Publications

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    ABSTRACT: Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS2 nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS2 nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS2-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS2-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS2-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS2 and the steel or DLC coating was observed.
    Tribology Letters 09/2014; 55(3). · 2.15 Impact Factor
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    ABSTRACT: The structural impact of ionic liquids was systematically investigated by assessing their behaviour as lubricant additives in two different biodegradable esters (refined rapeseed oil and synthetic ester). Cations were mainly based on derivatives of the tetraalkyl ammonium structure; the anions were alkyl sulfate, methyl sulfonate or bis(trifluoromethylsulfonyl)imide. Miscibility experiments showed that ultrasound treatment yielded the most stable emulsions. X-ray fluorescence was employed to determine maximum concentrations, at which ionic liquids formed stable mixtures in the base oil, either dissolved or as self-stabilized emulsions. Steel/steel contacts lubricated with such saturated rapeseed oil blends were investigated in a reciprocating sliding tester at room temperature and at 100°C. For most anions, a significant reduction of friction and wear was found at ionic liquid concentrations of less than 0.1 weight%. At substantially higher concentrations, a destabilisation of the ionic liquid–ester mixtures was observed, which was accompanied by a loss of wear-preventive properties. Copyright © 2014 John Wiley & Sons, Ltd.
    Lubrication Science 08/2014; · 0.68 Impact Factor
  • Matej Komelj, Mitjan Kalin, John Durham
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    ABSTRACT: Diamond-like carbon (DLC) is a promising material for tribology-based applications. We investigate the susceptibility of the DLC surface to some characteristic molecules that are potentially present in various lubricants by means of ab-initio calculations. We demonstrate that the strongest bond is formed between the oxygen atoms from the molecule and the metallic dopants present in the DLC. We present the first experimental evidence that proves the theoretical hypothesis. Copyright © 2014 John Wiley & Sons, Ltd.
    Lubrication Science 03/2014; · 0.68 Impact Factor
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    J. Kogovšek, M. Kalin
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    ABSTRACT: Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS2 nanotubes, MoS2 platelets (2 and 10 μm), WS2 nanotubes, WS2 fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS2 and WS2 nanotubes, which became somewhat shorter.
    Tribology Letters 02/2014; · 2.15 Impact Factor
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    ABSTRACT: The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out.
    Tribology Letters 01/2014; 53(1). · 2.15 Impact Factor
  • M. Kalin, M. Polajnar
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    ABSTRACT: The importance of wetting is becoming increasingly obvious and its control is inevitable in many engineering applications, including tribology and interface nanotechnology. However, the relations between the key parameters affecting surface–liquid wetting behaviour under realistic conditions are not very well understood, especially for typical engineering materials and lubricants (oils), often leading to exceptions and contradictions, which impede their use in engineering models and theories, and so the possible optimisation of the interfaces of engineering systems. In this paper we present the correlations between the contact angle, the spreading, the surface tension and the surface energy of fourteen frequently used engineering materials belonging to four different classes of materials (steel, DLC coatings, ceramics, and polymers) wetted with four different liquids: three oils (a non-polar synthetic oil of two different viscosities and a polar natural-based oil) and water. The results represent systematically and consistently obtained data about the wetting-relevant parameters of the selected materials and lubricants and numerous correlations between them. However, the most striking result suggests that the spreading parameter correlates very linearly with the surface energy for all the materials and liquids studied, in both the adhesion-wetting and spreading-wetting regimes. The experimentally determined spreading vs. surface energy correlation functions that appear generally valid for a broad range of properties of the materials and oils can thus be applied as an engineering tool to tailor and design the required/desired wetting performance and nature of the solid–liquid interfaces. The spreading parameter SP – in contrast to the contact angle – was found to be a reliable and relevant parameter for describing the wetting of oils with selected engineering materials.
    Applied Surface Science 01/2014; 293:97–108. · 2.54 Impact Factor
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    ABSTRACT: Diamond-like carbon (DLC) coatings are notable for their excellent tribological properties. Our understanding of the lubrication of DLC coatings has improved drastically over the past decade. However, only a few details are known about the properties of the adsorbed layers on DLC, which crucially affect their tribological properties under lubricated conditions. In this work we used neutron reflectometry to determine the thickness and the density of adsorbed layers of alcohol molecules on several different types of DLC coatings, i.e., non-hydrogenated (a-C) and hydrogenated, of which both non-doped (a-C:H) and doped (a-C:H:F and a-C:H:Si) coatings were used. The results showed that a 0.9-nm-thick and relatively dense (≈45%) layer of alcohol adsorbed on the a-C coating. In contrast, no adsorption layer was found on the a-C:H, confirming the important role of hydrogen, which predominantly acts as a dangling-bond passivation source and affects the reactivity and tribochemistry of DLC coatings. The incorporation of F into a DLC coating also did not cause an increase in the adsorption ability with respect to alcohol molecules. On the contrary, the incorporation of Si increased the reactivity of the DLC coating so that a 1.3-nm-thick alcohol layer with a 35% bulk density was detected on the surface. We also discuss the very good agreement of the current results with the surface energy of selected coatings found in these experiments.
    Applied Surface Science 01/2014; · 2.54 Impact Factor
  • M. Kalin, M. Polajnar
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    ABSTRACT: This work shows the influence of solid–liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson’s linear correlation coefficient of 0.999). By tailoring the wetting and surface energy—achieved by varying the DLC/DLC contacts with different types of DLC coatings—the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A “Slip-inducing interaction model based on surface forces” is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid–liquid interface.
    Tribology Letters 11/2013; 52(2). · 2.15 Impact Factor
  • R. Simič, M. Kalin
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    ABSTRACT: Fatty acids are known to affect the friction and wear of steel contacts via adsorption onto the surface, which is one of the fundamental boundary-lubrication mechanisms. The understanding of the lubrication mechanisms of polar molecules on diamond-like carbon (DLC) is, however, still insufficient. In this work we aimed to find out whether such molecules have a similar effect on DLC coatings as they do on steel. The adsorption of hexadecanoic acid in various concentrations (2-20 mmol/l) on DLC was studied under static conditions using an atomic force microscope (AFM). The amount of surface coverage of the adsorbed fatty-acid molecules was analysed. In addition, tribological tests were performed to correlate the wear and friction behaviours in tribological contacts with the adsorption of molecules on the surface under static conditions. A good correlation between the AFM results and the tribological behaviour was observed. We confirmed that fatty acids can adsorb onto the DLC surfaces and are, therefore, potential boundary-lubrication agents for DLC coatings. The adsorption of the fatty acid onto the DLC surfaces reduces the wear of the coatings, but it is less effective in reducing the friction. Tentative adsorption mechanisms that include an environmental species effect, a temperature effect and a tribochemical effect are proposed for DLC and steel surfaces based on our results and few potential mechanisms found in literature.
    Applied Surface Science 10/2013; · 2.54 Impact Factor
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    ABSTRACT: Ionic liquids are low-melting-point salts composed entirely of ions, and many of them are liquids at room temperature. In recent years, studies have indicated that they might be good candidates for lubricants, either in neat or additive form. In this work, a sulfate-based ionic liquid with a pyrollidinium cation was studied as neat lubricant and as additive for glycerol in lubrication of steel–steel contacts. Glycerol was chosen as the base oil because of its high polarity, which allows full miscibility with polar ionic liquids. Tests were performed on an oscillating friction and wear tribometer. The coefficients of friction and wear were measured. The tests were run at room temperature, 50 °C, 100 °C and 150 °C. By using profilometry, optical microscopy, scanning electron microscope and atomic force microscopy (AFM) analyses, it was shown that the ionic liquid plays an important role in the friction and wear reduction, as well as in the smoothening of the worn surface.
    Lubrication Science 07/2013; · 0.68 Impact Factor
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    ABSTRACT: MoS2 and WS2 nanoparticles, on the one hand, and DLC coatings, on the other, are receiving increasing attention for tribological applications. However, investigations that combine DLC coatings and lubrication using nanoparticles are very scarce and the understanding of such tribological systems remains limited. In this work we looked at such a lubrication system by studying the effects of surface roughness and running-in on the behaviour of DLC-coated contacts in all lubrication regimes. We also present a model of how the surface roughness influences the mechanism of lubrication for the MoS2 nanotubes. This model is based on a 2D 1:1 projection scale of dimensions of the surface profile, including its asperities, nanoparticles and film thicknesses, and is thus independent of the contacting materials.It was realized that the addition of the nanotubes to the base oil lowered the coefficient of friction of the DLC by more than 50% for the smooth, DLC-coated surfaces and up to 40% for the rough, DLC-coated surfaces. The nanoparticles were the most effective under boundary-lubrication conditions and had a negligible effect in the EHL regime. The surface roughness has a notable and two-fold effect: while the friction was lower on the smooth, compared to the rough, DLC-coated surfaces, the rough surfaces were better able to retain the nanoparticles within the contact during the running-in.
    Wear. 06/2013; 303(s 1–2):361–370.
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    ABSTRACT: Diamond-like carbon (DLC) coatings provide low friction and wear in the most demanding tribological contacts. However, their chemical reactivity with oil additives is poor and difficult to optimise. Moreover, even the partially effective, but high-SAPS (sulphuric ash, phosphor, sulphur) additives, will be phased out in the near future for environmental reasons. Based on recent advancements in the nanotechnology of inorganic MoS2 and WS2 nanoparticles, which lubricate through the low shear of basal planes, we propose a potential replacement of the current chemical-based lubrication with this novel, physical-based additive lubrication technology for poorly reactive DLC coatings. In our work, 30% less friction compared to steel surfaces and 50% less friction compared to the base oil was achieved by employing MoS2 nanotubes in the base oil in self-mated DLC contacts. This physical-based lubrication technology represents an innovative solution for highly effective but non-reactive surfaces and simultaneously provides green-lubrication performance.
    Wear. 06/2013; 303(s 1–2):480–485.
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    Mitjan Kalin
    Lubrication Science 06/2013; 25(4). · 0.68 Impact Factor
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    ABSTRACT: a b s t r a c t The main focus of this work is to show influences of the cooling rate on the microstructure and wear performance of Fe–Cr–C hardfacings. The coatings were produced by welding. The weld overlays were cooled from the molten stage in selected variations: (i) standard cooling – cooling on air at the steel welding table; (ii) passive cooling – cooling of the base material during the hardfacing process with a water cooled cooper plate with 12 1C; (iii) active cooling – cooling of the welding seam behind the main arc with CO 2 gas at temperature of −80 1C. For each cooling situation the temperature of the weld seam was measured in the melt pool with help of thermocouple. It was shown, that different cooling conditions can lead to the significant changes in the cooling-off time t 8/5 , dilution of the hardfacing with substrate and microstructural features of the hardfacings under investigation. Three different wear tests (three-body abrasion according to ASTM G65 standard, impact/abrasion and solid particle erosion) were performed to study the wear performances of Fe–Cr–C hardfacing. Testing results have shown that if the cooling-off time t 8/5 is too short it influences the brittleness and increases the residual stresses of the Fe–Cr–C hardfacing, which lowers its impact resistance. The effect of the cooling rate is in good correlation with wear test results and gives promise for further microstructure and process optimisation for the improved tribological performance. & 2013 Elsevier B.V. All rights reserved.
    Materials Science and Engineering A 04/2013; 576:243. · 2.11 Impact Factor
  • M. Kalin, R. Simič
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    ABSTRACT: Polar molecules are known to affect the friction and wear of steel contacts via adsorption onto the surface, which represents one of the fundamental boundary-lubrication mechanisms. Since the basic chemical and physical effects of polar molecules on diamond-like carbon (DLC) coatings have been investigated only very rarely, it is important to find out whether such molecules have a similar effect on DLC coatings as they do on steel. In our study the adsorption of hexadecanol in various concentrations (2-20 mmol/l) on DLC was studied under static conditions using an atomic force microscope (AFM). The amount of surface coverage, the size and the density of the adsorbed islands of alcohol molecules were analyzed. Tribological tests were also performed to correlate the wear and friction behaviours with the adsorption of molecules on the surface. In this case, steel surfaces served as a reference. The AFM was successfully used to analyze the adsorption ability of polar molecules onto the DLC surfaces and a good correlation between the AFM results and the tribological behaviour of the DLC and the steel was found. We confirmed that alcohols can adsorb physically and chemically onto the DLC surfaces and are, therefore, potential boundary-lubrication agents for the DLC coatings. The adsorption of alcohol onto the DLC surfaces reduces the wear of the coatings, but it is less effective in reducing the friction because of the already inherently low-friction properties of DLC. Tentative adsorption mechanisms that include the environmental species effect, the temperature effect and the tribological rubbing effect are proposed for DLC and steel surfaces.
    Applied Surface Science 04/2013; · 2.54 Impact Factor
  • M. Kalin, I. Velkavrh
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    ABSTRACT: Due to the specific surface properties of diamond-like-carbon (DLC) coatings their interactions with base oils and additives differ from those of conventional ferrous engineering materials such as steel. This relates not only to their reactions with additives, which were frequently investigated in the past, but also to the effects of base oils and the physical properties of these oils. In order to better understand the physical phenomena of base oils, in this study we analyse the influence of velocity and viscosity (through the Stribeck parameter) on the friction in DLC/DLC contacts for all lubrication regimes. The tribological tests were performed with various non-polar base oils and the behaviour of the DLC/DLC contacts was compared with the steel/steel contacts, where the lubrication mechanisms are well known. Several differences were found for all the lubrication regimes. However, the most surprising is that in the boundary-lubrication regime the Stribeck curve of the DLC contacts has the opposite, i.e., an “inverse”, shape to that of the steel contacts. Namely, the friction of the DLC contacts decreases at low Stribeck-parameter values, rather than showing an increase, as is known from the classical Stribeck-curve theory. This new finding shows that Stribeck curve shape depends strongly on the type of materials in contacts—not only on classical Stribeck parameters (velocity, viscosity, load).
    Wear. 01/2013; 297(s 1–2):911–918.
  • F. Majdič, I. Velkavrh, M. Kalin
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    ABSTRACT: Today, there are several water–hydraulic, power-control systems already available on the market. Their components are usually made of stainless steel, which ensures satisfactory performance under mild, conventional operating conditions. However, for more demanding operating conditions and long-term, low-friction and low-wear performance, they do not provide the required performance. One of the possible ways to improve the performance of stainless-steel components in water–hydraulic systems is to coat them with diamond-like carbon (DLC), since this material is well known for its excellent low-friction and low-wear characteristics and also provides very good performance under water-lubrication conditions. In this study, real-scale lifetime tests with 2.3 million cycles were performed on a hydraulic test rig with a proportional 4/3 directional control water–hydraulic valve. Two types of contacts in the valve were tested: the steel-spool/steel-sleeve and the DLC-spool/steel-sleeve. The wear behaviour of the valve was evaluated with a scanning electron microscope (SEM) and internal leakage measurements. In the real-scale lifetime tests the wear and the damage on the DLC-coated spool were significantly lower than on the steel spool. Furthermore, in agreement with this, the internal leakage in the DLC-spool/steel-sleeve valve was almost constant throughout the tests, while in the steel-spool/steel-sleeve valve the leakage slowly, but steadily, increased. The steel/steel and DLC/steel contacts were also separately evaluated in pin-on-disc model tribological tests under water-lubricated conditions for a comparison and for a better understanding of the tribological mechanisms. In agreement with the real-scale tests, the DLC/steel contact showed improved friction and wear performance in comparison with the steel/steel contact.
    Wear. 01/2013; 297(s 1–2):1016–1024.
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    M. Kalin, A. Pogačnik
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    ABSTRACT: In any attempt to theoretically calculate the real contact area for 3D engineering surfaces, a criterion is needed to identify the relevant asperity-peaks that carry the load in tribological contacts. In our recent work, we investigated how different, available 2D criteria affect the properties of the theoretically determined asperity-peaks in 2D surfaces. In this work, however, we focused on a 3D surface characterisation. The effect of different asperity-peak identification criteria on the properties of the asperity-peaks (numbers, radii and heights) is studied in the 3D domain. Several different criteria that take into account the number of neighbouring points, the distances between them (lateral resolution) and their heights were evaluated for real measured surfaces with five different surface roughnesses in the broad engineering range of arithmetic surface roughness from Sa=0.005 µm to Sa=0.529 µm. From the results it follows that all three chosen asperity-peak identification criteria (5PP-3D, 9PP-3D and 9PP-R-3D) result in reliable asperity-peak properties, and none of them can be favoured based on a theoretical evaluation only. There are, however, important differences between them. The data resolution in the x and y directions has a very important influence on the numbers, radii and heights of the asperity-peaks, and the results suggest that the data's lateral resolution, below 1 µm, should be used for the relevant asperity-peak identification.
    Wear. 01/2013; 308(s 1–2):95–104.
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    ABSTRACT: We report on the successful realization of a contactless, non-perturbing, displacement-measuring system for characterizing the surface roughness of polymer materials used in tribological applications. A single, time-dependent, scalar value, dubbed the collective micro-asperity deformation, is extracted from the normal-displacement measurements of normally loaded polymer samples. The displacement measurements with a sub-nanometer resolution are obtained with a homodyne quadrature laser interferometer. The measured collective micro-asperity deformation is critical for a determination of the real contact area and thus for the realistic contact conditions in tribological applications. The designed measuring system senses both the bulk creep as well as the micro-asperity creep occurring at the roughness peaks. The final results of our experimental measurements are three time-dependent values of the collective micro-asperity deformation for the three selected surface roughnesses. These values can be directly compared to theoretical deformation curves, which can be derived using existing real-contact-area models.
    Sensors 01/2013; 13(1):703-20. · 2.05 Impact Factor
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    ABSTRACT: In this work, we report on the BuMepyr-MeSO4 and Et3MeN-MeSO4 ionic liquids that were synthesized and used as additives in a glycerol model lubricant for steel/steel contacts. Tests were performed with three different ionic liquid concentrations, i.e. 0.625 wt%, 2.5 wt% and 8 wt%, as well as in glycerol without any ionic liquid (neat glycerol) and in neat ionic liquids (100%) at 100 °C. The wear and friction were measured and the worn surfaces were examined with scanning electron microscopy and atomic force microscopy. The results show a reduction of the wear and friction with the use of ionic liquids as additives, when compared to the neat glycerol. With an increasing ionic-liquid concentration in the glycerol, the friction was observed to decrease and the wear to increase. In this work, however, the results obtained for neat ionic liquids represent the lowest values in terms of both friction and wear.
    ARCHIVE Proceedings of the Institution of Mechanical Engineers Part J Journal of Engineering Tribology 1994-1996 (vols 208-210) 11/2012; 226(11):923-932. · 0.63 Impact Factor

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