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Strategy for simultaneously increasing both hardness and toughness in ZrB2-rich Zr1-xTaxBy thin films
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
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2019 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 37, no 3, article id 031506Article in journal (Refereed) Published
Abstract [en]

Refractory transition-metal diborides exhibit inherent hardness. However, this is not always sufficient to prevent failure in applications involving high mechanical and thermal stress, since hardness is typically accompanied by brittleness leading to crack formation and propagation. Toughness, the combination of hardness and ductility, is required to avoid brittle fracture. Here, the authors demonstrate a strategy for simultaneously enhancing both hardness and ductility of ZrB2-rich thin films grown in pure Ar on Al2O3(0001) and Si(001) substrates at 475 degrees C. ZrB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a ZrB2 target, while Zr1-xTaxBy alloy films are grown, thus varying the B/metal ratio as a function of x, by adding pulsed high-power impulse magnetron sputtering (HiPIMS) from a Ta target to deposit Zr1-xTaxBy alloy films using hybrid Ta-HiPIMS/ZrB2-DCMS sputtering with a substrate bias synchronized to the metal-rich portion of each HiPIMS pulse. The average power P-Ta (and pulse frequency) applied to the HiPIMS Ta target is varied from 0 to 1800W (0 to 300 Hz) in increments of 600W (100 Hz). The resulting boron-to-metal ratio, y = B/(Zr+Ta), in as-deposited Zr1-xTaxBy films decreases from 2.4 to 1.5 as P-Ta is increased from 0 to 1800W, while x increases from 0 to 0.3. A combination of x-ray diffraction (XRD), glancing-angle XRD, transmission electron microscopy (TEM), analytical Z-contrast scanning TEM, electron energy-loss spectroscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and atom-probe tomography reveals that all films have the hexagonal AlB2 crystal structure with a columnar nanostructure, in which the column boundaries of layers with 0 amp;lt;= x amp;lt; 0.2 are B-rich, whereas those with x amp;gt;= 0.2 are Ta-rich. The nanostructural transition, combined with changes in average column widths, results in an similar to 20% increase in hardness, from 35 to 42 GPa, with a simultaneous increase of similar to 30% in nanoindentation toughness, from 4.0 to 5.2MPa root m. Published by the AVS.

Place, publisher, year, edition, pages
A V S AMER INST PHYSICS , 2019. Vol. 37, no 3, article id 031506
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-159001DOI: 10.1116/1.5093170ISI: 000472182400035OAI: oai:DiVA.org:liu-159001DiVA, id: diva2:1338109
Note

Funding Agencies|Swedish Research Council VR [2014-5790, 2018-03957, 642-2013-8020]; Knut and Alice Wallenbergs foundation [KAW 2015.0043]; VINNOVA [2018-04290]; Aforsk Foundation [16-359]; Carl Tryggers Stiftelse [CTS 15: 219, CTS 17: 166, CTS 14: 431]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]

Available from: 2019-07-19 Created: 2019-07-19 Last updated: 2021-12-28
In thesis
1. Atom Probe Tomography of Hard Nitride and Boride Thin Films
Open this publication in new window or tab >>Atom Probe Tomography of Hard Nitride and Boride Thin Films
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hard ceramic thin films, including TiSiN, ZrAlN, ZrB2, and ZrTaB2, with applications for wear-resistant coatings, have been studied using atom probe tomography and correlated with several other analytical techniques, including X-ray diffraction, electron microscopy, and elastic recoil detection analysis. Outstanding obstacles for quantitative atom probe tomography of ceramic thin films have been surmounted.

Mass spectral overlaps in TiSiN, which make 28Si indistinguishable from 14N, was resolved by isotopic substitution with 15N, and the nanostructural distribution of elements was thus revealed in 3-D, which enabled the identification of additional structural elements within the nanostructured Ti0.81Si0.1915N film. Improvements to the growth model of TiSiN by cathodic arc deposition was suggested.

A self-organized nanolabyrinthine structure of ZrAlN, consisting of standing lamellae of fcc-ZrN and hexagonal AlN, was investigated with focus on the onset and limits of the self-organization. The local crystallographic orientational relationships were (001)ZrN || (0001)AlN and <110>ZrN || <2-1-10>AlN. Close to the MgO substrates, a smooth transition region was formed, going from segregated and disordered to the self-organized nanolabyrinthine structure. With increased growth temperature, coarse (111)-oriented ZrN grains occasionally precipitated and locally replaced the nanolabyrinthine structure. Significant local magnification effects rendered the Zr and N signals unusable, thereby inhibiting quantitative compositional analysis of the constituent phases, but the nanostructure was resolved using the Al signal.

Ceramic materials are often affected by correlated evaporation, which can result in losses due to the detector dead-time/space. A compositional correction procedure was suggested, tested against an established procedure, and applied to ZrB2. The correction was found to be less dependent on the isotope abundances and background correction compared to the established procedure. While losses due to dead-time/space occur in atom probe tomography of all materials, the correlative field evaporation behavior of ceramics significantly increases the compositional error. The evaporation behavior of ZrB2 was therefore thoroughly investigated and evidence of preferential retention, correlated evaporation, and inhomogeneous field distributions at a low-index pole was presented. The high mass resolution, relatively low multiple events percentage, and quality of the co-evaporation correlation data was partly attributed to the crystal structure and film orientation, which promoted a layer-by-layer field evaporation.

The evaporation behavior of the related ZrTaB2 films was found to be similar to that of ZrB2. The distribution of Ta in relation to Zr was investigated, showing that the column boundaries were both metal- and Ta-rich, and that there was a significant amount of Ta in solid solution within the columns.

In addition, an instrumental artefact previously not described in atom probe tomography was found in several of the materials investigated in this thesis. The artefact consists of high-density lines along the analysis direction, which cannot be related to pole artefacts. The detection system of the atom probe was identified as the cause, because the artefact patterns on detector histograms coincided with the structure of the microchannel plate. Inconsistencies in the internal boundaries of the microchannel plate multifibers from the manufacturing process can influence the signal to the detector and locally increase the detection efficiency in a pattern characteristic to the microchannel plate in question.

Altogether, this thesis shows that atom probe tomography of nitride and boride thin films is burdened by several artefacts and distortions, but that relevant material outcomes can nevertheless be achieved by informed choices of film isotopic constituents and analytical parameters, exclusion of heavily distorted regions (such as pole artefacts), and the use of compositional correction procedures when applicable.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. p. 79
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1995
Keywords
Materials science, Atom probe tomography (APT), Ceramics, Nitrides, Borides
National Category
Physical Sciences Materials Engineering Ceramics
Identifiers
urn:nbn:se:liu:diva-159187 (URN)10.3384/diss.diva-159187 (DOI)9789176850435 (ISBN)
Public defence
2019-09-13, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2019-08-02 Created: 2019-08-01 Last updated: 2019-09-20Bibliographically approved
2. Multifunctional transition metal diboride thin films grown by magnetron sputtering with metal-ion irradiation
Open this publication in new window or tab >>Multifunctional transition metal diboride thin films grown by magnetron sputtering with metal-ion irradiation
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 27
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1874
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-164911 (URN)10.3384/lic.diva-164911 (DOI)9789179298777 (ISBN)
Presentation
2020-05-07, Schrödinger, F-Building, Campus Valla, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2014-5790, 2018-03957, 642-2013-8020,Vinnova, 2018-04290ÅForsk (Ångpanneföreningen's Foundation for Research and Development), #16-359Carl Tryggers foundation , CTS 15:219, CTS 17:166, CTS 14:431
Note

Ytterligare forskningsfinansiärer: Knut and Alice WallenbergFoundation for a Fellowship Grant and Project funding (KAW 2015.0043), Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO MatLiUNo. 2009 00971), Swedish Research Council VR-RFI (#2017-00646_9), Swedish Foundation for Strategic Research(contract RIF14-0053)

Available from: 2020-04-01 Created: 2020-04-01 Last updated: 2020-09-01Bibliographically approved
3. Multifunctional Transition-metal Diboride Coatings Synthesized by Magnetron sputtering with Synchronized Metal-ion Irradiation
Open this publication in new window or tab >>Multifunctional Transition-metal Diboride Coatings Synthesized by Magnetron sputtering with Synchronized Metal-ion Irradiation
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Refractory transition-metal diborides (TMB2), classified as ultra-high temperature ceramics, are promising materials for extreme thermal and chemical environments. There is a growing demand for employing TMB2 in high-temperature electrodes, advanced nuclear fission reactors, molten metal containment, refractory crucibles, thermocouple protection tubes in steel baths and aluminum reduction cells, reinforcement fibers, solar power, aerospace, and armor applications. Magnetron-sputter-deposited TMB2 have recently received increasing attention as the next class of hard ceramic protective thin films. These layers usually crystallize in a hexagonal AlB2 crystal structure (P6/mmm, SG-191) in which B atoms form graphite-like honeycomb sheets between hexagonal-close-packed TM layers. The strong covalent bonding between TM and B atoms as well as within the honeycomb B sheets provides high melting temperature, hardness, and stiffness, while metallic bonding within TM layers results in good electrical and thermal conductivities. However, sputter-deposited TMB2 films suffer from several critical issues such as boron overstoichiometry, high brittleness, and low oxidation resistance. All of these aspects are addressed in the thesis.

In Paper 1, the common issue with sputter-deposited diboride thin films, i.e. the presence of excess B, is resolved by using high power impulse magnetron sputtering (HiPIMS). The B/Ti ratio in TiBx films, used as a model materials system, is controllably varied from 2.08 to 1.83 by adjusting the HiPIMS pulse length ton, while maintaining the average power and pulse frequency constant. As a result, the peak current density increases from 0.27 to 0.88 A/cm2, which leads to an increased gas rarefaction and, hence, higher metal-ion densities in the plasma. Film growth becomes then increasingly controlled by ionized target atoms, rather than neutral species. Since sputter-ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the B/Ti ratio in the films decreases a function of target peak current.

While TM diborides are inherently hard, that alone is not sufficient to prevent failure in applications involving high stresses, as hardness is typically accompanied by brittleness. In order to avoid brittle cracking, thin films must be both hard and relatively ductile, which is referred to as high toughness. In Paper 2, it is demonstrated that Zr1-xTaxBy thin films grown by hybrid high-power impulse and DC magnetron co-sputtering (Ta-HiPIMS/ZrB2-DCMS) with x ≥ 0.2 are not only hard, but also tough. The films with x ≥ 0.2 show a self-organized columnar core/shell nanostructure (see Paper 3), in which crystalline hexagonal Zr-rich stoichiometric Zr1-xTaxB2 cores are surrounded by narrow dense, disordered Ta-rich shells that are B-deficient.

The disordered shells have the structural characteristics of metallic-glass thin films, which exhibit both high strength and toughness. Hence, such a nanostructure combines the benefits of crystalline diboride nanocolumns, providing the high hardness, with the dense metallic-glasslike shells, which give rise to enhanced toughness.

The mechanical properties of Zr1-xTaxBy thin films annealed in Ar atmosphere are studied as a function of annealing temperature Ta up to 1200 °C in Paper 4. In-situ and ex-situ nanoindentation analyses reveal that all films undergo age hardening up to Ta = 800 °C, with the highest hardness achieved for Zr0.8Ta0.2B1.8 (45.5±1.0 GPa). The age hardening, which occurs without any phase separation or decomposition, can be explained by point-defect recovery that enhances chemical bond density. Although hardness decreases at Ta > 800 °C due mainly to recrystallization, column coarsening, and planar defect annihilation, all layers show hardness values above 34 GPa over the entire Ta range.

The oxidation resistance of TiBx thin films is addressed in Paper 5. In general, TMB2 suffer from rapid high-temperature oxidation, which is a critical issue for many applications. In this study, it is demonstrated that alloying the films with Al significantly increases the oxidation resistance with only a slight decrease in hardness. Contrary to bulk TiB2 synthesized by powder metallurgy processes, the oxidation products of TiB2 thin films do not contain the B2O3 oxide scale, which is usually observed below 1000 °C in air, and merely consists of a TiO2 phase. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale, which considerably decreases the oxygen diffusion rate by suppressing the oxidecrystallites coarsening.

To realize the goal of fully multifunctional diborides, Zr1-xCrxBy thin films grown by hybrid Cr-HiPIMS/ZrB2-DCMS co-sputtering are studied in Paper 6. These layers exhibit a unique combination of high hardness, toughness, wear, oxidation, and corrosion resistance.

The last paper (Paper 7) addresses the issue of efficient energy and resource consumption in industrial processes, which United Nations defines as one of the sustainable development goals. The idea here is to replace the conventionally used thermal-energy flux from resistive heaters with the irradiation by high mass metal ions (Hf+), which results in more efficient energy transfer to the deposited layer. We deposited Ti0.67Hf0.33B1.7 films using hybrid HfB2-HiPIMS/TiB2-DCMS co-sputtering at substrate temperature not exceeding 100 °C. Results reveal that dense layers can be achieved with high hardness values (> 40 GPa) even though no external substrate heating was used during the process.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 31
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2100
National Category
Ceramics Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-171848 (URN)9789179297749 (ISBN)
Public defence
2021-01-12, Röntgen, F-Building, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2014-5790; 2018-03957; 642-2013-8020Knut and Alice Wallenberg Foundation, KAW 2015.0043Vinnova, 2018-04290ÅForsk (Ångpanneföreningen's Foundation for Research and Development), #16-359Carl Tryggers foundation , CTS 15:219; CTS 17:166; CTS 14:431Swedish Research Council, #2017-00646_9
Note

Additional funding agencies: Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO MatLiU No. 2009 00971); Swedish Foundation for Strategic Research (contract RIF14-0053)

Available from: 2020-12-09 Created: 2020-12-09 Last updated: 2021-01-15Bibliographically approved

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