| 1. |
- Gangaprasad Rao, Smita, 1992-
(författare)
-
Phase formation in multicomponent films based on 3d transition metals
- 2021
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The need for materials that enhance life span, performance, and sustainability has propelled research in alloy design from binary alloys to more complex systems such as multicomponent alloys. The CoCrFeMnNi alloy, more commonly known as the Cantor alloy, is one of the most studied systems in bulk as well as thin film. The addition of light elements such as boron, carbon, nitrogen, and oxygen is a means to alter the properties of these materials. The challenge lies in understanding the process of phase formation and microstructure evolution on addition of these light elements. To address this challenge, I investigate multicomponent alloys based on a simplified version of the Cantor alloy.My thesis investigates the addition of nitrogen into a Cantor variant system as a step towards understanding the full Cantor alloy. Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤0.28 thin films were grown by reactive magnetron sputtering. The films showed a change in structure from fcc to mixed fcc+bcc and finally a bcc-dominant film with increasing nitrogen content. The change in phase and microstructure influenced the mechanical and electrical properties of the films. A maximum hardness of 11 ± 0.7 GPa and lowest electrical resistivity of 28 ± 5 μΩcm were recorded in the film with mixed phase (fcc+bcc) crystal structure.Copper was added as a fourth metallic alloying element into the film with the mixed fcc + bcc structure, resulting in stabilization of the bcc phase even though Cu has been reported to be a fcc stabilizer. The energy brought to the substrate increases on Cu addition which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness recorded by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.05. While it is generally believed that large amounts of Cu can be detrimental to thin film properties due to segregation, this study shows that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as for mechanical properties.This thesis thus provides insights into the phase formation of nitrogen-containing multicomponent alloys.
|
|
| 2. |
- Shu, Rui, 1990-
(författare)
-
Refractory High-entropy Alloy and Nitride Thin Films
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on understanding the process-structure-property relation-ships for several refractory-metal-based high-entropy alloys and nitrides synthesized by magnetron sputtering. The thesis begins with the growth of TiZrNbTaNx understoichiometric nitrides by controlling substrate temperature and nitrogen flow ratio fN. Their effects on microstructure and mechanical, electric, and electrochemical properties were investigated. TiZrNbTaN0.46 deposited at 400 ºC shows a NaCl-type structure with (001) preferred orientation and exhibits the highest corrosion resistance in 0.1 M H2SO4 aqueous solution. A stable passive region up to 3.0 V vs. Ag/AgCl could be achieved when x< 0.64. The densification effects were explored by ion energy for (TiZrTaMe)N1–x (Me = Hf, Nb, Mo, or Cr) films and by high-power impulse magnetron sputtering technique for TiNbCrAl films.The local chemical distortions in the TiZrTaNb-based system with different nitrogen content were investigated by X-ray absorption fine structure spectros-copy. The influence of crystallinity on superconducting transition behavior was studied in (TiZrHf)x(TaNb)1-x, (TiZrNbTa)1-xWx, and (TiZrNbTa)1-xVx systems. The highest superconducting transition temperature (Tc) reaches 8.05 K for the TiZrNbTa film (x=0). The superconducting transition temperature Tc of these films deposited at the fixed temperature decreases monotonically as a function of x, and Tc can be increased by elevating the deposition temperature. Furthermore, the structural stability and elemental segregation under Xe-ion irradiation of TiZrNbTaV-based HEA and HEN films, and high-entropy TiZrN-bTa/CrFeCoNi metallic and nitride multilayer coatings were investigated. The microstructure of TiZrNbTaVN film remain stable after irradiation at room temperature and 500 °C. The as-deposited TiZrNbTaV film exhibited an amorphous structure and became a bcc phase structure after irradiation at 500 °C. Thermal-induced and irradiation-induced grain growth resulted in a grain-size distribution. For the multilayer coatings, the microstructure of metallic multilayers was not stable and the interdiffusion or mixing of the constituent elements is prominent under ion irradiation and/or heat treatment. no diffusion and phase trans-formation were observed for the nitride multilayers after irradiation at 500 °C.
|
|
| 3. |
- Gangaprasad Rao, Smita, 1992-, et al.
(författare)
-
Phase formation and structural evolution of multicomponent (CrFeCo)Ny films
- 2021
-
Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 412
-
Tidskriftsartikel (refereegranskat)abstract
- The Cantor alloy (CoCrFeMnNi) and its variants, in bulk as well as thin films, have been extensively studied. They are known to exhibit cubic crystal structures and thermodynamic stability regardless of their complex chemical composition. Therefore, they may find use as hard, wear-resistant, corrosion and oxidation-resistant coatings. The addition of light elements, such as nitrogen, is known to help improve these properties further through processes such as amorphization and nitride compound formation. Here, we investigate the ternary CrFeCo system to study the effects of nitrogen addition. (CrFeCo)Ny multicomponent thin films are grown on silicon substrates by DC magnetron sputtering. Changes in crystal structure, morphology, mechanical and electrical properties with gradual increases of nitrogen in the film are described and discussed. Increased addition of nitrogen from 14 at.% to 28 at.% in the film leads to a transformation from an fcc to a bcc crystal structure, affects both the mechanical and electrical properties. XPS analysis shows the tendency of nitrogen to bond with Cr over other metals. The films display hardness values between 7 and 11 GPa with resistivities values ranging between 28 and 165 μΩ cm.
|
|
| 4. |
- Gharavi, Mohammad Amin, 1985-, et al.
(författare)
-
High thermoelectric power factor of pure and vanadium-alloyed chromium nitride thin films
- 2021
-
Ingår i: Materials Today Communications. - : ELSEVIER. - 2352-4928. ; 28
-
Tidskriftsartikel (refereegranskat)abstract
- Chromium-nitride based materials have shown unexpected promise as thermo-electric materials for, e.g., wasteheat harvesting. Here, CrN and (Cr,V)N thin films were deposited by reactive magnetron sputtering. Thermoelectric measurements of pure CrN thin films show a low electrical resistivity between 1.2 and 1.5 x 10(-3) Omega cm and very high values of the Seebeck coefficient and thermoelectric power factor, in the range between 370-430 mu V/K and 9-11 x 10(-3) W/mK(2), respectively. Alloying of CrN films with small amounts (less than 15 %) of vanadium results in cubic (Cr,V)N thin films. Vanadium decreases the electrical resistivity and yields powerfactor values in the same range as pure CrN. Density functional theory calculations of sub-stoichiometric CrN1-delta and (Cr,V)N1-delta show that nitrogen vacancies and vanadium substitution both cause n-type conductivity and features in the band structure typically correlated with a high Seebeck coefficient. The results suggest that slight variations in nitrogen and vanadium content affect the power factor and offers a means of tailoring the power factor and thermoelectric figure of merit.
|
|
| 5. |
- Shu, Rui, 1990-, et al.
(författare)
-
Effect of nitrogen content on microstructure and corrosion resistance of sputter-deposited multicomponent (TiNbZrTa)Nx films
- 2020
-
Ingår i: Surface & Coatings Technology. - LAUSANNE, SWITZERLAND : Elsevier BV. - 0257-8972 .- 1879-3347. ; 404
-
Tidskriftsartikel (refereegranskat)abstract
- Multicomponent (TiNbZrTa)Nx films were deposited on Si(100) substrates at room temperature using magnetron sputtering with a nitrogen flow ratio fN [fN = N2/(Ar + N2)], which was varied from 0 to 30.8%. The nitrogen content in the films varied between 0 and 45.2 at.%, i.e., x = 0 to 0.83. The microstructure was characterized by X-ray diffraction and electron microscopy. The metallic TiNbZrTa film comprised a dominant bcc solid-solution phase, whereas a single NaCl-type face-centred cubic structure was observed in all nitrogen-containing films (TiNbZrTa)Nx. The mechanical, electrical, and electrochemical properties of these films varied with nitrogen content. The maximum hardness was achieved at 22.1 ± 0.3 GPa when N = 43.0 at.%. The resistivities increased from 95 to 424 μΩcm with increasing nitrogen content. A detailed study of the variation of morphology and chemical bonding with nitrogen content was performed and the corrosion resistance of the TiNbZrTa nitride films was explored in 0.1 M H2SO4. While all the films had excellent corrosion resistances at potentials up to 2.0 V vs. Ag/AgCl, the metallic film and the films with low nitrogen contents (x < 0.60) exhibited an almost stable current plateau up to 4.0 V vs. Ag/AgCl. For the films with higher nitrogen contents (x ≥ 0.68), the current plateau was retained up to 2.0 V vs. Ag/AgCl, above which a higher nitrogen content resulted in a higher current. The decrease in the corrosion resistance at these high potentials indicate the presence of a potential-dependent activation effect resulting in an increased oxidation rate of the nitrides (present under the passive oxide film) yielding a release of nitrogen from the films. TEM results indicate that the oxide layer formed after this corrosion measurement was thick and porous for the film with x = 0.76, in very good agreement with the increased corrosion rate for this film. The results demonstrate that an increased nitrogen content in (TiNbZrTa)Nx system improves their mechanical properties with retained high corrosion resistance at potentials up to 2.0 V vs. Ag/AgCl in 0.1 M H2SO4. At even higher potentials, however, the corrosion resistance decreases with increasing nitrogen concentration for films with sufficiently high nitrogen contents (i.e. x ≥ 0.68).
|
|
| 6. |
- Shu, Rui, 1990-, et al.
(författare)
-
Effects of alloying and deposition temperature on phase formation and superconducting properties of TiZrTaNb-based high entropy-alloy films
- 2022
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 120:15
-
Tidskriftsartikel (refereegranskat)abstract
- A variety of bulk high-entropy alloy superconductors have been recently discovered; however, for thin films, only the TaNbHfZrTi highentropy alloy system has been investigated for its superconducting properties. Here, (TiZrNbTa)1-xWx and (TiZrNbTa)1-xVx superconducting films have been produced by DC magnetron sputtering at different growth temperatures. The phase formation and superconducting behavior of these films depend on the content of alloying x and deposition temperature. A single body-centered cubic (bcc) phase can be formed in the low x range with enough driving energy for crystallinity, but phase transition between amorphous or two bcc structures is observed when increasing x. The highest superconducting transition temperature Tc reaches 8.0 K for the TiZrNbTa film. The superconducting transition temperature Tc of these films deposited at the same temperature decreases monotonically as a function of x. Increasing deposition temperature to 400 °C can enhance Tc for these films while retaining nearly equivalent compositions. Our experimental observations suggest that Tc of superconducting high entropy alloys relate to the atomic radii difference and electronegativity difference of involved elements beyond the valence electron number.
|
|
| 7. |
- Shu, Rui, 1990-, et al.
(författare)
-
Influence of Metal Substitution and Ion Energy on Microstructure Evolution of High-Entropy Nitride (TiZrTaMe)N1-x (Me = Hf, Nb, Mo, or Cr) Films
- 2021
-
Ingår i: ACS APPLIED ELECTRONIC MATERIALS. - : American Chemical Society (ACS). - 2637-6113. ; 3:6, s. 2748-2756
-
Tidskriftsartikel (refereegranskat)abstract
- Multicomponent or high-entropy ceramics show unique combinations of mechanical, electrical, and chemical properties of importance in coating applications. However, generalizing controllable thin-film processes for these complex materials remains a challenge. Here, understoichiometric (TiZrTaMe)N1-x (Me = Hf, Nb, Mo, or Cr, 0.12 <= x <= 0.30) films were deposited on Si(100) substrates at 400 degrees C by reactive magnetron sputtering using single elemental targets. The influence of ion energy during film growth was investigated by varying the negative substrate bias voltage from similar to 10 V (floating potential) to 130 V. The nitrogen content for the samples determined by elastic recoil detection analysis varied from 34.9 to 43.8 at. % (0.12 <= x <= 0.30), and the metal components were near-equimolar and not affected by the bias voltage. On increasing the substrate bias, the phase structures of (TiZrTaMe)N1-x (Me = Hf, Nb, or Mo) films evolved from a polycrystalline fcc phase to a (002) preferred orientation along with a change in surface morphology from faceted triangular features to a dense and smooth structure with nodular mounds. All the four series of (TiZrTaMe)N1-x (Me = Hf, Nb, Mo, or Cr) films exhibited increasing intrinsic stress with increasing negative bias. The maximum compressive stress reached similar to 3.1 GPa in Hf- and Cr-containing films deposited at -130 V. The hardness reached a maximum value of 28.0 +/- 1.0 GPa at a negative bias >= 100 V for all the four series of films. The effect of bias on the mechanical properties of (TiNbZrMe)N1-x films can thus guide the design of protective high-entropy nitride films.
|
|
