Mitjan Kalin

Mechanical Engineering

Professor
36.45

Publications

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    M. Zalaznik · M. Kalin · S. Novak
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    ABSTRACT: The main aim of this research was to investigate the influence of different processing temperatures on the properties of pure poly-ether-ether-ketone (PEEK). In order to do this, we used a variation of compression moulding, which enabled us to use different processing temperatures, including temperatures below the melting point of PEEK. The PEEK samples were produced at temperatures below, around and above the melting point of PEEK and were compared to a commercially available PEEK material produced with a conventional procedure. The results of the dry-sliding tribological tests, the hardness measurements and the XRD analyses show that the processing temperature greatly influences the hardness and the crystallinity, which in turn affects the tribological behaviour.
    Full-text · Article · Feb 2016 · Tribology International
  • V. Pejaković · C. Tomastik · N. Dörr · M. Kalin
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    ABSTRACT: Commercially available ionic liquids were used as additives in the model lubricant fluid glycerol. The stability of the mixtures was controlled by measuring of turbidity. Tribological experiments have been performed on a reciprocating sliding tribometer in the boundary lubrication regime with three ionic liquid concentrations, as well as with neat glycerol and the neat ionic liquids at 100 °C. Wear and friction were measured, and the worn surfaces were examined with optical microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The results show the influence of the ionic liquid concentration and the anion alkyl chain length on the tribological behaviour. Significant improvement in friction and wear reduction at low ionic liquid concentrations was detected and attributed to sulfur species in tribofilm.
    No preview · Article · Jan 2016 · Tribology International
  • R. Simič · M. Kalin · J. Kovač · G. Jakša
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    ABSTRACT: Information about the interactions between lubricants and DLC coatings is scarce, despite there having been many studies over the years. In this investigation we used ToF-SIMS, XPS and contact-angle analyses to examine the adsorption ability and mechanisms with respect to two oiliness additives, i.e., hexadecanol and hexadecanoic acid, on an a-C:H coating. In addition, we analyzed the resistance of the adsorbed films to external influences like solvent cleaning. The results show that both molecules adsorb onto surface oxides and hydroxides present on the initial DLC surface and shield these structures with their hydrocarbon tails. This makes the surfaces less polar, which is manifested in a smaller polar component of the surface energy. We also showed that ultrasonic cleaning in heptane has no significant effect on the quantity of adsorbed molecules or on their chemical state. This not only shows the relatively strong adsorption of these molecules, but also provides useful information for future experimental work. Of the two examined molecules, the acid showed a greater adsorption ability than the alcohol, which explains some of the previously reported better tribological properties in the case of the acid with respect to the alcohol.
    No preview · Article · Dec 2015 · Applied Surface Science
  • Mitjan Kalin · Eva Oblak · Somayeh Akbari

    No preview · Article · Dec 2015 · Tribology International
  • Luis Bartolomé · Eva Oblak · Mitjan Kalin
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    ABSTRACT: The mechanical behaviour of zinc dialkyldithiophosphate (ZDDP) tribofilms on diamond-like carbon (DLC) coatings has been studied by combining the nano-indentation experimental data and finite-element modelling. Different constitutive models, whose analysis was performed using a global algorithm that consists of forward and inverse algorithms, were chosen to establish which of them more accurately predicts the actual mechanical behaviour of the ZDDP tribofilms. A non-homogeneous, dual structure, which was also verified by the local stiffness using the AFM force-modulation mode, was found. Based on the proposed algorithm, the mechanical properties of the ZDDP tribofilms were found, e.g. the elastic modulus E=26 GPa and the yield stress Y=1.55 GPa for the harder component and approximately E=1.65 GPa and Y=0.144 GPa for the softer component.
    No preview · Article · Nov 2015 · Tribology International
  • M. Polajnar · M. Kalin
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    ABSTRACT: In this paper, we show how the slide-to-roll ratio (SRR), the contact kinematics and the surface energy all have important effects on the elastohydrodynamic friction. As reported previously, diamond-like-carbon (DLC) contacts of the type DLC/DLC provide the lowest coefficient of friction, in particular those DLC materials with the lowest surface energies (three different DLC coatings were used in this study). A friction reduction of up to 48 %, compared to a steel/steel contact, was obtained. A surprising new finding from this investigation is that the friction in DLC contacts is significantly reduced for high SRRs, i.e. a difference of up to 27 % was measured in the DLC/DLC contacts when the SRR increased from 0.5 (rolling prevails) to 1.8 (sliding prevails). However, even more surprising is the effect of the SRR on the friction in mixed steel/DLC contacts, which is related to the contact design and its kinematics. We found that if the DLC is coated on the slower surface, a high SRR will not reduce the friction to any significant extent, typically about 5 %, and at most 13 %, compared to steel/steel contacts. However, a significant friction reduction is observed when applying a low-surface-energy DLC coating to the faster surface - on average about 20 % and as high as 33 %. In this case (a high SRR and DLC on the faster surface), mixed steel/DLC surfaces can experience a very similar friction as that seen for DLC/DLC contacts. Qualitatively, the same behaviour was found for both positive and negative SRR values.
    No preview · Article · Oct 2015 · Tribology Letters
  • Aljaž Iveković · Saša Novak · Marko Lukek · Mitjan Kalin
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    ABSTRACT: Aqueous electrophoretic deposition (EPD) is a colloidal processing technique which enables fast formation of bulk deposits. The addition of suitable surfactant would enable dispersion, and due to their effect on electrokinetic properties, also the deposition of hydrophobic particles of polyether ether ketone (PEEK). The electrophoretic properties and the conductivities of suspensions were analysed as a function of the addition of surfactants or the pH, and related to the properties of the as-prepared bulk deposits. A more in-depth understanding of the deposition process was achieved by monitoring the suspensions’ properties (pH, ζ and conductivity) before and after the deposition and observation of the deposits’ properties. We have shown that by using a concentrated suspension of PEEK (50 wt.%) with the addition of DBSA at pH > 4, thick deposits (>15 mm) were formed at moderate voltages (10–30 V cm−1) in 1 min. Although the PEEK polymer is hydrophobic by nature, aqueous EPD was shown to be an effective technique for shaping bulk parts. During the deposition unusual current increase was observed and was attributed to an increase in the conductivity due to the migration of ions and electrochemical reactions at the electrodes.
    No preview · Article · Sep 2015 · Journal of Materials Processing Technology
  • Eva Oblak · Mitjan Kalin
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    ABSTRACT: The properties of tribochemical films play an important, or even the key, role with respect to friction in boundary lubrication. While their chemical behavior has already been widely studied, their mechanical properties are much less well understood. However, their nanoscale mechanical properties and behavior may reveal important information about a correlation with the macroscopic friction behavior. In this investigation, we looked at steel, a-C:H and Si-DLC in contact with steel lubricated using two commercial oils containing different amount of SAPS additives (E6 and E7 grade) and a mineral base oil containing the ZDDP additive. The tribofilms were characterized using an atomic force microscope with seven different parameters, i.e., the topography (features morphology), tribofilm surface coverage, nano-roughness, adhesion, film thickness, lateral force, i.e., nanoscale friction, and the film stiffness through force modulation. The results confirmed the formation of tribofilms on all the selected coatings and showed that the film formation and its nanoscale properties are dependent on the coating and the additive. Two distinctive groups of parameters were identified: one closely related to the surface energy of the materials and the other clearly distinguishing the DLC coatings from the steel. The adhesion and tribofilm thickness were found to correlate directly with the macroscopic friction, while the other parameters have higher-order dependences.
    No preview · Article · Sep 2015 · Tribology Letters
  • Somayeh Akbari · Mitjan Kalin

    No preview · Conference Paper · Sep 2015
  • Somayeh Akbari · mitjan kalin

    No preview · Conference Paper · Sep 2015
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    ABSTRACT: Understanding contact between rough surfaces is of critical importance to the design of many engineering applications. Contact models rely on material properties and surface topography of the contacting surfaces as input parameters. Hence, the relevance of the contact models is dependent on their inherent assumptions and the accuracy with which the input parameters are determined. We have evaluated the difference between the surface topography parameters calculated with a statistical and deterministic approach for actual engineering surfaces. We have found topography values that change up to 300% depending on the method used, and attribute this to the stringent definition of an asperity-peak in the case of deterministic analysis as opposed to statistical analysis, which not only considers asperity-peaks but also asperity-shoulders.
    No preview · Article · Sep 2015 · Tribology International
  • J. Jerina · M. Kalin
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    ABSTRACT: During the processes of forming aluminium alloys, adhesion, and in particular the formation of the aluminium-alloy transfer on the bearing surface of the die, is one of the main reasons for the failure of tools and the poor surface quality of products. The present work was focused on the transfer initiation of an aluminium alloy (EN AW-6060) and its evolution on a coated (CrN) and an uncoated, nitrided hot-work tool steel (AISI H13) at temperatures from room temperature to 500 °C. The contact was investigated in terms of the transferred aluminium alloy surface area size on the uncoated and coated tool-steel surfaces, the topography of the wear trace and the corresponding change in the coefficient of friction.
    No preview · Article · Jul 2015 · Wear
  • Somayeh Akbari · Mitjan kalin

    No preview · Conference Paper · Jun 2015
  • Somayeh Akbari · Mitjan Kalin

    No preview · Conference Paper · Jun 2015
  • M. Kalin · J. Jerina
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    ABSTRACT: Adhesion and, in particular, the transfer of aluminium alloys to the bearing surface of a die are two of the main reasons for tool failure and the poor surface quality of products, especially at elevated temperatures. The present work was focused on the EN-AW6060 aluminium alloy׳s transfer initiation and the evolution to an AISI H13 hot-work tool steel, as well as CrN and TiAlN coatings in a cross-cylinder, single-pass, dry-sliding contact at room (20 °C) and elevated temperatures (300-500 °C). The contact was investigated in terms of the surface area and volume of the transferred aluminium alloy to the tool steel׳s surface, the topography of the wear trace and the corresponding change in the coefficient of friction. The results show a strong dependence of the tribological properties of the investigated materials on the temperature and only a limited dependence on the sliding distance, especially for the TiAlN coating. At room temperature the lowest coefficient of friction and the smallest amount of material transfer were measured for the TiAlN coating. At higher temperatures both the CrN and TiAlN coatings showed similar friction values and amounts of transferred aluminium alloy, while the nitrided hot-work tool steel exhibited an inferior tribological performance.
    No preview · Article · May 2015 · Wear
  • Source
    M. Zalaznik · S. Novak · M. Huskić · M. Kalin
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    ABSTRACT: The addition of different concentrations (2–10 wt.%) of molybdenum disulfide (MoS2) to a poly–ether–ether–ketone matrix has been studied in terms of the thermal, mechanical and tribological properties of the materials. The results of dry-sliding tribological tests, differential scanning calorimetry and scanning electron microscope–energy-dispersive X-ray (EDS) analyses show that the concentration of MoS2 influences the tribological, mechanical and thermal properties. With the highest concentration of MoS2 (10 wt.%), the coefficient of friction was reduced by as much as 25%, while the maximum reduction in the wear rate was ~20%, which required 5 wt.% of MoS2. The most important parameter when it comes to achieving an improved tribological behaviour was found to be the combination of a high hardness and a sufficient quantity of transfer film being formed. Copyright © 2015 John Wiley & Sons, Ltd.
    Full-text · Article · Apr 2015 · Lubrication Science
  • Source
    Vladimir Pejakovic · Amaya Igartua · Mitjan Kalin
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    ABSTRACT: Current work presents the investigation of frictional behaviour of ionic liquid lubricant mixtures under mixed slide to roll ratio. On the contrary to the previous study, which focuses on determination of the most suitable ionic liquid additive for identical ionic liquid weight concentration in lubricant mixture, this work had two scopes. The first one was to determine the optimal chemical composition of ionic liquid additive by investigating the lubricant mixtures with identical molar ratio, and the second one, to optimise additive concentration for certain ionic liquid structure. The changes were observed by two means. Namely, in frictional behaviour, ionic liquid concentration plays significant role. On the contrary, for the mixtures with identical molar concentration, the chemical structures with longer alkyl substituent do not always exhibit improvement. Experiments also revealed correlation between ionic liquid structures, concentration and wear. Atomic force microscopy (AFM) analysis of worn surfaces confirmed the above statements.
    Full-text · Article · Feb 2015 · Lubrication Science
  • Source
    M. Kalin · M. Zalaznik · S. Novak
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    ABSTRACT: High-performance polymers show a great potential to replace machined metal components in a wide variety of applications owing to their promising mechanical, thermal and tribological properties. However, further improvements in their properties are still needed. The aim of this study was to evaluate and compare the effect of the material type and the morphology of well-known solid lubricant nanoparticles on the tribological properties of poly-ether-ether-ketone (PEEK) composites. Different nanoparticles were added to the PEEK matrix: WS2 fullerene-like (WS₂F), WS2 needle-like (WS₂N), carbon nanotubes (CNT) and graphene nanopowder (GNP). The results of dry-sliding tribological tests show that the material and the morphology of the nanoparticles have a crucial effect on both the wear and the coefficient of friction, primarily by affecting their macroscopic hardness, as well as the thickness and the surface coverage of their transfer films. The WS₂-based nanoparticles outperformed the carbon-based nanoparticles in terms of wear performance; compared to PEEK, WS2F improved the wear rate by 10% and the WS2N by 60%, while the carbon-based particles deteriorated the wear behaviour by 20% (CNT) and as much as three times in the case of the GNP. The addition of WS₂F nanoparticles and the CNT resulted in a lower coefficient of friction compared to the WS₂N particles and the GNP.
    Full-text · Article · Jan 2015 · Wear
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    ABSTRACT: This study examines bacterial adhesion on titanium-substrates used for bone implants. Adhesion is the most critical phase of bacterial colonization on medical devices. The surface of titanium was modified by hydrothermal treatment (HT) to synthesize nanostructured TiO2-anatase coatings, which were previously proven to improve corrosion resistance, affect the plasma protein adsorption, and enhance osteogenesis. The affinity of the anatase coatings toward bacterial attachment was studied by using a green fluorescent protein-expressing Escherichia coli (gfp-E. coli) strain in connection with surface photo-activation by UV irradiation. We also analized the effects of surface topography, roughness, charge and wettability. The results suggested the dominant effect of the macroscopic surface topography, as well as micro-asperity at the surface roughness scale, were produced during titanium machining and/or HT treatment. Macroscopic grooves provided a preferential site for bacteria deposit within the valleys, while the microscopic roughness of the valleys determined the actual interaction surface between bacterium and substrate, resulting in an "interlocking" effect and undesired high bacterial adhesion on non-treated titanium. In the case of TiO2-coated samples, the nanocrystals reduced the width between the micro-asperities and, so, added nano-roughness features. These factors decreased the contact area between the bacterium and the coating, with consequent lower bacterial adhesion (up to -50% less) in comparison to the non-treated titanium. On the other hand, the pronounced hydrophilicity of one of the HT-coated discs after pre-irradiation seemed to enhance the attachment of bacteria, although the increase was not statistically significant (p > 0.05). This observation may be explained by the acquired similar degree of wetting between gfp-E. coli and the coating. No correlation was found between the bacterial adhesion and the ζ-values of the samples in PBS, so the effect of surface charge was considered negligible in this study.
    Full-text · Article · Dec 2014 · ACS Applied Materials & Interfaces
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    Vladimir Pejaković · M. Kalin
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    ABSTRACT: Ionic liquids are getting increased attention due to their potential use as additives or lubricants. One of parameters that influence their performance is anion alkyl chain length. Three chain lengths in imidazolium sulfate ionic liquids were varied in three different types of tests in order to study the friction performance in all lubrication regimes at different shear rates. Results show significant effect of alkyl chain length on friction behaviour and surface topography. The shortest chain length ionic liquid had the poorest friction performance whilst longer alkyl chain lengths performed significantly better, which was evidenced by the electrical contact resistance. Although friction behaviour was not generally reduced with ionic liquid mixtures, clear tribological improvement was observed in high shear conditions depending on alkyl chain length. Atomic force microscopy (AFM) analysis revealed correlation between ionic liquid chemistry and surface topography of samples after the experiments, with significantly increased roughness for short chain ionic liquid mixture.
    Full-text · Article · Dec 2014 · Lubrication Science

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