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  • 301.
    Munktell, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Nyholm, Leif
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Björefors, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Towards high-throughput Corrosion Screening using Bipolar Electrochemistry2015Conference paper (Other academic)
    Abstract [en]

    In this work the concept of using arrays of bipolar electrodes for a fast and simple way to screen corrosion properties is presented.

    When an isolated conducting object is exposed to an external electric field the resulting potential difference across its surface can cause part of the total current to pass through said object. The result of this is that the object will act as an anode and a cathode simultaneously and that there will be a gradient in electrochemical potential ranging between the two end poles. The effect has proved useful in various industries and research fields with applications ranging from large scale chemical reactors to the modification of nano-objects1.

    We have previously demonstrated that bipolar electrochemistry can be used to generate gradients in corrosion damage on steel samples to evaluate their corrosion properties2.

    When a number of steel samples are placed together in an array configuration and subjected to an external field they all act as individual bipolar electrodes, and in this way many samples can be compared in the same experiment at the same time.  The shape of the gradient that forms on the anodic end of the samples will depend on the steels corrosion resistance.

    When all steel samples placed together are of the same type the BPE-induced gradients are all alike. If steels of different corrosion properties are placed together the least resistant steel will pass more of the current and therefore corrode further3. This effect can be used to enhance differences between samples. 

    Figure 1: Schematic of the BPE-array setup.

    Figure 2: Gradients on the same type of steel from different setups, arrows roughly indicates the length of the gradient.

      

     References

    [1]    G. Loget, D. Zigah, L. Bouffier, N. Sojic, A. Kuhn, Accounts of Chemical Research 2013, 46, 2513-2523.

    [2]    S. Munktell, M. Tydén, J. Högström, L. Nyholm, F. Björefors, Electrochemistry Communications 2013, 35, 274-277.

    [3]    S. Munktell, L. Nyholm, F. Björefors, Journal of Electroanalytical Chemistry 2015, 747, 77-82.

  • 302.
    Munktell, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Tydén, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Högström, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Nyholm, Leif
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Björefors, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Bipolar electrochemistry for high-throughput corrosion screening2013In: Electrochemistry communications, ISSN 1388-2481, E-ISSN 1873-1902, Vol. 34, p. 274-277Article in journal (Refereed)
    Abstract [en]

    It is demonstrated that bipolar electrochemistry can be used for high-throughput corrosion testing covering a wide potential range in one single experiment and that this, combined with rapid image analysis, constitutes a simple and convenient way to screen the corrosion behaviour of conducting materials and corrosion protective coatings. Stainless steel samples (SS304), acting as bipolar electrodes, were immersed in sulphuric and hydrochloric acid and exposed to an electric field to establish a potential gradient along the surface. In this way, the same steel sample was exposed to a wide range of cathodic and anodic conditions, ranging from potentials yielding hydrogen evolution to potentials well into the transpassive region. This wireless approach enables rapid simultaneous comparison of numerous samples, and also provides the opportunity to perform experiments on samples that are of a complex shape, or which otherwise are difficult to employ in standard electrochemical corrosion tests.

  • 303.
    Nedfors, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Synthesis and Characterization of Multifunctional Carbide- and Boride-based Thin Films2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis present research on synthesis, microstructure, and properties of carbide- and boride- based thin films. The films have been synthesized by dc magnetron sputtering, and their microstructures have been characterized mainly by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and transmission electron microscopy.  One of the main objectives with this research has been to evaluate the thin films potential as materials for sliding electrical contact applications and this have influenced, which properties that have been evaluated.

    Co-sputtered Nb-C films have a microstructure comprising of nanocrystalline NbCx  (nc-NbCx) grains embedded in a matrix of amorphous C (a-C). A thinner a-C matrix form in the Nb-C films compared to the well-studied Ti-C system. As a consequence, the Nb-C films have a higher hardness and conductivity than previously studied Ti-C sputtered under similar conditions. The promising electrical contact properties are attained for reactively sputtered Nb-C films under industrial conditions, at deposition rates two orders of magnitude higher. A reduction in crystallinity is seen when Si is added to the Nb-C films and amorphous films forms at Si content > 25 at.%. The alloying of Si was however not beneficial for the electrical contact properties.

    Substoichiometric CrB2-x (B/Cr = 1.5) and NbB2-x (B/Nb = 1.8) films are achieved when deposited from MeB2 targets. Boron segregates to grain boundaries forming a B-rich tissue phase. This result in superhardness for the NbB2-x films (42 ± 4 GPa) as well as a low friction attributed to the formation of a boric acid film. Carbon forms a solid solution in the MeB2 grains as well as segregating to grain boundaries forming an amorphous BCx (a-BCx) phase when alloyed to CrB2-x and NbB2-x films. The formation of the a-BCx phase drastically improves the electrical contact resistance of the NbB2-x films. However, the mechanical properties are degraded, which result in a high friction and wear rate.

    It was in TEM studies of the metastable amorphous structures for the Nb-Si-C films found that the electron beam induces crystallization. Hence, great care is required when studying these types of metastable structures.

    List of papers
    1. Structural, mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering
    Open this publication in new window or tab >>Structural, mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering
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    2011 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 206, no 2-3, p. 354-359Article in journal (Refereed) Published
    Abstract [en]

    Niobium-carbide nanocomposite coatings with a carbon content varying from 43 to 64 at.% were deposited by dual DC magnetron sputtering. X-ray diffraction, x-ray photoelectron spectroscopy and electron microscopy showed that all coatings consisted of nanometer sized NbC grains embedded in a matrix of amorphous carbon. Mechanical properties and electrical resistivity showed a strong dependency on the amount of amorphous carbon (a-C) and NbC grain size in the coating. The highest hardness (23 GPa), elastic modulus (295 GPa) and the lowest resistivity (260 mu Omega cm) were measured for the coating with about 15% of a-C phase. Contact resistance measurements using a crossed cylinder set-up showed lowest contact resistance for the coating containing 33% a-C (140 mu Omega at a contact force of 100 N), which is comparable to a Ag reference (45 mu Omega at a contact force of 100 N). Comparison with TiC-based nanocomposites studied under similar conditions showed that the Nb-C system has less tendency to form a-C and that lowest contact resistance is obtained at comparable amounts of a-C phase in both material systems (33% for Nb-C compared to 35% for Ti-C). With these good electrical contact properties, the Nb- C nanocomposites can be considered as a potential material for electrical contact applications.

    Keywords
    Nanocomposite, Niobium carbide, Electrical contact properties, Mechanical properties, Magnetron sputtering
    National Category
    Natural Sciences Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-160716 (URN)10.1016/j.surfcoat.2011.07.021 (DOI)000295386900023 ()
    Available from: 2011-11-02 Created: 2011-10-31 Last updated: 2017-12-08Bibliographically approved
    2. Reactive sputtering of NbCx-based nanocomposite coatings: an up-scaling study
    Open this publication in new window or tab >>Reactive sputtering of NbCx-based nanocomposite coatings: an up-scaling study
    Show others...
    2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 253, p. 100-108Article in journal (Refereed) Published
    Abstract [en]

    Nanocomposite Nb-C coatings, with a C/Nb ratio of 0.93 - 1.59, have been deposited by reactive sputtering in a commercial sputtering system at deposition rates of up to 200 nm/min. The coatings are compared to non-reactively sputtered Nb-C in lab-scale equipment at deposition rates two orders of magnitude lower. X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy are used to conclude that all coatings consist of nanocrystalline NbCx grains (nc-NbCx) embedded in a matrix of amorphous C (a-C). The coating performance was evaluated in terms of their mechanical, tribological, and electrical properties. The chemical stability of the coatings was evaluated by exposure to a flowing mixture of corrosive gases. It is found that the coatings have comparable microstructure and performance to the coatings deposited by non-reactive sputtering. The high deposition rate and presence of different C-radicals on the coating surface during film growth for the reactively sputtered coatings is believed to result in a smaller NbCx grain size compared to the non-reactively sputtered coatings (reactive process: 10 – 3 nm, non-reactive process: ~75 – 3 nm). This difference results in a thinner a-C matrix of about 0.2 nm, which is not varying with C content for the reactively sputtered coatings. The thinner a-C matrix is reflected in coating properties, with a higher conductivity and slightly higher hardness. The coating richest in C content (C/Nb ratio 1.59) shows the lowest friction (0.23), wear rate (0.17x10-6 mm3/mN), and contact resistance before (11 mΩ at 10 N) and after (30 mΩ at 10 N) the chemical stability test. These results imply that nc-NbCx/a-C coatings of this composition are a good candidate for electrical contact applications, and that up-scaling of the process is achievable.

    Keywords
    thin film, carbide, electrical contacts, contact resistance, friction
    National Category
    Materials Chemistry
    Research subject
    Chemistry with specialization in Materials Chemistry
    Identifiers
    urn:nbn:se:uu:diva-219028 (URN)10.1016/j.surfcoat.2014.05.021 (DOI)000339150200014 ()
    Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
    3. Characterization of amorphous and nanocomposite Nb-Si-C thin films deposited by DC magnetron sputtering
    Open this publication in new window or tab >>Characterization of amorphous and nanocomposite Nb-Si-C thin films deposited by DC magnetron sputtering
    Show others...
    2013 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 545, p. 272-278Article in journal (Refereed) Published
    Abstract [en]

    Two series of Nb-Si-C thin films of different composition have been deposited using DC magnetron sputtering. In the first series the carbon content was kept at about 55 at.% while the Si/Nb ratio was varied and in the second series the C/Nb ratio was varied instead while the Si content was kept at about 45 at.%. The microstructure is strongly dependent on Si content and Nb-Si-C films containing more than 25 at.% Si exhibit an amorphous structure as determined by X-ray diffraction. Transmission electron microscopy, however, induces crystallisation during analysis, thus obstructing a more detailed analysis of the amorphous structure. X-ray photo-electron spectroscopy suggests that the amorphous films consist of a mixture of chemical bonds such as Nb-Si, Nb-C, and Si-C. The addition of Si results in a hardness decrease from 22 GPa for the binary Nb-C film to 18 - 19 GPa for the Si-containing films, while film resistivity increases from 211 mu Omega cm to 3215 mu Omega cm. Comparison with recently published results on DC magnetron sputtered Zr-Si-C films, deposited in the same system using the same Arplasma pressure, bias, and a slightly lower substrate temperature (300 degrees C instead of 350 degrees C), shows that hardness is primarily dependent on the amount of Si-C bonds rather than type of transition metal. The reduced elastic modulus on the other hand shows a dependency on the type of transition metal for the films. These trends for the mechanical properties suggest that high wear resistant (high H/E and H-3/E-2 ratio) Me-Si-C films can be achieved by appropriate choice of film composition and transition metal. 

    Keywords
    Magnetron sputtering, Carbide, Amorphous structure, Structure characterization, Mechanical properties, Electrical properties
    National Category
    Materials Chemistry
    Identifiers
    urn:nbn:se:uu:diva-209841 (URN)10.1016/j.tsf.2013.08.066 (DOI)000324820800045 ()
    Available from: 2013-10-28 Created: 2013-10-28 Last updated: 2017-12-06
    4. Beam-induced crystallization of amorphous Me-Si-C (Me = Nb or Zr) thin films during transmission electron microscopy
    Open this publication in new window or tab >>Beam-induced crystallization of amorphous Me-Si-C (Me = Nb or Zr) thin films during transmission electron microscopy
    Show others...
    2013 (English)In: MRS Communications, ISSN 2159-6859, Vol. 3, no 3, p. 151-155Article in journal (Refereed) Published
    Abstract [en]

    We report that an electron beam focused for high-resolution imaging rapidly initiates observable crystallization of amorphous Me-Si-C films. For 200-keV electron irradiation of Nb-Si-C and Zr-Si-C films, crystallization is observed at doses of similar to 2.8 x 10(9) and similar to 4.7 x 10(9) e(-)/nm(2), respectively. The crystallization process is driven by atomic displacement events, rather than heating from the electron beam as in situ annealing (400-600 degrees C) retains the amorphous state. Our findings demand a critical analysis of alleged amorphous and nanocrystalline ceramics including reassessing previous reports on nanocrystalline Me-Si-C films for possible electron-beam-induced crystallization effects.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-210740 (URN)10.1557/mrc.2013.31 (DOI)000325095100007 ()
    Available from: 2013-11-14 Created: 2013-11-14 Last updated: 2015-07-07Bibliographically approved
    5. Superhard NbB2-x thin films deposited by dc magnetron sputtering
    Open this publication in new window or tab >>Superhard NbB2-x thin films deposited by dc magnetron sputtering
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    2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 257, p. 295-300Article in journal (Refereed) Published
    Abstract [en]

    We have deposited weakly textured substoichiometric NbB2-x thin films by magnetron sputtering from a NbB2 target. The films exhibit superhardness (42 ± 4 GPa), previously only observed in overstoichiometric TiB2 thin films, and explained by a self-organized nanostructuring, where thin TiB2 columnar grains hinder nucleation and slip of dislocations and a B-rich tissue phase between the grains prevent grain-boundary sliding. The wide homogeneity range for the NbB2 phase allows a similar ultra-thin B-rich tissue phase to form between thin (5 – 10 nm) columnar NbB2-x grains also for films with a B/Nb atomic ratio of 1.8, as revealed here by analytical aberration-corrected scanning transmission electron microscopy. Furthermore, a coefficient of friction of 0.16 is measured for a NbB2-x film sliding against stainless steel with a wear rate of 4x10-7 mm3/Nm. X-ray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film.

    Keywords
    boride, structure characterization, mechanical properties, friction, tribological properties
    National Category
    Materials Chemistry
    Research subject
    Chemistry with specialization in Materials Chemistry
    Identifiers
    urn:nbn:se:uu:diva-219031 (URN)10.1016/j.surfcoat.2014.07.087 (DOI)000344423100029 ()
    Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
    6. Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering
    Open this publication in new window or tab >>Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering
    Show others...
    2014 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 32, no 4, p. 041503-Article in journal (Refereed) Published
    Abstract [en]

    The high wear resistance, high chemical inertness, and high electrical conductivity of magnetron-sputtered transition metal diborides (MeB2) make them a candidate material for sliding electrical contacts. However, their high hardness makes it difficult to penetrate surface oxides, resulting in a high electrical contact resistance. We have in this study investigated how the contact resistance can be improved by the formation of softer Nb-B-C films. The Nb-B-C films were deposited by magnetron sputtering and shown to exhibit a nanocomposite microstructure consisting of nanocrystalline NbB2-x grains with a solid solution of C separated by an amorphous BCx phase. The formation of the BCx phase reduces the hardness from 41 GPa for the NbB2-x film to 19 GPa at 36 at.% C. As a consequence the contact resistance is drastically reduced and the lowest contact resistance of 35 mΩ (contact force 5 N) is achieved for a film containing 30 at.% C. However, crack formation and subsequent delamination and fragmentation is observed for the C-containing Nb-B-C films in tribology tests resulting in high friction values for these films.

    Keywords
    boride, structural characterization, electrical properties, mechanical properties, friction, nanocomposite
    National Category
    Materials Chemistry
    Research subject
    Chemistry with specialization in Materials Chemistry
    Identifiers
    urn:nbn:se:uu:diva-219032 (URN)10.1116/1.4875135 (DOI)000338718400015 ()
    Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
    7. Characterization of magnetron sputtered Cr-B and Cr-B-C thin films for electrical contact applications
    Open this publication in new window or tab >>Characterization of magnetron sputtered Cr-B and Cr-B-C thin films for electrical contact applications
    Show others...
    2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 266, p. 167-176Article in journal (Refereed) Published
    Abstract [en]

    We have deposited Cr-B and Cr-B-C thin films by co-sputtering from chromium boride and carbon targets. The binary Cr-B films consist of nanocrystalline and substoichiometric CrB2 - x grains (B/Cr atomic ratio <= 1.5) with a (101)-texture, where B segregates to the grain boundaries forming a B-rich tissue phase. A hardness of 25 GPa is measured for these films. They have a low wear resistance, attributed to a (101)-texture and limited adhesion. As a consequence, wear debris in the CrB2 - x wear track from delaminated film and steel-to-steel contact between the exposed substrate and the counter surface result in a high friction (0.52-0.78 against stainless steel) making the Cr-B films unsuitable as sliding electric contacts. Cr-B-C films, on the other hand, form a two phase amorphous structure at >17 at.% C consisting of an amorphous Cr-rich phase containing both B and C and an amorphous matrix phase containing mainly B and C. The addition of C improves the adhesion and tribological properties and a coefficient of friction of 0.12 is obtained at 38 at.% C. The improved tribological properties are explained by the formation of the matrix phase, which acts as a solid lubricant forming a graphite-like tribofilm during ball-on-disc test. However, the formation of an amorphous structure is not beneficial for the electrical contact resistance, which increases from 0.5 Omega for the Cr-B film to 1.5 and 2.3 Omega for the Cr-B-C films containing 17 and 26 at% C, respectively. Finally, the importance of a chemical analysis of the chromium boride sputtering target composition is discussed.

    Keywords
    boride, CrB, CrBC, nanocomposite, amorphous, mechanical properties, electrical properties
    National Category
    Materials Chemistry
    Research subject
    Chemistry with specialization in Materials Chemistry
    Identifiers
    urn:nbn:se:uu:diva-219033 (URN)10.1016/j.surfcoat.2015.02.037 (DOI)000352664600022 ()
    Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
    8. Ti-B-C nanocomposite coatings deposited by magnetron sputtering
    Open this publication in new window or tab >>Ti-B-C nanocomposite coatings deposited by magnetron sputtering
    Show others...
    2012 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 258, no 24, p. 9907-9912Article in journal (Refereed) Published
    Abstract [en]

    Ti-B-C nanocomposite coatings with a B content of 8-17 at.% have been deposited by magnetron sputtering from B4C, Ti, and C targets. X-ray diffraction, photoelectron spectroscopy, and electron microscopy show that the coatings consist of nanocrystalline (nc) TiC: B embedded in a matrix of amorphous (a) C, BCx, TiOx and BOx. The fraction of amorphous phase scales with the Ti concentration, where the matrix predominantly consists of free C with some BCx in coatings with a C/Ti ratio > 1, while the matrix predominantly consists of BCx with some free C in coatings with a C/Ti ratio < 1. nc-TiC:B/a-BCx/a-C coatings with low amount of free C exhibit a contact resistance comparable to the contact resistance of an Ag sputtered coating at loads of similar to 1 N against an Au probe, despite the O content of similar to 16 at.%.

    Keywords
    Nanocrystalline, Electron microscopy, Contact resistance, B4C, TiC
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-181831 (URN)10.1016/j.apsusc.2012.06.049 (DOI)000307729600064 ()
    Available from: 2012-10-02 Created: 2012-10-01 Last updated: 2017-12-07Bibliographically approved
  • 304.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Primetzhofer, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Wang, Liping
    Lu, Jun
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Characterization of magnetron sputtered Cr-B and Cr-B-C thin films for electrical contact applications2015In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 266, p. 167-176Article in journal (Refereed)
    Abstract [en]

    We have deposited Cr-B and Cr-B-C thin films by co-sputtering from chromium boride and carbon targets. The binary Cr-B films consist of nanocrystalline and substoichiometric CrB2 - x grains (B/Cr atomic ratio <= 1.5) with a (101)-texture, where B segregates to the grain boundaries forming a B-rich tissue phase. A hardness of 25 GPa is measured for these films. They have a low wear resistance, attributed to a (101)-texture and limited adhesion. As a consequence, wear debris in the CrB2 - x wear track from delaminated film and steel-to-steel contact between the exposed substrate and the counter surface result in a high friction (0.52-0.78 against stainless steel) making the Cr-B films unsuitable as sliding electric contacts. Cr-B-C films, on the other hand, form a two phase amorphous structure at >17 at.% C consisting of an amorphous Cr-rich phase containing both B and C and an amorphous matrix phase containing mainly B and C. The addition of C improves the adhesion and tribological properties and a coefficient of friction of 0.12 is obtained at 38 at.% C. The improved tribological properties are explained by the formation of the matrix phase, which acts as a solid lubricant forming a graphite-like tribofilm during ball-on-disc test. However, the formation of an amorphous structure is not beneficial for the electrical contact resistance, which increases from 0.5 Omega for the Cr-B film to 1.5 and 2.3 Omega for the Cr-B-C films containing 17 and 26 at% C, respectively. Finally, the importance of a chemical analysis of the chromium boride sputtering target composition is discussed.

  • 305.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Tengstrand, O.
    Flink, A.
    Andersson, A. M.
    Eklund, P.
    Hultman, L.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Reactive sputtering of NbCx-based nanocomposite coatings: An up-scaling study2014In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 253, p. 100-108Article in journal (Refereed)
    Abstract [en]

    Nanocomposite Nb-C coatings, with a C/Nb ratio of 0.93-1.59, have been deposited by reactive sputtering in a commercial sputtering system where the C is supplied from an acetylene gas at deposition rates of up to 200 nm/min. The coatings are compared to non-reactively sputtered Nb-C coatings deposited from Nb and C targets in lab-scale equipment at deposition rates two orders of magnitude lower. X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy are used to conclude that all coatings consist of nanoctystalline Nbc(x) grains (nc-NbCx) embedded in a matrix of amorphous C (a-C). The coating performance was evaluated in terms of their mechanical, tribological, and electrical properties. The chemical stability of the coatings was evaluated by exposure to a flowing mixture of corrosive gases. It is found that the coatings have comparable microstructure and performance to the coatings deposited by non-reactive sputtering. The high deposition rate and presence of different C-radicals on the coating surface during film growth for the reactively sputtered coatings are believed to result in a smaller NbCx grain size compared to the non-reactively sputtered coatings (reactive process: 10-3 nm, non-reactive process: similar to 75-3 nm). This difference results in a thinner a-C matrix of about 0.2 nm, which is not varying with C content for the reactively sputtered coatings. The thinner a-C matrix is reflected in coating properties, with a higher conductivity and slightly higher hardness. The coating richest in C content (C/Nb ratio 1.59) shows the lowest friction (0.23), wear rate (0.17 x 10(-6) mm(3)/mN), and contact resistance before (11 m Omega at 10 N) and after (30 m Omega at 10 N) the chemical stability test. These results imply that nc-NbCx/a-C coatings of this composition are a good candidate for electrical contact applications, and that up-scaling of the process is achievable. 

  • 306.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Tengstrand, O.
    Lu, J.
    Eklund, P.
    Persson, P.
    Hultman, L.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Suberhard NbB2-x thin films deposited by dc magnetron sputtering2014In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 257, p. 295-300Article in journal (Refereed)
    Abstract [en]

    We have deposited weakly textured substoichiometric NbB2 −x thin films by magnetron sputtering from an NbB2 target. The films exhibit superhardness (42 ± 4 GPa), previously only observed in overstoichiometric TiB2 thin films, and explained by a self-organized nanostructuring, where thin TiB2 columnar grains hinder nucleation and slip of dislocations and a B-rich tissue phase between the grains prevent grain-boundary sliding. The wide homogeneity range for the NbB2 phase allows a similar ultra-thin B-rich tissue phase to form between thin (5–10 nm) columnar NbB2 −x grains also for films with a B/Nb atomic ratio of 1.8, as revealed here by analytical aberration-corrected scanning transmission electron microscopy. Furthermore, a coefficient of friction of 0.16 is measured for an NbB2 −x film sliding against stainless steel with a wear rate of 5 × 10− 7 mm3/Nm. X-ray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film.

  • 307.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Tengstrand, Olof
    Eklund, Per
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering2014In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 32, no 4, p. 041503-Article in journal (Refereed)
    Abstract [en]

    The high wear resistance, high chemical inertness, and high electrical conductivity of magnetron-sputtered transition metal diborides (MeB2) make them a candidate material for sliding electrical contacts. However, their high hardness makes it difficult to penetrate surface oxides, resulting in a high electrical contact resistance. We have in this study investigated how the contact resistance can be improved by the formation of softer Nb-B-C films. The Nb-B-C films were deposited by magnetron sputtering and shown to exhibit a nanocomposite microstructure consisting of nanocrystalline NbB2-x grains with a solid solution of C separated by an amorphous BCx phase. The formation of the BCx phase reduces the hardness from 41 GPa for the NbB2-x film to 19 GPa at 36 at.% C. As a consequence the contact resistance is drastically reduced and the lowest contact resistance of 35 mΩ (contact force 5 N) is achieved for a film containing 30 at.% C. However, crack formation and subsequent delamination and fragmentation is observed for the C-containing Nb-B-C films in tribology tests resulting in high friction values for these films.

  • 308.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Tengstrand, Olof
    Flink, Axel
    Andersson, Anna
    Eklund, Per
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Reactive sputtering of NbCx-based nanocomposite coatings: an up-scaling study2014In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 253, p. 100-108Article in journal (Refereed)
    Abstract [en]

    Nanocomposite Nb-C coatings, with a C/Nb ratio of 0.93 - 1.59, have been deposited by reactive sputtering in a commercial sputtering system at deposition rates of up to 200 nm/min. The coatings are compared to non-reactively sputtered Nb-C in lab-scale equipment at deposition rates two orders of magnitude lower. X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy are used to conclude that all coatings consist of nanocrystalline NbCx grains (nc-NbCx) embedded in a matrix of amorphous C (a-C). The coating performance was evaluated in terms of their mechanical, tribological, and electrical properties. The chemical stability of the coatings was evaluated by exposure to a flowing mixture of corrosive gases. It is found that the coatings have comparable microstructure and performance to the coatings deposited by non-reactive sputtering. The high deposition rate and presence of different C-radicals on the coating surface during film growth for the reactively sputtered coatings is believed to result in a smaller NbCx grain size compared to the non-reactively sputtered coatings (reactive process: 10 – 3 nm, non-reactive process: ~75 – 3 nm). This difference results in a thinner a-C matrix of about 0.2 nm, which is not varying with C content for the reactively sputtered coatings. The thinner a-C matrix is reflected in coating properties, with a higher conductivity and slightly higher hardness. The coating richest in C content (C/Nb ratio 1.59) shows the lowest friction (0.23), wear rate (0.17x10-6 mm3/mN), and contact resistance before (11 mΩ at 10 N) and after (30 mΩ at 10 N) the chemical stability test. These results imply that nc-NbCx/a-C coatings of this composition are a good candidate for electrical contact applications, and that up-scaling of the process is achievable.

  • 309.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Tengstrand, Olof
    Flink, Axel
    Eklund, Per
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Characterization of amorphous and nanocomposite Nb-Si-C thin films deposited by DC magnetron sputtering2013In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 545, p. 272-278Article in journal (Refereed)
    Abstract [en]

    Two series of Nb-Si-C thin films of different composition have been deposited using DC magnetron sputtering. In the first series the carbon content was kept at about 55 at.% while the Si/Nb ratio was varied and in the second series the C/Nb ratio was varied instead while the Si content was kept at about 45 at.%. The microstructure is strongly dependent on Si content and Nb-Si-C films containing more than 25 at.% Si exhibit an amorphous structure as determined by X-ray diffraction. Transmission electron microscopy, however, induces crystallisation during analysis, thus obstructing a more detailed analysis of the amorphous structure. X-ray photo-electron spectroscopy suggests that the amorphous films consist of a mixture of chemical bonds such as Nb-Si, Nb-C, and Si-C. The addition of Si results in a hardness decrease from 22 GPa for the binary Nb-C film to 18 - 19 GPa for the Si-containing films, while film resistivity increases from 211 mu Omega cm to 3215 mu Omega cm. Comparison with recently published results on DC magnetron sputtered Zr-Si-C films, deposited in the same system using the same Arplasma pressure, bias, and a slightly lower substrate temperature (300 degrees C instead of 350 degrees C), shows that hardness is primarily dependent on the amount of Si-C bonds rather than type of transition metal. The reduced elastic modulus on the other hand shows a dependency on the type of transition metal for the films. These trends for the mechanical properties suggest that high wear resistant (high H/E and H-3/E-2 ratio) Me-Si-C films can be achieved by appropriate choice of film composition and transition metal. 

  • 310.
    Nedfors, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Tengstrand, Olof
    Lu, Jun
    Persson, Per
    Eklund, Per
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Superhard NbB2-x thin films deposited by dc magnetron sputtering2014In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 257, p. 295-300Article in journal (Refereed)
    Abstract [en]

    We have deposited weakly textured substoichiometric NbB2-x thin films by magnetron sputtering from a NbB2 target. The films exhibit superhardness (42 ± 4 GPa), previously only observed in overstoichiometric TiB2 thin films, and explained by a self-organized nanostructuring, where thin TiB2 columnar grains hinder nucleation and slip of dislocations and a B-rich tissue phase between the grains prevent grain-boundary sliding. The wide homogeneity range for the NbB2 phase allows a similar ultra-thin B-rich tissue phase to form between thin (5 – 10 nm) columnar NbB2-x grains also for films with a B/Nb atomic ratio of 1.8, as revealed here by analytical aberration-corrected scanning transmission electron microscopy. Furthermore, a coefficient of friction of 0.16 is measured for a NbB2-x film sliding against stainless steel with a wear rate of 4x10-7 mm3/Nm. X-ray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film.

  • 311.
    Nedumkandathil, Reji
    et al.
    Stockholm Univ, Dept Mat & Environm Chem, SE-10691 Stockholm, Sweden..
    Kranak, Verina F.
    Stockholm Univ, Dept Mat & Environm Chem, SE-10691 Stockholm, Sweden..
    Johansson, Robert
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ångström, Jonas
    Balmes, Oliver
    Lund Univ, MAX Lab 4, Box 118, SE-22100 Lund, Sweden..
    Andersson, Mikael S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Haussermann, Ulrich
    Stockholm Univ, Dept Mat & Environm Chem, SE-10691 Stockholm, Sweden..
    Hydrogenation induced structure and property changes in GdGa2016In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, p. 184-191Article in journal (Refereed)
    Abstract [en]

    Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.

  • 312.
    Niklasson, Lovisa
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Characterization of the gas composition inside NiMH batteries during charge using GC-MS2018Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The aim of the project was to develop a method to measure and studythe degree of activation of the negative electrode (MH) in a NiMH battery.This was done by characterization of the gases produced during charge of a battery – O2 and H2 – using a Gas Chromatograph. The current applied in the very first charge of the battery was varied in order to examine how this affects the gas evolution. In the developed method, batteries were charged to 8Ah with 9A, after which a gas sample was taken and analyzed with Gas Chromatography. An additional goal was to use the method to examine the difference in activation between virgin and recycled negative electrode material. A module charged stepwise with 0.07C followed by 0.2C had the lowest share of H2 after two cycles, indicated best activation. However, a higher amount of H2 in the beginning of the activation process could possibly enhance the degree of activation during the following cycles. The method indicated that the module with recycled MH was better activated than the virgin MH. To improve the technique, repeated measurements to get better statistics should be done. Gas samples should be taken at dV/dt=0 in order to take samples at same SoC. The charge current should be adjusted so that the same C rate is always used. This would make the results easier to interpret.

  • 313.
    NISKANEN, K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    MAGNUSSON, J
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    NORDBLAD, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    SVEDLINDH, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    ULLSTROM, AS
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    LUNDSTROM, T
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    ANTI-MEISSNER EFFECT AND LOW-FIELD MAGNETIC-RELAXATION IN SINTERED BI-22121994In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 194, p. 1549-1550Article in journal (Refereed)
    Abstract [en]

    A detailed study of the low field magnetic properties of some sintered Bi-2212 samples has been performed. For sufficiently low applied magnetic fields, typically for fields H < 0.2 Oe, the field-cooled magnetisation becomes positive at some temperature below T(C) (anti-Meissner effect), contrary to the usual Meissner effect. The field dependence of this effect is comparable to the predicted behaviour for a system in an orbital glass state. Additionally, the relaxation of the zero-field-cooled magnetisation displays an unexpected field dependence, the relaxation rate S = partial-derivative M/partial-derivative log(t) increases as H(alpha) with alpha almost-equal-to 0.9 at very low fields. In contrast to the behaviour found in sintered YBCO, this result is not accounted for in conventional flux creep models, where the prediction is alpha greater-than-or-equal-to 2.

  • 314.
    NISKANEN, K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    NORLING, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    NORDBLAD, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    SVEDLINDH, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    LUNDGREN, L
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    FOLSTER, J
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    LUNDSTROM, T
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    FIELD AND TIME-DEPENDENCE OF THE MAGNETIC-SUSCEPTIBILITY OF SINGLE-CRYSTAL YBA2CU3O71989In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 162, p. 1637-1638Article in journal (Refereed)
  • 315.
    Nyberg, Harald
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Sundberg, Jill
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Särhammar, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Gustavsson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Kubart, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Jansson, Ulf
    Jacobson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings2013In: 19th International Conference on Wear of Materials 2013; 14-18 April 2013; Portland, OR, USA, Elsevier, 2013, no 1-2 SIConference paper (Refereed)
    Abstract [en]

    The disulphides of tungsten and molybdenum are known for their low friction properties when used as solid lubricants. Due to their low hardness, their load bearing capacity when used as thin films is poor. When carbon is added to a WS2 coating, both of these shortcomings are improved, and a structure consisting of nanocrystals of WS2, and possibly tungsten carbide, in a matrix of amorphous carbon is formed. In this study, an attempt is made for further increasing the hardness of such coatings, by addition of Ti, a strong carbide former. A number of W–S–C(–Ti) coatings were deposited using magnetron co-sputtering, and characterised with regard to chemical composition, structure and tribological properties. It was seen that addition of Ti significantly increased the hardness of the coatings, while maintaining their excellent low friction properties in dry atmosphere. However, the coatings with Ti showed extremely high initial friction, a feature not seen for the coatings without Ti. The mechanisms behind this running-in behaviour were investigated by studying surfaces at early stages of wear. It was observed that tribofilms formed during sliding for the coatings containing Ti consisted mainly of TiO2, with platelets of WS2 appearing in the contact only after prolonged sliding. For the pure W–S–C coatings, WS2 was observed in the sliding interface almost instantly at the onset of sliding.

  • 316.
    Nyberg, Harald
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Sundberg, Jill
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Särhammar, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Gustavsson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Kubart, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Jansson, Ulf
    Jacobson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings2013In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 302, no 1-2 SI, p. 987-997Article in journal (Refereed)
    Abstract [en]

    The disulphides of tungsten and molybdenum are known for their low friction properties when used as solid lubricants. Due to their low hardness, their load bearing capacity when used as thin films is poor. When carbon is added to a WS2 coating, both of these shortcomings are improved, and a structure consisting of nanocrystals of WS2, and possibly tungsten carbide, in a matrix of amorphous carbon is formed. In this study, an attempt is made for further increasing the hardness of such coatings, by addition of Ti, a strong carbide former. A number of W–S–C(–Ti) coatings were deposited using magnetron co-sputtering, and characterised with regard to chemical composition, structure and tribological properties. It was seen that addition of Ti significantly increased the hardness of the coatings, while maintaining their excellent low friction properties in dry atmosphere. However, the coatings with Ti showed extremely high initial friction, a feature not seen for the coatings without Ti. The mechanisms behind this running-in behaviour were investigated by studying surfaces at early stages of wear. It was observed that tribofilms formed during sliding for the coatings containing Ti consisted mainly of TiO2, with platelets of WS2 appearing in the contact only after prolonged sliding. For the pure W–S–C coatings, WS2 was observed in the sliding interface almost instantly at the onset of sliding.

  • 317.
    Nyberg, Harald
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Sundberg, Jill
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Särhammar, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Jacobson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Tribochemical formation of sulphide tribofilms from a Ti-C-S coating sliding against different counter surfaces2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 56, no 3, p. 563-572Article in journal (Refereed)
    Abstract [en]

    Tribochemically active Ti-C-S coatings are nanocomposite coatings containing a S-doped titanium carbide, from which S can be released in a tribological contact. This work studies tribochemical reactions between a Ti-C-S coating and various counter surface materials, and their effect on the tribological performance. Tribological tests were performed in a ball-on-disc set-up, using balls of five different materials as sliding partners for the coating: 100Cr6 steel, pure W, WC, 316-L steel and Al2O3. For W balls, a WS2 tribofilm was formed, leading to low friction (down to A mu = 0.06). Furthermore, increasing normal load on the W balls was found to lead to a strong decrease in A mu and earlier formation of the low-friction WS2 tribofilm. Similar WS2 and MoS2 tribofilms were, however, not formed from WC- and Mo-containing 316-L balls. The performance when using WC and Al2O3 balls was significantly worse than for the two steel balls. It is suggested that this is due to sulphide formation from Fe, analogous to formation of anti-seizure tribofilms from S-containing extreme pressure additives and steel surfaces. The tribochemical activity of Ti-C-S coatings, with the possibility of S release, is thus beneficial not only for pure W counter surfaces, but also for Fe-based sliding partners.

  • 318.
    Nygard, Magnus Moe
    et al.
    Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, NO-2027 Kjeller, Norway.
    Ek, Gustav
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Karlsson, Dennis
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sorby, Magnus H.
    Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, NO-2027 Kjeller, Norway.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Hauback, Bjorn C.
    Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, NO-2027 Kjeller, Norway.
    Counting electrons - A new approach to tailor the hydrogen sorption properties of high-entropy alloys2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 175, p. 121-129Article in journal (Refereed)
    Abstract [en]

    We have investigated the structure and hydrogen storage properties of a series of quaternary and quintary high-entropy alloys related to the ternary system TiVNb with powder X-ray diffraction (PXD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and manometric measurements in a Sieverts apparatus. The alloys have body-centred cubic (bcc) crystal structures and form face-centred cubic (fcc) metal hydrides with hydrogen-to-metal ratios close to 2 by hydrogenation. The onset temperature for hydrogen desorption, T-onset, decreases linearly with the valence-electron concentration, VEC. Moreover, the volumetric expansion per metal atom from the bcc alloys to the fcc hydrides, [(V/Z)(fcc) - (V/Z)(bcc)]/(V/Z)(bcc), increases linearly with the VEC. Therefore, it seems that a larger expansion of the lattice destabilizes the metal hydrides and that this effect can be tuned by altering the VEC. Kissinger analyses performed on the DSC measurements indicate that the destabilization is a thermodynamic rather than kinetic effect. Based upon these insights we have identified TiVCrNbH8 as a material with suitable thermodynamics for hydrogen storage in the solid state. This HEA-based hydride has a reversible hydrogen storage capacity of 1.96 wt% H at room temperature and moderate H-2-pressures. Moreover, it is not dependent on any elaborate activation procedure to absorb hydrogen.

  • 319.
    Nygren, Kristian
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Magnetron Sputtering of Nanocomposite Carbide Coatings for Electrical Contacts2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Today’s electronic society relies on the functionality of electrical contacts. To achieve good contact properties, surface coatings are normally applied. Such coatings should ideally fulfill a combination of different properties, like high electrical conductivity, high corrosion resistance, high wear resistance and low cost. A common coating strategy is to use noble metals since these do not form insulating surface oxides. However, such coatings are expensive, have poor wear resistance and they are often applied by electroplating, which poses environmental and human health hazards.

    In this thesis, nanocomposite carbide-based coatings were studied and the aim was to evaluate if they could exhibit properties that were suitable for electrical contacts. Coatings in the Cr-C, Cr-C-Ag and Nb-C systems were deposited by magnetron sputtering using research-based equipment as well as industrial-based equipment designed for high-volume production. To achieve the aim, the microstructure and composition of the coatings were characterized, whereas mechanical, tribological, electrical, electrochemical and optical properties were evaluated. A method to optically measure the amount of carbon was developed.

    In the Cr-C system, a variety of deposition conditions were explored and amorphous carbide/amorphous carbon (a-C) nanocomposite coatings could be obtained at substrate temperatures up to 500 °C. The amount of a-C was highly dependent on the total carbon content. By co-sputtering with Ag, coatings comprising an amorphous carbide/carbon matrix, with embedded Ag nanoclusters, were obtained. Large numbers of Ag nanoparticles were also found on the surfaces. In the Nb-C system, nanocrystalline carbide/a-C coatings could be deposited. It was found that the nanocomposite coatings formed very thin passive films, consisting of both oxide and a-C.

    The Cr-C coatings exhibited low hardness and low-friction properties. In electrochemical experiments, the Cr-C coatings exhibited high oxidation resistance. For the Cr-C-Ag coatings, the Ag nanoparticles oxidized at much lower potentials than bulk Ag. Overall, electrical contact resistances for optimized samples were close to noble metal references at low contact load. Thus, the studied coatings were found to have properties that make them suitable for electrical contact applications.

    List of papers
    1. Influence of deposition temperature and amorphous carbon on microstructure and oxidation resistance of magnetron sputtered nanocomposite Cr-C films
    Open this publication in new window or tab >>Influence of deposition temperature and amorphous carbon on microstructure and oxidation resistance of magnetron sputtered nanocomposite Cr-C films
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    2014 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 305, p. 143-153Article in journal (Refereed) Published
    Abstract [en]

    It is known that mechanical and tribological properties of transition metal carbide films can be tailored by adding an amorphous carbon (a-C) phase, thus making them nanocomposites. This paper addresses deposition, microstructure, and for the first time oxidation resistance of magnetron sputtered nanocomposite Cr C/a-C films with emphasis on studies of both phases. By varying the deposition temperature between 20 and 700 C and alternating the film composition, it was possible to deposit amorphous, nanocomposite, and crystalline Cr C films containing about 70% C and 30% Cr, or 40% C and 60% Cr. The films deposited at temperatures below 300 degrees C were X-ray amorphous and 500 C was required to grow crystalline phases. Chronoamperometric polarization at +0.6 V vs. Ag/AgCl(sat. KG) in hot 1 mM H-2 SO4 resulted in oxidation of Cr C, yielding Cr203 and C, as well as oxidation of C. The oxidation resistance is shown to depend on the deposition temperature and the presence of the a-C phase. Physical characterization of film surfaces show that very thin C/Cr2O3/Cr C layers develop on the present material, which can be used to improve the oxidation resistance of, e.g. stainless steel electrodes. (C) 2014 Elsevier B.V. All rights reserved.

    Keywords
    Chromium carbide, Magnetron sputtering, Nanocomposite, Deposition temperature, Carbon oxidation
    National Category
    Physical Chemistry Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-227992 (URN)10.1016/j.apsusc.2014.03.014 (DOI)000336525400020 ()
    Available from: 2014-07-04 Created: 2014-07-02 Last updated: 2017-12-30
    2. Growth and characterization of chromium carbide films deposited by high rate reactive magnetron sputtering for electrical contact applications
    Open this publication in new window or tab >>Growth and characterization of chromium carbide films deposited by high rate reactive magnetron sputtering for electrical contact applications
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    2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 260, p. 326-334Article in journal (Refereed) Published
    Abstract [en]

    Chromium carbide films with different phase contents were deposited at 126±26 °C by industrial high rate reactivemagnetron sputtering, using both direct current magnetron sputtering (DCMS) and high power impulsemagnetron sputtering (HiPIMS). Film structure and properties were studied by SEM, XRD, TEM, XPS, NRA, Raman spectroscopy, nanoindentation, unlubricated reciprocating sliding experiments, and a laboratory setup to measure electrical contact resistance. The films consisted of amorphous a-CrCy, a nanocrystalline minority phase of metastable cubic nc-CrCx, and a hydrogenated graphite-like amorphous carbon matrix (a-C:H). The DCMS and HiPIMS processes yielded films with similar phase contents and microstructures, as well as similar functional properties. Low elastic modulus, down to 66 GPa, indicated good wear properties via a hardness/elastic modulus (H/E) ratio of 0.087. Unlubricated steady-state friction coefficients down to 0.13 were obtained for films with 69 at.% carbon, while the electrical contact resistance could be reduced by two orders of magnitude by addition of a-C:H phase to purely carbidic films. The present films are promising candidates for sliding electrical contact applications.

    Place, publisher, year, edition, pages
    Elsevier: , 2014
    Keywords
    Direct current magnetron sputtering, High power impulse magnetron sputtering, Reactive sputtering, Amorphous chromium carbide, Solid lubricant, Contact resistance
    National Category
    Physical Chemistry Condensed Matter Physics Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-240742 (URN)10.1016/j.surfcoat.2014.06.069 (DOI)000347584300045 ()
    Conference
    The 41st International Conference on Metallurgical Coatings and Thin Films, APR 28-MAY 02, 2014, San Diego, CA
    Funder
    Swedish Foundation for Strategic Research Swedish Research Council
    Available from: 2015-01-08 Created: 2015-01-08 Last updated: 2017-12-05Bibliographically approved
    3. Structure and properties of Cr-C/Ag films deposited by magnetron sputtering
    Open this publication in new window or tab >>Structure and properties of Cr-C/Ag films deposited by magnetron sputtering
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    2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 281, p. 184-192Article in journal (Refereed) Published
    Abstract [en]

    Cr-C/Ag thin films with 0-14 at% Ag have been deposited by magnetron sputtering from elemental targets. The samples were analyzed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to study their structure and chemical bonding. A complex nanocomposite structure consisting of three phases; nanocrystalline Ag, amorphous CrCx and amorphous carbon is reported. The carbon content in the amorphous carbide phase was determined to be 32-33 at% C, independent of Ag content Furthermore, SEM and XPS results showed higher amounts of Ag on the surface compared to the bulk. The hardness and Young's modulus were reduced from 12 to 8 GPa and from 270 to 170 GPa, respectively, with increasing Ag content. The contact resistance was found to decrease with Ag addition, with the most Ag rich sample approaching the values of an Ag reference sample. Initial tribological tests gave friction coefficients in the range of 0.3 to 0.5, with no clear trends. Annealing tests show that the material is stable after annealing at 500 degrees C for 1 h, but not after annealing at 800 degrees C for 1 h. In combination, these results suggest that sputtered Cr-C/Ag films could be potentially applicable for electric contact applications.

    Place, publisher, year, edition, pages
    Elsevier, 2015
    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-247276 (URN)10.1016/j.surfcoat.2015.09.054 (DOI)000366072200024 ()
    Funder
    Swedish Foundation for Strategic Research , RMA11-0029Swedish Research Council, 621-2011-3492
    Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2017-12-04Bibliographically approved
    4. The influence of nanoeffects on the oxidation of magnetron sputtered Cr-C/Ag thin films containing silver nanoparticles
    Open this publication in new window or tab >>The influence of nanoeffects on the oxidation of magnetron sputtered Cr-C/Ag thin films containing silver nanoparticles
    2017 (English)In: Chemelectrochem, ISSN 2196-0216, Vol. 4, no 2, p. 418-429Article in journal (Refereed) Published
    Abstract [en]

    Well-controlled functionalization of carbide-based nanocomposite films with noble-metal surface nanoparticles of different sizes may lead to new materials with novel multifunctional properties. In this work, magnetron sputtering was used to deposit nanocomposite films comprising amorphous chromium carbide (a-CrCx), amorphous carbon (a-C), and a minority of silver in the form of embedded nanoclusters. Up to 510(10) surface nanoparticles per cm(2) with different size distributions were also found to be formed, owing to the diffusion of silver from the bulk of the film. The influences of these conductive nanoparticles on the electrochemical behavior of the films were investigated in dilute sulfuric acid. Although silver is a noble metal, the oxidation potential of the nanoparticles was about 0.4V more negative than the Ag+/Ag standard potential, meaning that the nanoparticles were oxidized in the Cr passive potential region. While this effect can mainly be explained by a low concentration of Ag+ in the electrolyte, the sizes of the nanoparticles and interactions with the matrix were also found to be important. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to analyze the surface chemistries. As Ag can be replaced by other noble metals, the concept is of general interest for further studies.

    Keywords
    Nanoelectrochemistry, Noble metals, Carbides, Nanoparticles, Thin films
    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-302060 (URN)10.1002/celc.201600615 (DOI)000394905900026 ()
    Funder
    Swedish Foundation for Strategic Research Swedish Research Council
    Available from: 2016-08-29 Created: 2016-08-29 Last updated: 2017-04-25Bibliographically approved
    5. Passive films on nanocomposite carbide coatings for electrical contact applications
    Open this publication in new window or tab >>Passive films on nanocomposite carbide coatings for electrical contact applications
    2017 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 52, no 13, p. 8231-8246Article in journal (Refereed) Published
    Abstract [en]

    Nanocomposite transition metal carbide/amorphous carbon coatings (Me-C/a-C) deposited by magnetron sputtering have excellent electrical contact properties. The contact resistance can be as low as that of noble metal coatings, although it is known to vary by several orders of magnitude depending on the deposition conditions. We have investigated a nanocrystalline niobium carbide/amorphous carbon (NbC (x) /a-C:H) model system aiming to clarify factors affecting the contact resistance for this group of contact materials. For the first time, the surface chemistry is systematically studied, by angle-resolved X-ray photoelectron spectroscopy, and in extension how it can explain the contact resistance. The coatings presented a mean oxide thickness of about 1 nm, which could be grown to 8 nm by annealing. Remarkably, the contact resistances covered four orders of magnitude and were found to be exponentially dependent on the mean oxide thickness. Moreover, there is an optimum in the amount of a-C:H phase where the contact resistance drops very significantly and it is thus important to not only consider the mean oxide thickness. To explain the results, a model relying on surface chemistry and contact mechanics is presented. The lowest contact resistance of a nanocomposite matched that of a gold coating at 1 N load (vs. gold), and such performance has previously not been demonstrated for similar nanocomposite materials, highlighting their useful properties for electrical contact applications.

    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-302061 (URN)10.1007/s10853-017-1039-0 (DOI)000399422000050 ()
    Funder
    Swedish Foundation for Strategic Research VINNOVASwedish Research Council, VR 2011-3492
    Available from: 2016-08-29 Created: 2016-08-29 Last updated: 2017-05-23Bibliographically approved
    6. Optical methods to quantify amorphous carbon in carbide-based nanocomposite coatings
    Open this publication in new window or tab >>Optical methods to quantify amorphous carbon in carbide-based nanocomposite coatings
    2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 638, p. 291-297Article in journal (Other academic) Published
    Abstract [en]

    We report how the total carbon content and the amorphous carbon (a-C) phase fraction in transition metal carbide/a-C nanocomposite coatings can be obtained using optical methods, which are much more practical for industrial use than conventional X-ray photoelectron spectroscopy. A large set of carbon-containing nanocomposite coatings deposited using different magnetron sputtering techniques were analyzed by X-ray photoelectron spectroscopy, reflectance spectrophotometry, and spectroscopic ellipsometry. The chemical composition and the a-C phase fraction were determined by X-ray photoelectron spectroscopy for each coating and results are presented for the Ti-C, Cr-C, and Nb-C systems. The composition and the a-C phase fraction are correlated to optical reflectance in the visible range, by parametrization in L*a*b* color space, and by ellipsometry primary data. Results show that it is possible to rapidly estimate the composition and the a-C fraction using these optical methods. We propose that optical methods have promising use in the industry as a cost-efficient technique for characterization of carbide-based coatings.

    Place, publisher, year, edition, pages
    Elsevier, 2017
    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-302062 (URN)10.1016/j.tsf.2017.07.053 (DOI)000411775900038 ()
    Available from: 2016-08-29 Created: 2016-08-29 Last updated: 2017-12-20Bibliographically approved
  • 320.
    Nygren, Kristian
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Impact Coatings AB, Westmansgatan 29, 582 16 Linköping, Sweden.
    Andersson, Anna Mikaela
    Eklund, Per
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Passive films on nanocomposite carbide coatings for electrical contact applications2017In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 52, no 13, p. 8231-8246Article in journal (Refereed)
    Abstract [en]

    Nanocomposite transition metal carbide/amorphous carbon coatings (Me-C/a-C) deposited by magnetron sputtering have excellent electrical contact properties. The contact resistance can be as low as that of noble metal coatings, although it is known to vary by several orders of magnitude depending on the deposition conditions. We have investigated a nanocrystalline niobium carbide/amorphous carbon (NbC (x) /a-C:H) model system aiming to clarify factors affecting the contact resistance for this group of contact materials. For the first time, the surface chemistry is systematically studied, by angle-resolved X-ray photoelectron spectroscopy, and in extension how it can explain the contact resistance. The coatings presented a mean oxide thickness of about 1 nm, which could be grown to 8 nm by annealing. Remarkably, the contact resistances covered four orders of magnitude and were found to be exponentially dependent on the mean oxide thickness. Moreover, there is an optimum in the amount of a-C:H phase where the contact resistance drops very significantly and it is thus important to not only consider the mean oxide thickness. To explain the results, a model relying on surface chemistry and contact mechanics is presented. The lowest contact resistance of a nanocomposite matched that of a gold coating at 1 N load (vs. gold), and such performance has previously not been demonstrated for similar nanocomposite materials, highlighting their useful properties for electrical contact applications.

  • 321.
    Nygren, Kristian
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Andersson, Matilda
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Högström, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Fredriksson, Wendy
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Edström, Kristina
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Nyholm, Leif
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Influence of deposition temperature and amorphous carbon on microstructure and oxidation resistance of magnetron sputtered nanocomposite Cr-C films2014In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 305, p. 143-153Article in journal (Refereed)
    Abstract [en]

    It is known that mechanical and tribological properties of transition metal carbide films can be tailored by adding an amorphous carbon (a-C) phase, thus making them nanocomposites. This paper addresses deposition, microstructure, and for the first time oxidation resistance of magnetron sputtered nanocomposite Cr C/a-C films with emphasis on studies of both phases. By varying the deposition temperature between 20 and 700 C and alternating the film composition, it was possible to deposit amorphous, nanocomposite, and crystalline Cr C films containing about 70% C and 30% Cr, or 40% C and 60% Cr. The films deposited at temperatures below 300 degrees C were X-ray amorphous and 500 C was required to grow crystalline phases. Chronoamperometric polarization at +0.6 V vs. Ag/AgCl(sat. KG) in hot 1 mM H-2 SO4 resulted in oxidation of Cr C, yielding Cr203 and C, as well as oxidation of C. The oxidation resistance is shown to depend on the deposition temperature and the presence of the a-C phase. Physical characterization of film surfaces show that very thin C/Cr2O3/Cr C layers develop on the present material, which can be used to improve the oxidation resistance of, e.g. stainless steel electrodes. (C) 2014 Elsevier B.V. All rights reserved.

  • 322.
    Nygren, Kristian
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Andersson, Matilda
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Högström, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Fredriksson, Wendy
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Corrosion resistance of amorphous, nanocomposite, and crystalline Cr-C films deposited by magnetron sputteringManuscript (preprint) (Other academic)
  • 323.
    Nygren, Kristian
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.