TY - THES

T1 - Development of high entropy alloy thin films for hydrogen permeation barriers

AU - Kirchmair, Magdalena

N1 - no embargo

PY - 2024

Y1 - 2024

N2 - The central role of hydrogen in the ongoing global energy transition underlines the urgency of developing robust material concepts for the hydrogen economy and infrastructure to ensure a safe and efficient handling of hydrogen. Due to its remarkable strength and high toughness, steel is the most commonly used material in all areas of hydrogen technology, including generation, storage, transportation, and utilization. High alloyed steels such as austenitic stainless steels are favored for this kind of application due to their beneficial properties, but their high costs stimulate the search for alternative options. Low alloyed steels provide a more economical option regarding costs, but they suffer from increased susceptibility to hydrogen embrittlement (HE), a phenomenon that leads to degradation of the mechanical properties due to hydrogen diffusion. Hydrogen permeation barriers (HPBs) have emerged as promising solutions to reduce the risks of HE. Several materials, including metals, nitrides and oxides have been suggested as suitable HPBs. High entropy alloys (HEAs), representing a rather new material class, show also promising properties for the use as HPBs. HEAs, are gaining increased attention in the field of engineering and materials science, since as a result of their multi-principal element design concept they exhibit exceptional mechanical, physical and chemical properties. They benefit from their so-called HEA core effects, where for the application as HPB the sluggish diffusion effect is of particular importance. This thesis delves into the scope of HEA thin films, with special emphasis on MoNbTaW-based HEAs supplemented by a fifth element, Ti or Zr, termed as refractory high entropy alloys (RHEAs). The RHEA thin film HPBs are synthesized by two different physical vapor deposition (PVD) methods, direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) with the aim to reduce HE in steel substrates. In addition, a TiN film grown by DCMS, representing an already well-established coating system, serves as reference material. The experimental results highlight the differences in terms of deposition rates, elemental composition, residual stresses, and microstructural features depending on the deposition technique, parameters and employed film system. The HPB efficiency of the films is evaluated by using a recently introduced in-situ electrochemical nanoindentation method, applying a side charging setup. The findings from the in-situ nanoindentation measurements indicate that the MoNbTaWZr thin film deposited via HiPIMS could be a promising prospect for HPB applications, since during hydrogen charging no hardness increase within the steel substrate can be observed. However, some RHEA films exhibit rather poor adhesion and thus show only limited HPB performance. This study provides the fundamental basis for future investigations to assess the full potential of HEA thin films for HPBs applications.

AB - The central role of hydrogen in the ongoing global energy transition underlines the urgency of developing robust material concepts for the hydrogen economy and infrastructure to ensure a safe and efficient handling of hydrogen. Due to its remarkable strength and high toughness, steel is the most commonly used material in all areas of hydrogen technology, including generation, storage, transportation, and utilization. High alloyed steels such as austenitic stainless steels are favored for this kind of application due to their beneficial properties, but their high costs stimulate the search for alternative options. Low alloyed steels provide a more economical option regarding costs, but they suffer from increased susceptibility to hydrogen embrittlement (HE), a phenomenon that leads to degradation of the mechanical properties due to hydrogen diffusion. Hydrogen permeation barriers (HPBs) have emerged as promising solutions to reduce the risks of HE. Several materials, including metals, nitrides and oxides have been suggested as suitable HPBs. High entropy alloys (HEAs), representing a rather new material class, show also promising properties for the use as HPBs. HEAs, are gaining increased attention in the field of engineering and materials science, since as a result of their multi-principal element design concept they exhibit exceptional mechanical, physical and chemical properties. They benefit from their so-called HEA core effects, where for the application as HPB the sluggish diffusion effect is of particular importance. This thesis delves into the scope of HEA thin films, with special emphasis on MoNbTaW-based HEAs supplemented by a fifth element, Ti or Zr, termed as refractory high entropy alloys (RHEAs). The RHEA thin film HPBs are synthesized by two different physical vapor deposition (PVD) methods, direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) with the aim to reduce HE in steel substrates. In addition, a TiN film grown by DCMS, representing an already well-established coating system, serves as reference material. The experimental results highlight the differences in terms of deposition rates, elemental composition, residual stresses, and microstructural features depending on the deposition technique, parameters and employed film system. The HPB efficiency of the films is evaluated by using a recently introduced in-situ electrochemical nanoindentation method, applying a side charging setup. The findings from the in-situ nanoindentation measurements indicate that the MoNbTaWZr thin film deposited via HiPIMS could be a promising prospect for HPB applications, since during hydrogen charging no hardness increase within the steel substrate can be observed. However, some RHEA films exhibit rather poor adhesion and thus show only limited HPB performance. This study provides the fundamental basis for future investigations to assess the full potential of HEA thin films for HPBs applications.

KW - high entropy alloys

KW - refractory high entropy alloys

KW - hydrogen permeation barriers

KW - physical vapor deposition

KW - direct current magnetron sputtering

KW - high-power impulse magnetron sputtering

KW - in-situ electrochemical nanoindentation

KW - side charging setup

KW - MoNbTaTiW

KW - MoNbTaWZr

KW - TiN

KW - thin film

KW - Hochentropielegierung

KW - Refraktär-Hochentropielegierungen

KW - Wasserstoffpermeationsbarrieren

KW - physikalischen Gasphasenabscheidung

KW - Gleichstrom-Magnetronsputtern

KW - Hochleistungsimpuls-Magnetronsputtern

KW - in-situ elektrochemischen Nanoindentationsmethode

KW - side charging setup

KW - MoNbTaTiW

KW - MoNbTaWZr

KW - TiN

KW - Dünnschicht

M3 - Master's Thesis

ER -