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Superhard NbB2-x thin films deposited by dc magnetron sputtering
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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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.

Place, publisher, year, edition, pages
2014. Vol. 257, 295-300 p.
Keyword [en]
boride, structure characterization, mechanical properties, friction, tribological properties
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-219031DOI: 10.1016/j.surfcoat.2014.07.087ISI: 000344423100029OAI: oai:DiVA.org:uu-219031DiVA: diva2:698059
Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Synthesis and Characterization of Multifunctional Carbide- and Boride-based Thin Films
Open this publication in new window or tab >>Synthesis and Characterization of Multifunctional Carbide- and Boride-based Thin Films
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
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:nbn:se:uu:diva-219040 (URN)978-91-554-8886-4 (ISBN)
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

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