019In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344Article in journal (Refereed)
    Abstract [en]

    Developing a fully green lubricant is an urgent need due to the growing consciousness of environmental protection and dwindling resources. In this work, fully green gel lubricants were developed out of cellulose derivatives as gelator and mixture of water and poly(ethylene glycol) 200 (PEG 200) as the base fluid. The non-ionic hydroxyethyl cellulose (HEC) and anionic sodium carboxymethyl cellulose (NaCMC) were chosen to understand the effect of ionic and non-ionic gelators on the thermal, rheological and the tribological properties of the gel lubricant. HEC or NaCMC is demonstrated as effective additive to reduce wear, stabilize friction coefficient and enhance the thermal stability of developed lubricants. It is shown that anionic gelator will result in producing lower friction and wear in comparison to non-ionic gelator, which may be attributed to the possible tribo-film formation due to the negative charge in the NaCMC molecules and its larger molecular weight.

  • 40.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Scientific Computing with Applications in Tribology: A course compendium2019Other (Other academic)
    Abstract [en]

    This compendium comprises models and numerical solution procedure for tribological interfaces. It describes the tribological contact and the classical lubrication regimes. A thorough derivation of the Reynolds equation, governing the fluid pressure, from the Navier-Stokes momentum equations and the continuity equation for conservation of mass, is presented along with its analytical solution for the infinitely wide linear slider bearing.

    The compilation of the compendium was conducted by the first author during his tenure as Professor at the Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology and by the second author during his tenure as a postdoctoral researcher at the same division.

    Although the compilation of this text is the work solely of the authors, the models and solution procedure presented herein is joint development of many good colleagues and co-authors. Our sincere gratitude is extended towards them all.

  • 41.
    Lan, Xiaoyu
    et al.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Ma, Xiaofeng
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Wang, Lei
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Gu, Qun
    Department of Chemistry, Edinboro University of Pennsylvania, Edinboro, United States.
    Wu, Linlin
    College of Materials Science and Engineering, Nanjing Tech University, Nanjing, P. R. China.
    Gu, Xiaoli
    College of Chemical Engineering, Nanjing Forestry University, Nanjing, P. R. China.
    Luo, Zhenyang
    College of Science, Nanjing Forestry University, Nanjing , P. R. China.
    Self-Assembly of Diblock Copolymers Containing Thermo- and Photoresponsive Lower Critical Solution Temperature Phase Behavior Polymer with Tunable Assembly Temperature in an Ionic Liquid Mixture2019In: ACS Omega, E-ISSN 2470-1343, Vol. 4, no 6, p. 11229-11236Article in journal (Refereed)
    Abstract [en]

    This work prepared a type of diblock copolymer with thermo- and photosensitivity in ionic liquids (ILs). P(N,N-dimethylacrylamide) (compatible with ILs) was prepared as one segment, while butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) were copolymerized as another segment P(AzoMA-r-BA) with stimuli responsiveness. The diblock copolymer showed tunable lower critical micellization temperature (LCMT) in two mixed imidazole ionic liquids. The value of LCMT depends on not only the conformation status of the azo group in copolymers but also the azo group content in copolymers and mixed ratio of ionic liquids. Based on this tunable LCMT, photoinduced micellization/demicellization can be achieved near room temperature by alternate irradiation with visible and ultraviolet light, and it is totally reversible.

  • 42.
    Torres, Hector
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. AC2T research GmbH, Wiener Neustadt, Austria.
    Rodriguez Ripoll, M.
    AC2T research GmbH, Wiener Neustadt, Austria.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel2019In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 269, p. 79-90Article in journal (Refereed)
    Abstract [en]

    In recent years, the use of Al/Si coatings has become widespread in hot stamping in order to protect the work piece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperature can lead to unstable friction and damage both the tool and the work piece.

    In the present study, self-lubricating coatings with the addition of silver and MoS2 have been prepared by means of laser cladding deposition, aiming at their use in hot stamping in order to decrease friction and wear. The coatings were evaluated at high temperatures against Al-Si-treated boron steel using two different testing configurations featuring open and closed tribosystems. A significant reduction in friction for the self-lubricating claddings were observed along with decreased material transfer. This could be beneficial for hot stamping applications as it can ensure the stability of the process while preventing surface damage to the work piece. Additionally, closed configuration tribotesting has been found to underestimate friction and wear of the tool/work piece system, thus making it less suited for the lab-scale simulation of hot metal forming compared to open configuration tribometers. This finding has been considered relevant as many references in the available literature still report the use of closed configuration tribometers.

  • 43.
    Torres, Hector
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. AC2T research GmbH.
    Self-Lubricating Laser Claddings in the Context of Hot Metal Forming2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.

    In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.

    In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS2 and WS2. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.

    During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.

    Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS2 to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS2 to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.

  • 44.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. The University of Akron, Akron, USA.
    Ma, Xiaofeng
    Nanjing Forestry University, Nanjing, PR China.
    Guo, Xiaojing
    Chinese Academy of Sciences, Shanghai, PR China.
    Chen, Minjiao
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ji, Tuo
    The University of Akron, Akron, USA.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhu, Jiahua
    The University of Akron, Akron, USA.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Structural strategies to design bio-ionic liquid: Tuning molecular interaction with lignin for enhanced lubrication2019In: Journal of Molecular Liquids, ISSN 0167-7322, E-ISSN 1873-3166, Vol. 280, p. 49-57Article in journal (Refereed)
    Abstract [en]

    Lignin strengthened ionic liquids (ILs) have shown high potential to be used as high performance green lubricants. Strengthened lignin-ILs molecular interaction is an effective approach to improve their lubrication properties. The molecular interactions of ILs’ cation and anion containing different functional groups with lignin and efficiency on the lubricating properties have rarely been studied yet. In this work, a series of novel green lubricants with dissolved lignin in [Choline][Amino Acid] ([CH][AA]), [Tetramethylammonium][Glycine] ([N 1111 ][Gly]) and [Tetrabutylammonium][Glycine] ([N 4444 ][Gly]) ILs have been synthesized and their tribological properties were systematically investigated. The longer alkyl chain in cation without reciprocal H-bond interaction between ILs’ cation and anion has the positive effect on the anti-wear properties. In addition, the less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs, which enhances lubrication film strength and thus resulting in the better tribological property of ILs/lignin green lubricants. Specifically, the wear volume loss of the steel disc lubricated by [N 4444 ][Gly] with the addition of 15% lignin is only 12% of the one lubricated by pure [N 4444 ][Gly]. This work presents a method to tune molecular interaction between lignin and ILs via the structural design of ILs’ cation and anion, which are revealed as the key factor that bridges the individual components and improves overall lubricating properties. 

  • 45.
    Wang, Hongdong
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Tribology, Tsinghua University, Beijing, China. Applied Materials Division, Argonne National Laboratory, Argonne, Illinois , United States.
    Liu, Yuhong
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Liu, Wenrui
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Wang, Kunpeng
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Li, Jinjin
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Ma, Tianbao
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Levent Eryilmaz, Osman
    Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Erdemir, Ali
    Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States.
    Luo, Jianbin
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets2019In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 11, no 22, p. 20249-20256Article in journal (Refereed)
    Abstract [en]

    It was previously proved that the existence of a large amount of hydrogen ions in water-based lubricants can easily lead to a superlubric state; however, it was also shown that these hydrogen ions could cause severe corrosion. As part of a large family of attractive clays, layered double hydroxides (LDHs) possess excellent tribological properties in water-based lubrication systems. In the present work, two different kinds of LDHs are dispersed in polyalkylene glycol (PAG) aqueous solutions, in two distinct forms: ultrathin nanosheets (ULDH-NS) of ca. 60 nm wide and ca. 1 nm thick (single or double layer) and nanoparticles (LDH-NP) of ca. 19.73 nm wide and ca. 8.68 nm thick. We find that the addition of ULDH-NS greatly shortens (as much as 85%) the running-in period prior to reaching the superlubricity regime and increases the ultimate load-bearing capacity by about four times. As compared to the fluid film thickness of the lubricating PAG solution, their ultrathin longitudinal dimension will not impair or influence the fluid film coverage in the contact zone. The analysis of sliding solid surfaces and the atomic force microscope microscale friction test demonstrate that the adsorption of ULDH-NS enables the sliding solid surfaces to be polished and protected because of their relatively weak interlayer interaction and increased adhesion effect. Owing to their superior tribological properties as lubricant additives, ultrathin LDH nanosheets hold great potential for enabling liquid superlubricity in industrial applications in the future.

  • 46.
    Mishra, Tanmaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Nordin, Björn
    Bosch Rexroth Mellansel AB, Mellansel, Sweden.
    Svanbäck, Daniel
    Bosch Rexroth Mellansel AB, Mellansel, Sweden.
    Tervakangas, Sanna
    DIARC-Technology Oy, Espoo, Finland.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication2019In: Tribology - Materials, Surfaces & Interfaces, ISSN 1751-5831, E-ISSN 1751-584X, Vol. 13, no 2, p. 120-129Article in journal (Refereed)
    Abstract [en]

    Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.

  • 47.
    Moghaddam, Pouria Valizadeh
    et al.
    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.
    Vuorinen, Esa
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The role of retained austenite in dry rolling/sliding wear of nanostructured carbide-free bainitic steels2019In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 428-429, p. 193-204Article in journal (Refereed)
    Abstract [en]

    The dry rolling/sliding wear of nanostructured bainite has been investigated and compared with that of a conventional quenched and tempered bearing steel. In order to elucidate the role of retained austenite on the wear performance, high silicon hypereutectoid bearing steel with an identical alloy composition was heat treated to obtain different microstructures with similar hardness and different amounts of retained austenite. The results indicate that the nanostructured bainite can meet the minimum hardness requirements for bearing applications. Moreover, the nanostructured bainite outperformed the tempered martensitic steel in terms of wear resistance. The work hardening capacity and thus wear resistance increases due to the transformation of retained austenite into martensite. The results of XRD analyses show that the higher stability of retained austenite and strength of bainitic ferrite leads to better wear performance. It is demonstrated that the stability of retained austenite outweigh the influence of retained austenite content on wear resistance. Adhesion and oxidation were identified as the main wear mechanisms. In addition to microstructure, surface oxidation also plays a prominent role in determining the wear resistance. 

  • 48.
    Torres, Hector
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. AC2T Research GmbH, Wiener Neustadt, Austria.
    Caykara, Tugce
    Rojacz, H.
    AC2T Research GmbH, Wiener Neustadt, Austria.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rodríguez Ripoll, M.
    AC2T Research GmbH, Wiener Neustadt, Austria.
    The tribology of Ag/MoS2-based self-lubricating laser claddings for high temperature forming of aluminium alloys2019In: Wear, ISSN 0043-1648, E-ISSN 1873-2577Article in journal (Refereed)
    Abstract [en]

    In recent years, the use of aluminium alloys in the automotive industry has gained significant attention due to their specific strength, corrosion resistance and recyclability. However, their forming at high temperature in processes like hot stamping is challenging due to the poor tribological behaviour of aluminium alloys, which is the source of severe adhesive wear and a poor surface quality of the finished product.

    In an effort to overcome these tribological problems, iron- and nickel-based self-lubricating laser claddings with the addition of solid lubricants such as silver and molybdenum disulfide have been evaluated under conditions representative of hot stamping against the aluminium alloy AA6082. It has been found that self-lubricating claddings decrease friction and counter body wear at high temperatures compared to alloys commonly used in forming tools such as grade 1.2367 steel. Furthermore, nickel-based self-lubricating claddings have shown a better tribological behaviour than their iron-based counterparts, due to the formation of a nickel-based sulfide layer on the counter body. It is thus expected that the implementation of self-lubricating claddings can improve the quality of the final product while reducing the need for added lubricant during the hot stamping of aluminium alloys.

  • 49.
    Decrozant-Triquenaux, Justine
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pelcastre, Leonardo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Courbon, Cedric
    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.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological Behaviour of PVD Coated Tool Steels in Hot Forming of Aluminium Alloys2019In: CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel, 2019Conference paper (Refereed)
    Abstract [en]

    Aluminium alloys are commonly used as light-weight materials in the automotive industry. Thisnon-ferrous family of metal alloys offers high versatility of properties and designs. In order to reduceweight and improve safety, grades with high strength-to-weight ratio, such as the 6XXX and 7XXXalloy series, are increasingly implemented in vehicles. These alloys, however, exhibit low formabilityand experience considerable springback when formed at low temperatures, and thus have to beformed at elevated temperatures. Severe adhesion and galling are known to be critical tribologicalchallenges in hot forming of aluminium. During the forming operation, adhesion and transfer ofaluminium onto the die surface take place. This phenomenon has a detrimental effect on the surfaceproperties, geometrical tolerances of the formed parts and maintenance of the dies. The influence ofsurface engineering as well as lubricant composition on adhesion and galling has not been sufficientlyinvestigated. Diamond-like-Carbon and Chromium Nitride PVD coatings applied on the toolsteel have shown promising results for reducing aluminium transfer at high-temperatures, especiallyin the presence of a lubricant. However, the interaction between lubricants and PVD coatings duringhot forming of aluminium alloys is not yet fully understood. The present study thus aims at characterisingthe high temperature tribological behaviour of selected PVD coatings and a lubricant duringsliding against an aluminium alloy. The objectives are to select promising lubricant-coating combinationsfor the given application and to study their tribological response in a high-temperature reciprocatingfriction and wear tester. The tests were carried out at 300°C, under dry and lubricatedconditions, in order to study the friction and wear performance. Uncoated tool steel reference testswere performed under dry and lubricated conditions and lubricated tests using DLC, CrN, CrTiNand CrAlN coated tool steel were performed and compared to the reference results. The initial andworn surfaces were analysed with white light 3D optical interferometry, scanning electron microscopy(SEM) and energy dispersive spectroscopy (EDS) with a view to understand the wear mechanisms.

  • 50.
    Rodiouchkina, Maria
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pelcastre, Leonardo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lind, Jonna
    Berglund, Kim
    Kassman Rudolphi, Åsa
    Larsson, Roland
    Tribological behaviour of self-lubricating polymercomposite bearing materials during long-time dry sliding2019In: Tribological behaviour of self-lubricating polymercomposite bearing materials during long-time dry sliding, 2019Conference paper (Refereed)
    Abstract [en]

    Thermosets and thermoplastics containing PTFE are used in marine and hydropower applications due to their long service life and self-lubricity in dry and water-lubricated contacts. Their tribological performance is usually extrapolated from short, accelerated tests, which induces risks of inaccurate or incorrect interpretation.

    The aim of this study is to investigate the tribological behaviour and development of transfer layers during long sliding tests (160 hours), corresponding to years of operation. To mimic operational shutdowns, the tests were stopped every 20 hours to study the evolution of the surface topography and transfer layers.

    The wear rates of both materials decreased significantly with time. For the thermoplastic, COF decreased due to accelerated material transfer after 80 hours, highlighting the importance of long duration tests. The thermoset showed the highest transfer amount after 20 h and cause severe abrasive wear on the counter surface accompanied by a COF increase.