| 1. |
- Bakhit, Babak, 1983-
(författare)
-
Multifunctional Transition-metal Diboride Coatings Synthesized by Magnetron sputtering with Synchronized Metal-ion Irradiation
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 2. |
- Folkenant, Matilda
(författare)
-
Synthesis and Characterization of Amorphous Carbide-based Thin Films
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, research on synthesis, structure and characterization of amorphous carbide-based thin films is presented. Crystalline and nanocomposite carbide films can exhibit properties such as high electrical conductivity, high hardness and low friction and wear. These properties are in many cases structure-related, and thus, within this thesis a special focus is put on how the amorphous structure influences the material properties.Thin films within the Zr-Si-C and Cr-C-based systems have been synthesized by magnetron sputtering from elemental targets. For the Zr-Si-C system, completely amorphous films were obtained for silicon contents of 20 at.% or higher. Modeling of these films, as well as experimental results suggest that the films exhibit a network-type structure where the bond types influence the material properties. Higher hardness and resistivity were observed with high amounts of covalent Si-C bonds.Several studies were performed in the Cr-C-based systems. Cr-C films deposited in a wide composition range and with substrate temperatures of up to 500 °C were found to be amorphous nanocomposites, consisting of amorphous chromium carbide (a-CrCx) and amorphous carbon (a-C) phases. The carbon content in the carbidic phase was determined to about 30-35 at.% for most films. The properties of the Cr-C films were very dependent of the amount of a-C phase, and both hardness and electrical resistivity decreased with increasing a-C contents. However, electrochemical analysis showed that Cr-C films deposited at higher substrate temperature and with high carbon content exhibited very high oxidation resistance. In addition, nanocomposite films containing Ag nanoparticles within an amorphous Cr-C matrix were studied in an attempt to improve the tribological properties. No such improvements were observed but the films exhibited a better contact resistance than the corresponding binary Cr-C films. Furthermore, electrochemical analyses showed that Ag nanoparticles on the surface affected the formation of a stable passive film, which would make the Cr-C/Ag films less resilient to oxidation than the pure Cr-C films.
|
|
| 3. |
- Sangiovanni, Davide G., 1979-
(författare)
-
Transition Metal Nitrides : Alloy Design and Surface Transport Properties using Ab-initio and Classical Computational Methods
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Enhanced toughness in brittle ceramic materials, such as transition metal nitrides (TMN), is achieved by optimizing the occupancy of shear-sensitive metallic electronic-states. This is the major result of my theoretical research, aimed to solve an inherent long-standing problem for hard ceramic protective coatings: brittleness. High hardness, in combination with high toughness, is thus one of the most desired mechanical/physical properties in modern coatings. A significant part of this PhD Thesis is dedicated to the density functional theory (DFT) calculations carried out to understand the electronic origins of ductility, and to predict novel TMN alloys with optimal hardness/toughness ratios. Importantly, one of the TMN alloys identified in my theoretical work has subsequently been synthesized in the laboratory and exhibits the predicted properties.The second part of this Thesis concerns molecular dynamics (MD) simulations of Ti, N, and TiNx adspecies diffusion on TiN surfaces, chosen as a model material, to provide unprecedented detail of critical atomic-scale transport processes, which dictate the growth modes of TMN thin films. Even the most advanced experimental techniques cannot provide sufficient information on the kinetics and dynamics of picosecond atomistic processes, which affect thin films nucleation and growth. Information on these phenomena would allow experimentalists to better understand the role of deposition conditions and fine tune thin films growth modes, to tailor coatings properties to the requirements of different applications. The MD simulations discussed in the second part of this PhD Thesis, predict that Ti adatoms and TiN2 admolecules are the most mobile species on TiN(001) terraces. Moreover, these adspecies are rapidly incorporated at island descending steps, and primarily contribute to layer-by-layer growth. In contrast, TiN3 tetramers are found to be essentially stationary on both TiN(001) terraces and islands, and thus constitute the critical nuclei for three-dimensional growth.
|
|
| 4. |
- Torres, Hector
(författare)
-
Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS2 and WS2. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS2 to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS2 to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.
|
|
| 5. |
- Flink, Axel, 1979-
(författare)
-
Growth and Characterization of Ti-Si-N Thin Films
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Utvecklingen inom materialforskningen går mot att framställa avancerade material vilka är skräddarsydda för olika tillämpningar. Detta har medfört att det blir allt mer populärt att belägga ytor med ett eller flera tunna lager med syfte att förbättra materialegenskaperna. Användningsområden för ytbeläggningar går att hitta inom allt från vardagliga produkter såsom teflonbeläggningar av stekpannor, förgyllning av smycken till avancerad halvledarteknik för att åstadkomma energieffektiva lysdioder. Det enskilt största tillämpningsområdet för tunna filmer är dock som skyddande skikt för verktyg inom skärande bearbetning. Utvecklingen går stadigt mot högre skärhastigheter och därmed ökade temperaturer, idagsläget kan området där verktyget och arbetsmaterialet är i kontakt nå temperaturer på mellan 800-1000 °C utan att förlora nämnvärt i styrka. Detta har gjorts möjligt genom att belägga skären med någon eller några μm (tusendels mm) av lämpligt keramiskt material i avseende att öka motståndskraften för nötning vid bearbetning vid höga temperaturer.I den här avhandlingen har tunna filmer studerats med det övergripande målet att förbättra egenskaper hos verktyg för skärande metallbearbetning genom att öka motståndskraften hos materialen mot mekanisk och kemisk nötning vid höga temperaturer. Materialsystemet som undersökts är Ti-Si-N, där tunna filmer av både legeringar och tvåfassystem har syntetiserats och egenskapskarakteriserats. Legeringarna är belagda med varierande Si-halt från 0 till 10 atomprocent och avsedda för att studera strukturella, termiska och mekaniska egenskaper. De framställdes med en teknik som kallas arcförångning, där man i ett vakuumsystem frigör högenergetiskt material i det här fallet av Ti och Si som förångas från en solid yta kallad target. Atomerna joniseras genom kollisioner med elektroner och reagerar på sin väg mot substratet med kvävgas. Väl framme vid substratet, kondenserar jonerna och bilder den tunna filmen. Filmerna består av två strukturtyper, den första är en fast lösning där Si atomer upp till 5 at.% ersätter Ti atomer i TiN. I det andra fallet så segregerar Si till korngränserna. Värmebehandlingsexperiment visar att Si bildar SiNx som kapslar in TiN-korn vid temperaturer upp till 1000 °C. Hårdhetstester visar att filmerna bibehåller sin hårdhet upp till 1000 °C tack vare fasomvandlingen. Även vid 1100 °C är hårdheten hög. Dessa skikt besitter alltså egenskaper som gör dem väldigt användbara inom tillämpningar för skärande bearbetning.Nanostrukturerade materials egenskaper beror på dess mikrostruktur snarare än på de grundämnen som ingår, detta exemplifieras av TiN-SiNx-nanokompositer bestående av nanokristallina TiN-korn inbäddade i några få atomlager SiNx, där materialegenskaperna helt och hållet beror på kornstorleken på TiN-kornen och tjockleken på SiNx-lagren. Ökas tjockleken på SiNx minskar hårdheten. Dessa filmer har mycket goda mekaniska egenskaper och är ett av de hårdaste materialen som finns. Nyckeln till den höga hårdheten hos skikten ligger i att bilda starka bindningar mellan TiN och SiNx. Hur dessa ser ut vet man dock inte eftersom strukturen på SiNx gränsytan inte är känd. Anledningen är att den är svår att avbilda på grund av dess krökta form och begränsade volym. I denna avhandling har TiN/SiNx multilager belagts, dvs. en lagrad struktur TiN alternerad med SiNx. Dessa filmer framställdes med sputtring, en teknik som liknar arcförångning men där man istället accelerera positivt laddade joner mot Ti och Si targets med en hög negativ potential som frigör Ti och Si. I multilagren varierades SiNx-lagrets tjocklek mellan endast några få atomlager för att göra en förenklad modell av gränsytan hos nanokompositen och med atomupplöst transmissionselektronmikroskopi samt hårdhetsmätningar konstateras sedan att de hårdaste filmerna var de där kristallin SiNx stabiliseras mellan TiNkorn. Vidare studerar jag SiNx/TiN ytor med sveptunnelmikroskopi och täthetsfunktionalteori (en kvantmekanisk simuleringsmetod). Mina resultat visar SiNx och bindningarna till TiN är mycket mer komplicerade än vad man tidigare trott, då de kan vara kristallina och anta komplexa rekonstruktioner. Detta bidrar till den starka bindningen mellan TiN och SiNx vilket i sin tur förklarar varför materialen blir så hårda.
|
|
| 6. |
- Schnitter, Claudia, 1989-
(författare)
-
Epitaxial thin films of group 4 transition metal diborides
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group 4 transition metal diboride films were deposited epitaxially onto different substrates, typically at 900 °C, using direct-current magnetron sputtering from compound targets of ZrB2, TiB2, and HfB2. Epitaxial ZrB2 has been deposited on Al2O3(0001), 4H-SiC(0001), and Si(100) describing the epitaxial relationships on Si(100) for the first time, where two relationships were observed from pole-figure analysis, namely A) in-plane: ZrB2[001] ∥ Si[110] and ZrB2[110] ∥ Si[110], out-of-plane: ZrB2(100) ∥ Si(100), and B) in-plane: ZrB2[11] ∥ Si[110] and the same rotated 90° around the 102 axis, out-of- plane: ZrB2(102) ∥ Si(100). Composition analysis by time-of-flight elastic recoil detection analysis revealed typically B-rich ZrB2 and TiB2, and stoichiometric to Hf-rich HfB2. For ZrB2, the application of an additional external magnetic field during growth influenced the B-to-Zr ratio towards being stoichiometric. Rocking curve measurements of ZrB2 deposited onto Si(100) reveal a higher crystal order in the 100-oriented domains, compared to the 102-oriented domains. In ZrB2 films annealed to temperatures in the range of 1100-1500 °C, rocking curve measurements of the symmetric 001 reflection as well as the asymmetric 101 reflection reveal increased order with increased temperature. This phenomenon occurred at lower temperature when the annealing was performed in H2 compared to Ar.The morphology in plan-view transmission electron microscopy reflects the composition of the film: TiB2.5 has B-rich areas around the grain boundaries, forming an almost continuous network around the grains. ZrBx films with x between 2.0 and 2.3 also contain B-rich regions, though to a smaller extent and mostly in areas where more than two grains adjoin each other. In Hf-rich HfB1.8, no B-rich areas were observed. Scanning transmission electron micrographs and a combination of B electron energy loss spectroscopy and energy dispersive X-ray analysis Hf distribution maps revealed Hf-rich areas in a close-to single crystalline matrix. The hardness of epitaxial HfB2 films is reported to 33 and 36 GPa for films deposited onto Al2O3 and SiC, respectively. These values are slightly higher than reported for bulk HfB2. ZrB2 films on SiC decrease in hardness to 38, 37, and 30 GPa, upon annealing in Ar up to 1100, 1300, and 1500 °C, respectively. In summary, the knowledge is expanded about epitaxially grown group 4 transition metal diborides in terms of their Chemical composition, crystallographic orientations, microstructure, electrical and mechanical properties, as well as their response to heat treatment.
|
|
