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Synthesis and Characterization of Multifunctional Carbide- and Boride-based Thin Films
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2014 (English)Doctoral 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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 79 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1127
Keyword [en]
thin film, coating, magnetron sputtering, nanocomposite, boride, carbide, electrical properties, mechanical properties
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-219040ISBN: 978-91-554-8886-4 (print)OAI: oai:DiVA.org:uu-219040DiVA: diva2:698085
Public defence
2014-04-04, Polhemssalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-03-13 Created: 2014-02-20 Last updated: 2014-04-29
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, 354-359 p.Article 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.

Keyword
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
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2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 253, 100-108 p.Article 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.

Keyword
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
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2013 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 545, 272-278 p.Article 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. 

Keyword
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
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2013 (English)In: MRS Communications, ISSN 2159-6859, Vol. 3, no 3, 151-155 p.Article 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, 295-300 p.Article 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.

Keyword
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
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2014 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 32, no 4, 041503- p.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.

Keyword
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
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2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 266, 167-176 p.Article 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.

Keyword
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
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2012 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 258, no 24, 9907-9912 p.Article 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.%.

Keyword
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

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