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Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

The design and development of new multifunctional materials that exhibit a combination of high hardness and ductility, as well as a high corrosion resistance and thermal stability, is one of the key challenges in the field of material science. The focus of this thesis is on the development of novel multifunctional magnetron sputtered CrNbTaTiW–C based thin films. Carbon was selected as an alloying element to investigate if it could modify the microstructure (via grain refinement) and improve the properties (e.g. the hardness and ductility).

TaW-rich and near-equimolar high entropy alloys in the CrNbTaTiW system were selected as starting points for this study. The latter alloys were predicted, based on empirical design rules, to form a single-phase solid solution. In contrast, thermodynamic calculations showed that the films at equilibrium should be composed of a mixture of several phases at temperatures below 1100 °C.  Experimentally, however, a single-phase bcc structure was observed for the deposited films and it was concluded that the films were kinetically and not entropy stabilised. A hypothesis is that the kinetics during sputtering allow a ’direct’ phase selection by tuning the process parameters and evidence of this was found in the HfNbTiVZr alloy system.

The CrNbTaTiW–C system is, however, complex and additional studies were carried out on the W–C and TaW–C systems. All metallic films crystallised in a bcc structure with a <110> texture and the column width of these films varied between 25 nm and 80 nm. The films were very hard (~ 13 GPa), which was explained by the small grain size. A single-phase bcc structure was also obtained upon the addition of 5-10 at.% carbon for all compositions except the near-equimolar CrNbTaTiW. X-ray diffraction indicated a unit cell expansion, which was attributed to the formation of a supersaturated solid solution. Additional atom probe tomography (APT) studies on selected samples confirmed the formation of such solid solutions. The supersaturated solid solution is not thermodynamically stable and an annealing study showed that heat treatment yielded segregation and clustering of carbon at the grain boundaries. The addition of carbon had a grain refining effect in the W–C system and the multicomponent CrNbTaTiW–C system. In general, the addition of carbon increased the hardness, which was mainly caused by a reduced grain size in line with the Hall-Petch relationship. Excellent mechanical properties of carbon supersaturated films were further confirmed in pillar tests on W–C films, which showed very high yield strength (~ 9 GPa) and no brittle fracture. The results show that carbon can be used as a chemical approach to control the grain size and properties of these films. 

Multicomponent carbides with a B1 structure were formed at high carbon concentrations (~ 40 at.%). The microstructure of these films depended strongly on the process parameters and a higher deposition temperature was found to increase the film density and hardness. The TaW-rich carbide exhibited a very high hardness of ~ 35 GPa and excellent corrosion resistance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. , p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1909
Keywords [en]
thin films magnetron sputtering, refractory metals, high entropy alloys, mechanical properties, transition metal carbides
National Category
Engineering and Technology
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-405323ISBN: 978-91-513-0884-5 (print)OAI: oai:DiVA.org:uu-405323DiVA, id: diva2:1402403
Public defence
2020-04-17, Ångströmslaboratoriet Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-03-26 Created: 2020-02-28 Last updated: 2020-03-26
List of papers
1. Magnetron Sputtering of Carbon Supersaturated W Films - A Chemical Approach to Increase Strength and Ductility
Open this publication in new window or tab >>Magnetron Sputtering of Carbon Supersaturated W Films - A Chemical Approach to Increase Strength and Ductility
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-404772 (URN)
Available from: 2020-02-26 Created: 2020-02-26 Last updated: 2020-02-28
2. Influence of Carbon on Microstructure and Mechanical Properties of Magnetron Sputtered TaW Coatings
Open this publication in new window or tab >>Influence of Carbon on Microstructure and Mechanical Properties of Magnetron Sputtered TaW Coatings
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-404744 (URN)
Available from: 2020-02-26 Created: 2020-02-26 Last updated: 2020-02-28
3. Hard and crack resistant carbon supersaturated refractory nanostructured multicomponent coatings
Open this publication in new window or tab >>Hard and crack resistant carbon supersaturated refractory nanostructured multicomponent coatings
Show others...
2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 14508Article in journal (Refereed) Published
Abstract [en]

The combination of ceramic hardness with high crack resistance is a major challenge in the design of protective thin films. High entropy alloys have shown in earlier studies promising mechanical properties with a potential use as thin film materials. In this study, we show that small amounts of carbon in magnetron-sputtered multicomponent CrNbTaTiW films can lead to a significant increase in hardness. The film properties were strongly dependent on the metal composition and the most promising results were observed for TaW-rich films. They crystallised in a bcc structure with a strong (110) texture and coherent grain boundaries. It was possible to deposit films with 8 at.% C in a supersaturated solid-solution into the bcc structure without carbide formation. A major effect of carbon was a significant grain refinement, reducing the column diameter from approximately 35 to 10 nm. This resulted in an increase in hardness from 14.7 to 19.1 GPa while the reduced E-modulus stayed constant at 322 GPa. The carbon-containing films exhibited extremely little plastic deformation around the indent and no cracks were observed. These results show that supersaturation of carbon into high entropy films can be a promising concept to combine superior hardness with high crack resistance.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-368107 (URN)10.1038/s41598-018-32932-y (DOI)000445894500011 ()30266967 (PubMedID)
Funder
Swedish Research Council, 621-2012-4359Swedish Research Council, 622-2008-405Swedish Foundation for Strategic Research , RMA11-0029
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2020-02-28Bibliographically approved
4. Elemental Distribution in CrNbTaTiW-C High Entropy Alloy Thin Films
Open this publication in new window or tab >>Elemental Distribution in CrNbTaTiW-C High Entropy Alloy Thin Films
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2019 (English)In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 25, no 2, p. 489-500Article in journal (Refereed) Published
Abstract [en]

The microstructure and distribution of the elements have been studied in thin films of a near-equimolar CrNbTaTiW high entropy alloy (HEA) and films with 8 at.% carbon added to the alloy. The films were deposited by magnetron sputtering at 300 degrees C. X-ray diffraction shows that the near-equimolar metallic film crystallizes in a single-phase body centered cubic (bcc) structure with a strong (110) texture. However, more detailed analyses with transmission electron microscopy (TEM) and atom probe tomography (APT) show a strong segregation of Ti to the grain boundaries forming a very thin Ti-Cr rich interfacial layer. The effect can be explained by the large negative formation enthalpy of Ti-Cr compounds and shows that CrNbTaTiW is not a true HEA at lower temperatures. The addition of 8 at.% carbon leads to the formation of an amorphous structure, which can be explained by the limited solubility of carbon in bcc alloys. TEM energy-dispersive X-ray spectroscopy indicated that all metallic elements are randomly distributed in the film. The APT investigation, however, revealed that carbide-like clusters are present in the amorphous film.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
atom probe tomography, carbon clustering, high entropy alloy, segregation, thin film
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-385570 (URN)10.1017/S1431927618016264 (DOI)000466756600025 ()30712522 (PubMedID)
Conference
Atom Probe Tomography and Microscopy (APT and M) Conference, JUN 10-15, 2018, Gaithersburg, MD
Funder
Swedish Research Council, 621-2012-4359Swedish Research Council, 622-2008-405Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research , RMA11-0029
Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2020-02-28Bibliographically approved
5. Synthesis and characterization of multicomponent (CrNbTaTiW)C films for increased hardness and corrosion resistance
Open this publication in new window or tab >>Synthesis and characterization of multicomponent (CrNbTaTiW)C films for increased hardness and corrosion resistance
Show others...
2018 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 149, p. 51-62Article in journal (Refereed) Published
Abstract [en]

Multicomponent carbide thin films of (CrNbTaTiW)C (30–40 at.% C) with different metal contents were depos-ited at different temperatures using non-reactive DC magnetron sputtering. The lattice distortion for the metallattice was estimated to vary from about 3 to 5%. Most films crystallized in the cubic B1 structure but Ta/W-rich films deposited at 600 °C exhibited a tetra gonal distortion. X-ray diffraction results sh ow that near-equimolar films exhibited a strong (111) texture. In contrast, Ta/W-rich films exhibited a shift from (111) to(100) texture at 450 °C. The in-plane relationship was determined to MC(111)[-12-1]//Al2O3(001)[110] with alattice mismatch of about 11% along the Al2O3[110] direction. A segregation of Cr to the grain boundaries was ob-served in all films. The microstructure was found to be the most important factor for high hardness. Less denseNb-rich and near-equimolar films deposited at low tem peratures exhib ited the low est hardnes s (12 GPa),while very dense Ta/W-rich high temperature films were found to be the hardest (36 GPa). No correlation wasfound between the lattice distortion and the hardness. Corrosion studies revealed that the multicomponentfilms exhibited excellent corrosion resistance, superior to that of a reference hyper-duplex stainless steel, in1.0 M HCl.

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry; Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-335985 (URN)10.1016/j.matdes.2018.03.068 (DOI)000431007500006 ()
Funder
Swedish Research Council, 621-2012-4359Swedish Research Council, 622-2008-405Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research , RMA11-0029
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2020-02-28Bibliographically approved
6. Influence of Deposition Temperature on the Phase Evolution of HfNbTiVZr High-Entropy Thin Films
Open this publication in new window or tab >>Influence of Deposition Temperature on the Phase Evolution of HfNbTiVZr High-Entropy Thin Films
Show others...
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 4, article id 587Article in journal (Refereed) Published
Abstract [en]

In this study, we show that the phase formation of HfNbTiVZr high-entropy thin films is strongly influenced by the substrate temperature. Films deposited at room temperature exhibit an amorphous microstructure and are 6.5 GPa hard. With increasing substrate temperature (room temperature to 275 degrees C), a transition from an amorphous to a single-phased body-centred cubic (bcc) solid solution occurs, resulting in a hardness increase to 7.9 GPa. A higher deposition temperature (450 degrees C) leads to the formation of C14 or C15 Laves phase precipitates in the bcc matrix and a further enhancement of mechanical properties with a peak hardness value of 9.2 GPa. These results also show that thin films follow different phase formation pathways compared to HfNbTiVZr bulk alloys.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
high-entropy alloys, physical vapour deposition (PVD), metallic glass
National Category
Metallurgy and Metallic Materials Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-380494 (URN)10.3390/ma12040587 (DOI)000460793300037 ()30781407 (PubMedID)
Funder
Swedish Research Council, 2018-04834
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2020-02-28Bibliographically approved

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