Phase formation in
multicomponent
films based on 3d
transition metals
Linköping studies in Science and TechnologyLicentiate Thesis No. 1904
Smita G. Rao
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FACULTY OF SCIENCE AND ENGINEERING
Linköping studies in science and technology, Licentiate Thesis No. 1904, 2021 Department of Physics, Chemistry and Biology (IFM)
Linköping University
SE-581 83 Linköping, Sweden
www.liu.se
Linköping Dissertation on 22/04/2021 Thesis No. 1904 Linköping Studies in Science and Technology
Phase formation in multicomponent
films based on 3d transition metals
Smita G. Rao
Department of Physics, Chemistry and Biology (IFM) Linköping University, Sweden
©Smita G. Rao, 2021
Published article is used under Creative Commons BY license 4.0
Printed in Sweden by LiU-Tryck, Linköping, Sweden, 2021
ISBN: 978-91-7929-668-1 ISSN: 0282-7971
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
i
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.
iii
Populärvetenskaplig Sammanfattning
En legering bildas när två eller flera metaller kombineras tillsammans. Stål är till exempel en legering av exempelvis 90% järn, 3% kol och 7% andra metaller som krom, molybden och nickel. Den allmänna regeln för legering har varit att ha en metall i mycket högre koncentrationer jämfört med de andra. Denna uppfattning ändrades 2004 då så kallde högentropilegeringar upptäcktes. En definition för sådana legeringar är” En legering tillverkad med fem eller flera grundämnen i ungefär lika koncentrationer”. Den första högentropilegeringen har kallats “Cantor-legeringen” efter Brian Cantor, den person som upptäckte den. Denna legering gjordes av krom (Cr), mangan (Mn), järn (Fe), kobolt (Co) och nickel (Ni) i samma koncentration (dvs 20% vardera). Medan bulklegeringen har visat många intressanta egenskaper undersöker forskare att utveckla beläggningar eller tunna filmer av dessa material.
En tunn film är ett lager av material med en tjocklek mellan 1 nm och några mikrometer (människohår är 60 μm tjockt). Nästan alla enheter, verktyg och maskiner vi använder i våra dagliga liv kräver skyddande eller dekorativa tunna filmer. Exempelvis skulle skärverktyg som används inom industrin kräva en film som skyddar verktyget från slitage och ökar dess livslängd. En bränslecell kan behöva tunna filmer på dess delar för att skydda dem från korrosion. Kantorlegeringsbaserade tunna filmer är intressanta kandidater i detta avseende. Tillsatsen av lätta grundämnen som bor, kol, kväve eller syre tillsammans med metallerna i Cantor-systemet är ett sätt att ändra egenskaperna hos dessa material. Utmaningen ligger dock i att utveckla en stabil kväveinnehållande tunn film och förstå vad tillsatsen av kväve ger systemet när det gäller mekaniska, elektriska och kemiska egenskaper. Detta är lättare när man arbetar med tre metaller (Cr, Fe och Co) i motsats till de fem (Cr, Mn, Fe, Co och Ni). Mitt arbete har främst fokuserat på att studera dessa förenklade Cantor-system. Jag har utvecklat tunna filmer av (CrFeCo)-baserade kväveinnehållande filmer och tittat på filmens struktur, mekaniska och elektriska egenskaper när mängden kväve ökas. Jag har dessutom tillsatt koppar (Cu) som ett ytterligare legeringsämne och studerat stabiliseringen av tunnfilmskristallstrukturen när mer Cu tillsätts.
iv
Preface
This licentiate thesis is a part of my Ph.D. research in the Energy Materials unit in the Thin Film Physics Division at the Department of Physics, Chemistry and Biology (IFM), at Linköping University.
This work is supported by the VINNOVA Competence Centre FunMat-II (grant no. 2016-05156), the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), and the Centre in Nanoscience and technology at LiTH, CeNano”). I have been enrolled in the Graduate School Agora Materiae.
Smita G. Rao Linköping, 2021
v
List of included papers
Paper I
Phase formation and structural evolution of multicomponent (CrFeCo)Ny films
Authors: Smita G. Rao, Rui Shu, Robert Boyd, Grzegorz Greczynski, Arnaud le Febvrier and Per Eklund
Accepted in Surface & Coatings Technology (2021) 127059
Author’s contribution: I was responsible for planning and conceptualizing the experiments, carried out the depositions and most of the characterization and analysis, and wrote the manuscript.
Paper II
The effects of copper on non-equiatomic (CrFeCo)1-yNy multicomponent thin films
Authors: Smita G. Rao, Rui Shu, Robert Boyd, Arnaud le Febvrier and Per Eklund
In manuscript
Author’s contribution: I was responsible for planning and conceptualizing the experiments, carried out the depositions and most of the characterization and analysis, and wrote the manuscript.
vii
Acknowledgement
Words cannot say how grateful I am for the help and support of the following people without whom this work could not have been possible. I thank:
Per Eklund, for the opportunity to be a part of the Energy Materials group. I am grateful to
have a supervisor who lets me put forth my ideas and allows me to find my way, for motivating me when I need it the most, and for teaching and exposing us to new developments within and outside the boundaries of our field of research during our meetings.
Arnaud le Febvrier, for always finding time to clarify my doubts, answer queries and
brainstorm. For patiently teaching us about Jessie, XRD and much more. For being the best co-supervisor, one could ask for.
Björn Wallner, my mentor for his input during the study plan meetings.
My office mates and friends, Rui Shu and Faezeh Alijan Farzad Lahiji, I am the luckiest person to work with the two of you. For all the fun times in office and interesting discussions. For all your help, especially when changing targets and tightening screws.
The TEM sample prep grievance group, my friends Qurat Ul-Ain, Janella Salamania, Maiara
Moreno, Tun Wei Hsu, Zhixing Wu, and Clara Linder. For the long lunches and fikas,
support and encouragement.
My group mates Satish Kumar Shanmugham, Eric Ekström, and Binbin Xin for your feedback during presentations and discussions.
Members of FunMat-II, for provoking my thought process with your questions. Robert Boyd for patiently helping me with TEM analysis.
All members of the thin film division. Particularly Grzegorz Greczynski and Babak Bakhit, for helping me with XPS and ERDA. Thomas Lingefelt, Harri Savimäki, and Per Sandström for technical support in the laboratory. Jens Birch, Fredrik Eriksson, Naureen Ghafoor,
Ahmed EL Ghazaly, Johan Neyman, Samiran Bairagi and Megan Dorri for all the
constructive comments during and after meetings.
Members of Agora Materiae, Caroline Brommesson in particular for organizing the monthly seminars and always trying to improve them.
My parents, my sister, Sanjana and boyfriend, Kartik for being there for me and encouraging me regardless of the 8000 km distance.
Contents
1. Introduction ... 1
2. High-entropy alloys ... 3
2.1Four core effects ... 3
2.1.1 High entropy effect ... 3
2.1.2Sluggish diffusion ... 4
2.1.3Lattice distortion ... 4
2.1.4Cocktail effect ... 5
2.2Materials system ... 5
2.2.2Nitride Material ... 6
2.3Batteries, fuel cell and bipolar plates ... 7
3 Methodology ... 9
3.1Thin-film deposition technique ... 9
3.2Sputtering ... 9
4 Characterization techniques ... 13
4.1X-ray diffraction (XRD) ... 13
4.2Electron microscopy techniques ... 14
4.2.2Scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS). ... 15
4.2.3Transmission electron microscopy (TEM) ... 15
4.3X-ray photoelectron spectroscopy (XPS) ... 16
4.4Ion beam analysis-Elastic recoil detection analysis (ERDA) ... 16
4.5Four-point probe measurement ... 18
4.6Nanoindentation ... 18
5 Summary of appended papers ... 21
6 Future work... 23
6.1Theoretical and experimental understanding of phase formation in multicomponent thin films based on 3d transition metals. ... 23
6.2Nitrogen-containing multicomponent Cr-Fe-Ni-Co-Mo thin films ... 24
6.3Multicomponent thin film growth and plasma characterization by solenoid coil assisted magnetron sputtering. ... 24
1
Chapter 1
1.
Introduction
Alloying of metals may have been discovered around 2500 BCE. It started with the production of bronze and other copper-containing alloys much before steel making. As technology progressed and more elements were discovered, alloying became more complex form binary systems to ternary and quaternary systems. Traditional alloys contain one principal element, and one or several alloying elements in small quantities. This definition or method of alloying was reconsidered in the 1980’s when the concept of multi-principal-element alloys, i.e, alloys with several main elements in approximately equal amounts, was conceived. In 2004 two separate groups, Cantor et al. and Yeh et
al. experimentally proved that alloys with multi-principal elements have unique properties and
surprisingly simple crystal structures [1,2]. Since then, research on multi-principal alloys has extended from bulk to thin films [3–6].
Cantor’s work presented the CoCrFeMnNi alloys which formed a fcc single-phase solution, termed the Cantor alloy[2]. Yeh’s work, on the other hand, introduced the broader and more popular but highly debated term “high-entropy alloy”[7]. This term was introduced based on the fact that the higher the number of elements in the alloy, higher the mixing entropy which would overcome enthalpy of intermetallic compound formation resulting in stable single phase solid solutions. Other than the high entropy of mixing, multiple element mixtures also resulted in sluggish diffusion, lattice distortion and the cocktail effect. Together they are more commonly called as the four core effects [8–10].
Thin film deposition is one of the most common methods of surface engineering for improved material properties. Sputtering is a common method for thin film deposition in works concerning multicomponent systems. This technique allows for good control over process parameters along with the possibility to upscale to industries. Sputtering also allows for the deposition of compound films (i.e., borides, carbides, nitrides, and oxides) [11]. While there are a few studies concerning the nitrides of the Cantor alloy, not many studies have been reported on simpler systems, sometimes referred to as medium-entropy alloys [12,13]. The role of nitrogen in phase formation remains to be ambiguous. This thesis introduces the world of multicomponent nitride thin films. The objective of the thesis is to address the question of how nitrogen and copper can affect the phase formation and stabilization in a simplified version of the Cantor system. The second chapter introduces the concept of high entropy alloys in more detail along with complexity involved in characterizing these materials. The
2
third chapter gives a detailed explanation of the sputter deposition processes. Chapter four provides a brief explanation of all the characterization techniques used in this thesis. Finally, chapter four discusses the various characterization techniques involved. My work has been carried out as part of the FunMat-II Competence Centre, funded by the Swedish Agency for Innovation Systems (VINNOVA), which aims at developing functional materials for cutting-tool, fuel-cell, and battery applications.
3
Chapter 2
2.
High-entropy alloys
2.1 Four core effects
2.1.1 High entropy effect
From thermodynamics we know that any system will try to reach an equilibrium or stable state by minimizing its Gibbs free energy (G) which can be describes as follows,
1
𝐺 = 𝐻 − 𝑇
Where, H is the enthalpy of the system and S the entropy. In the case of multi-principal alloys, the equilibrium state is dependent on the mixing free energy (ΔGmix). Equation 1 then can be written as,
2
∆𝐺𝑚𝑖𝑥= ∆𝐻𝑚𝑖𝑥− 𝑇∆𝑆𝑚𝑖𝑥
Here we see that the mixing enthalpy (∆𝐻𝑚𝑖𝑥) and mixing entropy (∆𝑆𝑚𝑖𝑥) compete with each other.
Furthermore, as the temperature is increased the mixing entropy becomes the more dominant factor influencing solid solution formation. ∆𝑆𝑚𝑖𝑥 can be estimated from the equation,
3
∆𝑆𝑚𝑖𝑥= −𝑅 ∑ 𝑐𝑖ln 𝑐𝑖 𝑁
𝑖=1
Where R is the ideal gas constant, N is the number of elements and ci is the atomic concentration of
the element i.
While configurational entropy may play an important role in the stabilization of phases it does not guarantee the formation of simple solid solution. This has been demonstrated in many studies and also by Cantor et al., who showed that equimolar alloys with 16 elements did not form stable solid solutions [2,14,15][16]. Furthermore, practical applications of these materials do not always require solid solution phases [17,18]. I will therefore primarily use the term multicomponent alloy as opposed to high entropy alloy.
4
2.1.2 Sluggish diffusion
Diffusion of atoms is slower in multicomponent alloys in comparison to the binary alloys as an effect of the differences in local atomic configuration. Tsai et al. were the first to study the diffusion kinetics in Co-Cr-Fe-Mn-Ni bulk alloys and stated that variation in the lattice potential energies (LPE) as a result of the different bonding mechanism between the atoms could change the diffusion kinetics. Low LPE sites act as traps and hinder atomic diffusion [19].
2.1.3 Lattice distortion
Zhang et al. proposed that distortions in the lattice due the atomic radii difference (δ) influence solid solution formation [20]. The atomic radii difference (δ) is calculated using the equation,
4 𝛿 = 100√∑ 𝑐𝑖(1 − 𝑟𝑖 𝑟) 2 𝑛 𝑖=1
where, ci and ri are the atomic percentage and atomic radius of the element i, respectively and r the average atomic radius [20].
Figure 1. Illustration depicting lattice distortions in a multicomponent system. Larger atoms tend to distort the lattice.
A study carried out by the combinatorial sputtering of quinary high entropy alloys showed that introducing elements of larger atomic radii such as Al promoted the stabilization of the bcc structure. The reason for this was the lower packing factor of a bcc (68%) in comparison to a fcc structure (74%). The less dense structure allows larger atoms to occupy the lattice without causing distortions [21].
5
2.1.4 Cocktail effect
The term “cocktail effect” is not primarily a physical concept; the term was derived from Ranganathan’s work entitled “Alloyed pleasures: Multimetallic cocktails” [22] and describes the interactions between each element which gives rise to surprising properties. Most mechanical properties exhibited by high entropy alloys can accounted to be a result of the cocktail effect. Figure 2 is an illustration of three of the common strengthening mechanism that arise as a result of the cocktail effect.
Figure 2. Strengthening process that arise as a consequence of multielement cocktails. Image adapted from reference [23].
2.2 Materials system
The Cantor alloy, which is a solid solution of Fe-Cr-Mn-Co-Ni in equimolar composition has been a foundation for many studies on multicomponent alloys. Today’s research has expanded to refractory metal and noble metal alloys. The addition of light elements such as boron, carbon, nitrogen, and oxygen to enhance properties has become popular [3,24–26]. While research in bulk alloy system has been progressing so has the concept of multicomponent thin films. It is interesting to note that research towards developing thin films started almost immediately after first two initial studies on HEA bulk alloys. Chen et al., work on “Nanostructured nitride films of multi-element high-entropy alloys by reactive DC sputtering” was not only the first paper to discuss high entropy thin films but also the first to discuss nitride thin films [27]. Multicomponent thin films are now attractive candidates as functional materials.
These alloys, although complex in terms of material composition, tend to form simple solid solutions which are generally cubic or hexagonal. Figure 3 is a schematic representation of the lattice structures of three common structures, body centered cubic (bcc), face centered cubic (fcc), and a hexagonal
6
closed packed (hcp). Each structure has a different packing factor based on the number of atoms a unit cell can accommodate. The bcc has a packing factor of 68% while the fcc and hcp both have packing a factor of 74%. Metals such as chromium and iron crystallize in a bcc structure when in pure form at room temperature and pressure (Iron: ferrite (bcc) at room temperature; austenite (fcc) at higher temperature). Nickel and copper are fcc-structured metals while cobalt is an example of an hcp structured element. Most high-entropy alloys made from the above-mentioned metals tend to form cubic structures where the metal atoms occupy the lattice positions.
Figure 3. Schematic representation of the three basic structures that are commonly observed in high entropy materials. a) Face centered cubic (fcc), (b) Body centered cubic (bcc), (c) Hexagonal closed packed (hcp). Face centered and body centered atoms marked in red.
2.2.2 Nitride Material
What would happen when a non-metallic element such as nitrogen is introduced into these complex systems? We start with the binary transition metals. The ability of a transition metal to form bonds with nitrogen (i.e. the heat of formation) decreases along a period as the d-orbital fills up. Therefore, early transition metals such as titanium, niobium, and hafnium form stable nitride compounds in comparison to cobalt, nickel, and copper. In the case of a stochiometric binary nitride one can expect the nitrogen atoms to occupy the octahedral voids of the lattice (Figure 4). The resulting crystal structure is generally fcc or hcp [3,28]. This tendency to form stable stoichiometric nitrides with fcc or hcp structures diminishes down the period. Fcc CrN only occurs in a narrow range of stoichiometry, Fe-N compound generally form at high pressures and so on. We now consider the Cantor alloy; on addition of nitrogen, one would expect it to occupy the octahedral voids of the fcc structure. However, several factors such as the metals formation enthalpy for nitride and oxide formation, the composition and the magnetic properties may influence this [29][30].
7
Figure 4. Model of a high entropy material where metal atoms occupy the lattice points (solid spheres) of a cubic system assuming there is no lattice distortion or defects. Interstitial sites marked with “A” can be occupied by the smaller anions such as nitrogen.
Determining the role of nitrogen in phase formation, mechanical, electrical properties of the group 7 and 8 transition metals (Cr, Fe, Co) has been the first question of my current research. Paper I and II discuss the development of nitrogen containing Cr-Fe-Co and Cr-Fe-Co-Cu thin films grown by
magnetron sputtering. The Cr-Fe-Co system was chosen based on its simplicity in comparison to the full Cantor alloy, the similarity in atomic sizes of metallic elements, their sputter rates as well as popularity in the field of multicomponent thin film growth. The phase formation and structural evolution with gradual increase in nitrogen is studied.
The second study was carried out as an extension of the first. Here we add Cu to the mix of metals. We study the occurrence of segregation and change in structural, mechanical, and electrical properties.
2.3 Batteries, fuel cell and bipolar plates
Climate change has driven research in renewable energy sources, the technology that runs on these sources and the materials required for their production and storage. The hydrogen fuel cell is one such example. A typical fuel cell consists of a stack of polymer electrolyte membrane (PEM) cells. A single PEM cell consists of two electrodes, a catalyst, and a polymer membrane. Within the stack each cell is separated by a bipolar plate. Other that being a separating layer the bipolar plate has the additional responsibility of regulating gasses and coolant within the stack [31]. Naturally, these plates are subjected to harsh environments and are required to be chemically and mechanically stable. A typical bipolar plate must be able to withstand operating temperatures of approximately 200 °C, corrode at rates less than 0.016 mA/cm2 and, have an electrical resistance less than 0.01Ω/cm2 [32].
8
Hermann et al., in their review on bipolar plates for PEM fuel cells explain the various classification and requirements of bipolar plates [33].
Figure 5. Classification of materials used for bipolar plates.
Traditional bipolar plates are made of graphite but due to their high cost and inferior mechanical properties, metal and alloy plates are becoming attractive candidates. Figure 5 is gives an idea of the different materials that can be used for bipolar plate manufacturing. Surface engineered stainless steel plates are now popular alternatives due to their superior mechanical, chemical, and electrical properties as well as comparative low cost. Thin film deposition is one of the most common and versatile types of surface engineering processes. Nitride coatings such as TiN and CrN have been widely studied [31,34,35] but of late alloy coatings, in particular multicomponent coatings, are being increasingly investigated due to the combination of properties that can be obtained.
9
Chapter 3
3 Methodology
This chapter discusses methods of thin film synthesis, in particular magnetron sputtering.
3.1 Thin-film deposition technique
Surface engineering of materials by deposition of coatings and thin films are an integral part of today’s technology. There are many ways to deposit coatings. A simple way to categorize them is based on the form of deposition, from solution (e.g., electroplating), deposition from powder (spraying), and atomistic techniques (Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD))[36–39].
PVD is important in industries due to its ease of use, possibility to upscale, and flexibility in material selection. The basic process is based on vaporization of metal elements by ion bombardment under high vacuum conditions and condensation on the substrate to form a coating. Thermal evaporation, sputtering, and cathodic arc deposition are three of the most common PVD techniques. The following section will be focused on sputtering as it is the chosen method of deposition in this study.
3.2 Sputtering
A sputtering system consists of a vacuum chamber inside which targets are placed at one end and the substrate to be coated at the other. The target is made of the material which is meant to be deposited. For example, to grow an iron thin film one would require an iron target. Depending on the geometry of the sputtering system, targets can be of different shapes and dimensions. Laboratory-scale deposition systems generally make use of 2-inch or 3-inch circular target. An inert gas is used for the sputtering process. In most cases argon (Ar) is used as the sputtering gas. Ar gas is inserted into the chamber and ionized to a become a plasma. This is done by applying a voltage between the cathode (target) and anode (substrate). Electrons are accelerated towards the anode in the process colliding into the Ar atoms. This collision causes the dissociation of Ar into Ar+ ions and electrons.
e- + Ar → Ar+ + 2e-
The resulting electrons cause a cascade of collisions, also known as the Townsend avalanche which ignites the plasma. Similar to the electrons being accelerated towards the anode, the ions in the plasma are accelerated towards the cathode. The cathode, i.e., the target is bombarded by the ions which
10
results in the ejection, or sputtering, of target material atoms. The sputtered material moves through the plasma at different angles to reach the substrate. The base pressure of the deposition system determines the mean free path of the which in turn influences the energy at which the sputtered species arrive at the substrate. Figure 6 is a schematic of the deposition system used to grow thin films mentioned in Paper I and II.
Figure 6. Schematic of the magnetron sputtering system used to grow CrFeCo multicomponent nitrogen containing thin films.
Plasma, being an ionized gas, can be influenced by the presence of a magnetic field. Confining the plasma with the help of permanent magnets around the target helps in increasing the sputter rates at low pressures of Ar in the chamber. The conventional way to sputter with magnets is known as magnetron sputtering. Around the 1980s, unbalanced magnetron sputtering was introduced and continues a popular technique of sputtering [40] [41]. In this technique, the outer ring of magnets which form one pole are stronger in comparison to the inner one which forms the opposite pole. This configuration extends the magnetic field lines towards the substrate which means that the plasma is no longer confined near the target. Figure 7 is a schematic of a typical magnet configuration in balanced magnetron sputtering vs unbalanced magnetron sputtering. Typically, the magnets are arranged in the type II unbalanced configuration where the central magnet is weaker than the surrounding magnets. This allows a fraction of the electron to escape the magnetic field and ionize with gas atoms.
11
In systems with multiple magnetrons the adjacent magnetrons can either be of the same polarity or opposite. The former is called a mirrored configuration and the latter is called closed field. In the mirrored configuration, the field lines are directed towards the chamber walls resulting in low plasma density towards the substrate. In a closed-field configuration, adjacent magnetrons are coupled. This results in higher plasma density toward the substrate [42]. Since the magnetic field lines go from the cathode center to the edges there is always inhomogeneous erosion of the cathode material forming a race-track. The substrate can also be subjected to bias, a floating potential or grounded. Depending on the magnitude of the applied voltage the ions will transfer different amounts of energy to the ad-atoms.
The present work was carried out in the unbalanced magnetron configuration. The chamber base pressure was lower than 4×10-7 Pa (3×10-9 Torr) after baking. The substrate temperature was set at
300° C with the substrate at floating potential (i.e., not electrically connected). A more detailed description of the deposition system can be found in reference [43].
13
Chapter 4
4 Characterization techniques
This chapter discusses the characterization techniques used in studying the thin films described in this work. The morphology and crystal structure of the thin films were studied using electron microscopy and X-ray diffraction techniques, respectively. The mechanical properties were investigated using nanoindentation techniques. Elastic recoil detection analysis (ERDA), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used for composition quantification. Finally, the electrical properties were obtained via four-point-probe measurements.
4.1 X-ray diffraction (XRD)
X-ray diffraction is a versatile non-destructive technique that can be used to study the crystal structure of thin films. A crystal is an ordered structure where the atoms are arranged in arrays with a period. When an electromagnetic wave having wavelength (𝜆) interacts with these arrays, elastic scattering, known as Thomson scattering, takes place [44]. In this case, the wavelength of the electromagnetic wave is conserved obeying Bragg’s law 𝑛𝜆 = 2𝑑ℎ𝑘𝑙 sin𝜃. Since the arrays act as a grating, constructive
interference can be observed in the diffractogram. The diffractograms can be used to get information related to the phase and structure of the sample. X-rays are used as the electromagnetic source as the wavelengths are of the same order of magnitude as the atomic spacing (d) (~0.5 Å -10 Å). The beam divergence and resolution can be controlled by use of slits.
Figure 8. illustration showing the scattering of coherent waves on two atoms of the crystalline material which is described by Bragg’s law.
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The 𝜃-2𝜃 Bragg-Brentano setup is the most commonly used X-ray diffraction setup where the incident and diffracted beams are symmetric. This allows the user to probe the crystal planes parallel to the sample surface. This setup is ideal when studying polycrystalline materials which do not have high texture, or preferred orientation. Figure 8 is an illustration explaining the principle of Bragg’s law where n is a positive integer (Bragg order), dhkl is the spacing between planes with Miller indices
hkl and 𝜃 is the angle between the incident X-rays and sample.
In the present work, XRD was used for determining the crystal structure as well as to calculate residual stress (σf) in the films. The sin2Ψ method and wafer curvature methods are used for stress
analysis, In the case of multicomponent thin films, the sin2Ψ method is not easy to use as it requires
prior knowledge of the samples Young’s modulus and Poisson’s ratio. Stress measurements in Paper
II were carried out by the wafer curvature method [45]. The principle of this method is to measure
the curvature of the substrate due to the compressive and tensile stress in the film. This is done by measuring a certain symmetrical reflection (Si 004 in Paper II) while the angle (ω) between the incident beam and sample is changed. The Stoney equation is used to calculate the stresses based on the curvature of the substrate [46,47].
5
𝜎𝑓× 𝑡𝑓=𝑀𝑠𝑡𝑠
2
6𝑅
Here, tf is film thickness, Ms is the bulk modulus of the substrate, ts is the thickness of the substrate
and, R (bending radius) =1/k, where k is the curvature.
4.2 Electron microscopy techniques
A focused electron beam scanned on the surface of the material can be used to study the surface as well as cross section and planar morphology of samples. The basic working principle of any electron microscopy technique is to produce an electron beam by thermal or field emission. The electrons are accelerated by applying a voltage and focused into a beam using electromagnetic lenses. This beam is used to probe the surface. Depending on the technique, transmitted, backscattered, or secondary electrons from the surface are detected to form images. The most common electron microscopy techniques are scanning electron microscopy and transmission electron. In the following sections we go through these techniques one by one
15
4.2.2 Scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS).
Scanning electron microscopy (SEM) is a non-destructive imaging technique [48]. The sample to be analyzed is cleaned and loaded into a vacuum chamber. An accelerated electron beam rastered along the sample surface. The electrons that are reflected or scattered are used to creating an image of the surface. The energy of the e- beam is generally set between ~1-20 kV and depends on the material being analyzed.
When the electronbeam hits the sample surface several interactions can take place. The most common are secondary electron emission, backscattered emission, inelastic backscattered emission, and characteristic X-ray emission. Each interaction is detected using specific detectors. Secondary electrons are emitted as a result of interactions between the electron beam and the atoms in the sample causing valence electrons to be ejected. The valence/secondary electrons are detected using a secondary electron emission detector. The elastically scattered electrons of the incident electron beam are known as backscattered electrons. They are detected using the back-scattering emission detector. The secondary electron and backscattered electrons are used to in imaging surface and cross section morphology. The characteristic X-ray on the other hand, are used in obtaining chemical information of the sample. These X-rays can be detected if the SEM is equipped with an energy dispersive X-ray spectroscopy detector (EDS). Figure 9 is an illustration of the various interaction that take place when the electrons interact with the sample.
4.2.3 Transmission electron microscopy (TEM)
TEM is similar to SEM in that it is based on accelerating electrons in order to probe the properties of materials. Typical beam energies used for TEM analysis range from approximately 100 keV to 400 keV which results in short wavelengths. While SEM requires almost no sample preparation TEM samples are required to be electron transparent. This means that sample thicknesses must be < 100 nm (electron transparent) for the high energy electrons to interact causing some of the electrons to be transmitted and some scattered.
In this thesis, SEM, EDS and TEM techniques have been used for analyzing the growth morphology, composition and phase analysis.
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Figure 9. “Pear of interaction” when electrons interact with matter.
4.3 X-ray photoelectron spectroscopy (XPS)
XPS is a technique used in quantitative analysis of elemental concentrations, chemical and electronic states of the material. Its operation is based on the photoelectric effect. The sample is irradiated with x-rays causing the emission of photoelectrons [49]. The kinetic energy Ek is measured and the binding
energy (Eb) which is specific to each element can be obtained from Einstein’s law of photoelectric
effect (Ek= hν–EB –φsp) [50]. XPS is a very surface sensitive technique since the mean free path of
the photoelectrons emitted is small. This technique has been used in Paper I to investigate the
bonding mechanism of nitrogen in the multicomponent system. However, it has not been used for quantification of the composition due to the overlapping of the spectra between Cr, Fe and Co. The spectrometer was calibrated in order to avoid problem with the C 1s peak of adventitious carbon [51,52].
4.4 Ion beam analysis-Elastic recoil detection analysis (ERDA)
Ion beam analysis is a family of techniques which includes, Rutherford backscattering (RBS), secondary-ion mass spectrometry (SIMS), proton induced X-ray emission (PIXI) and elastic recoil detection analysis (ERDA). These techniques make use of high energy ions to probe the surface of materials in order to obtain information regarding the composition and structure. In the present study ERDA has been used to analysis the composition of the thin films.
17
ERDA is an ion beam analysis technique used mainly in the determination of thin film composition. The principle behind the technique is the detection of lighter recoil atoms from the sample when irradiated with a heavier high-energy beam (iodine or chlorine beam). This technique is much more reliable when estimating the concentration of light elements such as, boron, oxygen, or nitrogen in the sample in comparison to EDS. The measurements are usually carried out with a 40 MeV iodine beam. When the iodine beam hits the sample at a grazing angle, the ions interact with the atoms of the sample and lose energy in the process emitting recoil atoms. A time-of-flight detector allows for the detection of these atoms in the forward direction. The recoiled atoms on the way to the detector suffer energy losses proportional to the distance travelled. Atoms of different masses result in different velocities of the recoil atoms. The mass (m) can be determined by measuring the velocity (v) and energy (ε) of the recoil by the equation,
6
𝜀 =𝑚𝑣
2
2
Figure 10 is a typical TOF-ERDA histogram obtained from a 3d transition metal multicomponent thin film. The recoil time of flight (ToF) spectrum from each element in the films is plotted against the energy spectrum. Since the atomic masses of Cr, Fe and Co do not vary to a great extent, they overlap in a single feature. This restricts us from being able to distinguish between individual metallic components. The compositions reported in Paper I and II are therefore a combination of EDS and
ERDA measurements where the metal concentrations are obtained from EDS and nitrogen from ERDA. The data is analyzed using the Potku software [53].
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4.5 Four-point probe measurement
Sheet resistance (𝜌𝑠) values obtained from the four-point probe can be to be used to determine the
resistivity (𝜌) of a sample. The instrument consists of four probes in series. A current (I) is driven through the outer two probes while the inner two measure the voltage difference (V). The film thickness (t) as well the correction factors (C’ and F) for the sample must be known in order to convert the sheet resistance to resistivity. The resistivity is calculated using the equation,
7 𝜌 = 𝜌𝑠𝑡 = 𝑉 𝐼𝑡 𝑑 𝑠𝐶 ′∗ 𝐹 (𝑡 𝑠)
Where, d is the sample width, and s is the distance between the tips (1 mm in my case). The correction factor depends on the sample geometry. A more detailed explanation can be found in reference [54].
4.6 Nanoindentation
Understanding the mechanical properties of thin films requires a different set of testing methods in comparison to bulk materials. Here, one is required to uncouple the effects of the substrate on the film. This has been achieved by techniques such as nanoindentation where it is possible to probe the mechanical properties of the thin film alone.
During an indentation event, a diamond tip of known dimensions and mechanical properties is allowed to penetrate the sample surface and retract. A loading-unloading curve as shown in Figure 11c is obtained.
Figure 11. Schematic representation of the cross section during an indentation event (a) during loading and (b) unloading. (c) load-displacement curve obtained from Me1-yNy thin film studied in Paper I.
19
This curve gives us an idea of the elastic and plastic deformations that occur during the indentation event. The maximum load (Pmax), the maximum displacement (h) and the residual displacement (hf)
are parameters that can be obtained from an initial analysis of the curve. The hardness of a material which is its resistance to plastic deformation caused by the indenter can be obtained from the contact area (A) of the indenter and the applied load (P).
8
𝐻 =𝑃 𝐴
The contact area can be estimated in two ways. The first is by optical microscopy if the indent is large enough. Most of the time this is not the case and the contact area must be estimated in from the contact depth (hc) and the geometry of the indenter. This method is known as the Oliver-Pharr method [55].
Figure 11 a and b gives an idea of how the various parameters for calculating the mechanical properties of a material are obtained. The equation to calculate the area of a Berkovich indenter is given below.
9
𝐴 = 24.49ℎ𝑐2
Figure 12. Berkovich indenter
hc in turn depends on the stiffness (S) of the material which can be calculate form the slope of the
unloading segment and is given by equation 9, where ϵ is a constant depending on the geometry of the indenter (Berkovich ϵ = 0.75[55])
20
10
ℎ𝑠= 𝜖 ×
𝑃𝑚𝑎𝑥
𝑑𝑃 𝑑𝐻⁄
Nanoindentation studies were carried out in both Paper I and II in order to estimate the hardness and elastic modulus of the sample. The mechanical properties were later corelated with the phase evolution of the films in both studies.
21
5 Summary of appended papers
The main objective of this thesis was to understand the process of phase formation and the role of nitrogen in multicomponent systems.
Figure 13. θ-2θ XRD pattern for (CrFeCo)1-yNy coatings on Si(100) substrates. Peaks from the metallic film
corresponding to hcp phase are marked with ¤. (b) High-resolution Cr 2p, Co 2p, Fe 2p, and N 1s core-level spectra recorded from Me1-yNy films. Reference line in fig. 13d corresponds to the extreme maxima recorded
in the series. Paper I
In Paper I, dc magnetron sputtering was used to grow Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤ 0.28)
thin films at 300°C. Nitrogen was added into the metal matrix by increasing the flow ratio (fN = N2/(Ar
+ N2))while sputtering. Introducing nitrogen changes the metallic hcp-structured film into a mixed
fcc/bcc structured film (Fig. 13a). With further addition of nitrogen, the films form a bcc dominant structure. This evolution of structure with nitrogen addition is puzzling as transition metal nitrides generally do not exist with a bcc structure. The stabilization of the bcc may be due to the fact that the films are not completely nitrided. The increasing addition of nitrogen may cause distortions to the fcc lattice therefore promoting the nucleation of bcc structured crystallites. Theoretically calculated phase diagrams of Fe-Co and Fe-Co-Cr metallic systems also demonstrate the competition between hcp, fcc and bcc phases where the stabilization of a certain phase can depend on the concentration of the elements. This is excluding the effects of local magnetic moments [56,57]. Adding nitrogen into such a system could induce distortions or influence the electronic concentration. The sputter deposition process may also play a role in the stabilization of phases which are in non-equilibrium conditions. The higher substrate temperature and the change in the atomic radii difference may also influence the phase formation.
22
The experiments carried out in Paper II were planned to study the effect of a non-nitride forming 3d
transition metal on the phase formation. One of the nitrogen-containing films from the first study was chosen as a starting point of the Cu addition study. This sample when analyzed by XRD and selected area electron diffraction (SAED) patterns was seen to have a mixed fcc + bcc structure, as observed
in Paper I. On addition of Cu, the bcc structure was found to be stabilized and observed by XRD
diffractograms as well as SAED. This is an interesting observation, since Cu is known to act as a fcc-stabilizing element. The addition of a metal such as Cu into this system would further cause distortions to the lattice. The bcc being more accommodative of distortions is the phase which stabilizes in the case of (CrFeCoCux)1-yNy films [58]. As the amount of Cu is increased to 12 at.% a
secondary phase starts to appear which was found to be fcc Cu segregating in the film. A comparative study on CrCoCuFeNi bulk as well as thin films has shown similar results where no segregation occurred in thin films up to 8 at.% [59]. In the case of Cu0.12 the segregation may be due the inability
of Cu to bond with the other elements in the thin film thus forming a secondary metallic fcc structure. It was seen that by adding Cu, the energy brought to the substrate increases which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness measured by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.04. The present study
showed that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as improving mechanical properties.
Figure 14. HAADF-STEM images of (CrFeCoCux)1-yNy films (a, b, c) with inset EDX maps; TEM bright field
images with inset SAED patterns (d, e, f), corresponding HRTEM images (g, h, i). Inset image in h indicates the d spacing of the 110 and 200 bcc planes. Paper II
23
6 Future work
The aim of this thesis was to focus on the synthesis and characterization of the CrFeCo system to lay a foundation for the upcoming studies. In my future work, we will be moving toward more complex systems similar to the Cantor alloy. The studies will focus on three aspects, theoretical understanding, synthesis and characterization, and industrial application oriented.
6.1 Theoretical and experimental understanding of phase formation in
multicomponent thin films based on 3d transition metals.
Understanding the phase transformations in low nitrogen content thin films only through experimental data has proven to be quite challenging. Studies by various researchers have been carried out on the full Cantor system by introducing nitrogen however no clear explanation can be found for the completion of phases [30]. DFT calculations although complex could provide a better understanding [56,57]. This study is in intended to be carried out in collaboration with theoreticians at Linköping University. My part in the study will be focused on developing and characterizing thin films by varying process parameters such as the deposition temperature, and nitrogen partial pressure. The aim is to create a map of sorts where one could access the experimental as well theoretical data of a particular composition.
Figure 15. Regions of interest for experimental and theoretical study marked in grey. Red line indicates the minimum at. % of nitrogen that can be obtained on reactive sputtering in the home-built sputter system. The outcome of this study will give us a better understanding of how nitrogen influences the stabilization of phases in the Cantor alloy and how it can be used to advantage for industrial application.
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6.2 Nitrogen-containing multicomponent Cr-Fe-Ni-Co-Mo thin films
The work carried out in Paper I and II has shown us that multicomponent thin films based on 3d transition metals Cr, Fe, Co and Cu should be built on, for their potential use in applications. A study on pure cobalt thin films has shown that not only do the films have a higher oxidation reduction reaction (ORR) current response that than steel substrates but can also be modified to become catalysts by anodization [60]. Introducing passivating elements such as Cr and Mo could help in further increasing the pitting potential and decreasing the corrosion current [61,62]. Moreover, fully nitrided 3d transition metal multicomponent film tend to have either a B1 NaCl structure or are amorphous in nature [29]. If increasing the nitrogen content in the film leads to amorphization, this could in turn help to improve the corrosion resistance of the film. Therefore, there is a need to investigate both the phase formation and growth mechanisms in Cr-Fe-Co-Cu-Mo-based films with different nitrogen content, and their electrochemical behavior.
6.3 Multicomponent thin film growth and plasma characterization by solenoid coil
assisted magnetron sputtering.
Paper I and II have indicated that the microstructure and phase play a role on the mechanical, and
electrical properties of multicomponent systems. A part of the study on Cr-Fe-Co-Cu-Mo-based films will be focused on developing an amorphous thin film by solenoid coil assisted dc magnetron sputtering. Analyzing the plasma with Langmuir and flat probs will help in understanding better the effects of the solenoid coil on the plasma and in turn the thin film properties. This study could also provide information for microstructural optimization of multicomponent thin films.
25
Reference
[1] A.J.B. Vincent, A study of three multicomponent alloys, BSc Part II Thesis, Univ. Sussex, UK. (1981).
[2] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A. 375–377 (2004) 213–218.
https://doi.org/10.1016/j.msea.2003.10.257.
[3] E. Lewin, Multi-component and high-entropy nitride coatings—A promising field in need of a novel approach, J. Appl. Phys. 127 (2020) 160901. https://doi.org/10.1063/1.5144154.
[4] T.S. Srivatsan, M. Gupta, X.H. Yan, Y. Zhang, A useful review of high entropy films, in: High Entropy Alloy., 2020: pp. 703–721. https://doi.org/10.1201/9780367374426-24.
[5] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Mater. 122 (2017) 448–511. https://doi.org/10.1016/j.actamat.2016.08.081.
[6] M. Tsai, J. Yeh, M. Tsai, J. Yeh, High-Entropy Alloys : A critical review, 3831 (2015) 107–123. https://doi.org/10.1080/21663831.2014.912690.
[7] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang,
Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303+274. https://doi.org/10.1002/adem.200300567. [8] S. Gorsse, D.B. Miracle, O.N. Senkov, Mapping the world of complex concentrated alloys, Acta
Mater. 135 (2017) 177–187. https://doi.org/10.1016/j.actamat.2017.06.027.
[9] R. Carroll, C. Lee, C.W. Tsai, J.W. Yeh, J. Antonaglia, B.A.W. Brinkman, M. Leblanc, X. Xie, S. Chen, P.K. Liaw, K.A. Dahmen, Experiments and model for serration statistics in low-entropy, medium-entropy, and high-entropy alloys, Sci. Rep. 5 (2015) 1–12.
https://doi.org/10.1038/srep16997.
[10] E.P. George, D. Raabe, R.O. Ritchie, High-entropy alloys, Nat. Rev. Mater. 4 (2019) 515–534. https://doi.org/10.1038/s41578-019-0121-4.
[11] P.J. Kelly, R.D. Arnell, Magnetron sputtering: A review of recent developments and applications, Vacuum. 56 (2000) 159–172. https://doi.org/10.1016/S0042-207X(99)00189-X.
[12] N. Chawake, J. Zálešák, C. Gammer, R. Franz, M.J. Cordill, J.T. Kim, J. Eckert, Microstructural characterization of medium entropy alloy thin films, Scr. Mater. 177 (2020) 22–26.
https://doi.org/10.1016/j.scriptamat.2019.10.001.
[13] F. Cao, P. Munroe, Z. Zhou, Z. Xie, Medium entropy alloy CoCrNi coatings: Enhancing hardness and damage-tolerance through a nanotwinned structuring, Surf. Coatings Technol. 335 (2018) 257–264. https://doi.org/10.1016/j.surfcoat.2017.12.021.
[14] F. Otto, Y. Yang, H. Bei, E.P. George, Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys, Acta Mater. 61 (2013) 2628–2638.
https://doi.org/10.1016/j.actamat.2013.01.042.
[15] M.C. Troparevsky, J.R. Morris, P.R.C. Kent, A.R. Lupini, G.M. Stocks, Criteria for predicting the formation of single-phase high-entropy alloys, Phys. Rev. X. 5 (2015) 11041.
https://doi.org/10.1103/PhysRevX.5.011041.
[16] O.N. Senkov, J.D. Miller, D.B. Miracle, C. Woodward, Accelerated exploration of multi-principal element alloys for structural applications, Calphad Comput. Coupling Phase Diagrams Thermochem. 50 (2015) 32–48. https://doi.org/10.1016/j.calphad.2015.04.009.
[17] N. Qu, Y. Chen, Z. Lai, Y. Liu, J. Zhu, The phase selection via machine learning in high entropy alloys, in: Procedia Manuf., Elsevier B.V., 2019: pp. 299–305.
https://doi.org/10.1016/j.promfg.2019.12.051.
26
overcome the strength-ductility trade-off, Nature. 534 (2016) 227–230. https://doi.org/10.1038/nature17981.
[19] K.Y. Tsai, M.H. Tsai, J.W. Yeh, Sluggish diffusion in Co-Cr-Fe-Mn-Ni high-entropy alloys, Acta Mater. 61 (2013) 4887–4897. https://doi.org/10.1016/j.actamat.2013.04.058.
[20] Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid-solution phase formation rules for multi-component alloys, Adv. Eng. Mater. 10 (2008) 534–538. https://doi.org/10.1002/adem.200700240. [21] S.A. Kube, S. Sohn, D. Uhl, A. Datye, A. Mehta, J. Schroers, Phase selection motifs in High Entropy
Alloys revealed through combinatorial methods: Large atomic size difference favors BCC over FCC, Acta Mater. 166 (2019) 677–686. https://doi.org/10.1016/j.actamat.2019.01.023.
[22] S. Ranganathan, Alloyed pleasures: Multimetallic cocktails, Curr. Sci. 85 (2003) 1404–1406. [23] B.X. Cao, C. Wang, T. Yang, C.T. Liu, Cocktail effects in understanding the stability and properties
of face-centered-cubic high-entropy alloys at ambient and cryogenic temperatures, Scr. Mater. 187 (2020) 250–255. https://doi.org/10.1016/j.scriptamat.2020.06.008.
[24] X.H. Yan, J.S. Li, W.R. Zhang, Y. Zhang, A brief review of high-entropy films, Mater. Chem. Phys. 210 (2018) 12–19. https://doi.org/10.1016/j.matchemphys.2017.07.078.
[25] C. Oses, C. Toher, S. Curtarolo, High-entropy ceramics, Nat. Rev. Mater. 5 (2020) 295–309. https://doi.org/10.1038/s41578-019-0170-8.
[26] J. Gild, Y. Zhang, T. Harrington, S. Jiang, T. Hu, M.C. Quinn, W.M. Mellor, N. Zhou, K. Vecchio, J. Luo, High-entropy metal diborides: A new class of high-entropy materials and a new type of ultrahigh temperature ceramics, Sci. Rep. 6 (2016) 1–10. https://doi.org/10.1038/srep37946. [27] T.K. Chen, M.S. Wong, T.T. Shun, J.W. Yeh, Nanostructured nitride films of multi-element
high-entropy alloys by reactive DC sputtering, Surf. Coatings Technol. 200 (2005) 1361–1365. https://doi.org/10.1016/j.surfcoat.2005.08.081.
[28] W. Lengauer, Nitrides: Transition metal solid-state chemistry, in: Encycl. Inorg. Bioinorg. Chem., 2015: pp. 1–24. https://doi.org/10.1002/9781119951438.eibc0146.pub2.
[29] R. Dedoncker, P. Djemia, G. Radnóczi, F. Tétard, L. Belliard, G. Abadias, N. Martin, D. Depla, Reactive sputter deposition of CoCrCuFeNi in nitrogen/argon mixtures, J. Alloys Compd. 769 (2018) 881–888. https://doi.org/10.1016/j.jallcom.2018.08.044.
[30] C. Sha, Z. Zhou, Z. Xie, P. Munroe, FeMnNiCoCr-based high entropy alloy coatings: Effect of nitrogen additions on microstructural development, mechanical properties and tribological performance, Appl. Surf. Sci. 507 (2020) 145101. https://doi.org/10.1016/j.apsusc.2019.145101. [31] C.K. Jin, K.H. Lee, C.G. Kang, Performance and characteristics of titanium nitride, chromium nitride,
multi-coated stainless steel 304 bipolar plates fabricated through a rubber forming process, Int. J. Hydrogen Energy. 40 (2015) 6681–6688. https://doi.org/10.1016/j.ijhydene.2015.03.080. [32] S. Pasupathi, J.C.C. Gomez, H. Su, H. Reddy, P. Bujlo, C. Sita, Recent Advances in
High-Temperature PEM Fuel Cells, Elsevier Inc., 2016. https://doi.org/10.1016/c2015-0-06917-9. [33] A. Hermann, T. Chaudhuri, P. Spagnol, Bipolar plates for PEM fuel cells: A review, in: Int. J.
Hydrogen Energy, Pergamon, 2005: pp. 1297–1302. https://doi.org/10.1016/j.ijhydene.2005.04.016. [34] E. Haye, F. Deschamps, G. Caldarella, M.L. Piedboeuf, A. Lafort, H. Cornil, J.F. Colomer, J.J.
Pireaux, N. Job, Formable chromium nitride coatings for proton exchange membrane fuel cell stainless steel bipolar plates, Int. J. Hydrogen Energy. 45 (2020) 15358–15365.
https://doi.org/10.1016/j.ijhydene.2020.03.248.
[35] M.P. Brady, B. Yang, H. Wang, J.A. Turner, K.L. More, M. Wilson, F. Garzon, The formation of protective nitride surfaces for PEM fuel cell metallic bipolar plates, JOM. 58 (2006) 50–57. https://doi.org/10.1007/s11837-006-0054-4.
[36] K.K. Schuegraf, K. Seshan, Handbook of thin film deposition processes and techniques: Principles, methods, equipment and applications, Angew. Chemie. 101 (2002) 646. www.williamandrew.com (accessed February 23, 2021).
27
[37] W. Kern, J.L. Vossen, Thin Film Processes II, 2012. https://doi.org/10.1016/C2009-0-22311-7. [38] M. Ohring, Materials science of thin films, Elsevier, 2001.
[39] S. Rossnagel, Sputtering and Sputter Deposition, in: Handb. Thin Film Depos. Process. Tech., Elsevier, 2001: pp. 319–348. https://doi.org/10.1016/b978-081551442-8.50013-4.
[40] S. Swann, Magnetron sputtering, Phys. Technol. 19 (1988) 67–75. https://doi.org/10.1088/0305-4624/19/2/304.
[41] B. Window, N. Savvides, Charged particle fluxes from planar magnetron sputtering sources, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 4 (1986) 196–202. https://doi.org/10.1116/1.573470. [42] P.J. Kelly, R.D. Arnell, The influence of magnetron configuration on ion current density and
deposition rate in a dual unbalanced magnetron sputtering system, Surf. Coatings Technol. 108–109 (1998) 317–322. https://doi.org/10.1016/S0257-8972(98)00566-0.
[43] A. le Febvrier, L. Landälv, T. Liersch, D. Sandmark, P. Sandström, P. Eklund, An upgraded ultra-high vacuum magnetron-sputtering system for ultra-high-versatility and software-controlled deposition, Vacuum. 187 (2021) 110137. https://doi.org/10.1016/j.vacuum.2021.110137.
[44] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing, 1956.
[45] A. Segmüller, I.C. Noyan, V.S. Speriosu, X-ray diffraction studies of thin films and multilayer structures, Prog. Cryst. Growth Charact. 18 (1989) 21–66. https://doi.org/10.1016/0146-3535(89)90024-5.
[46] G.C.A.M. Janssen, M.M. Abdalla, F. van Keulen, B.R. Pujada, B. van Venrooy, Celebrating the 100th anniversary of the Stoney equation for film stress: Developments from polycrystalline steel strips to single crystal silicon wafers, Thin Solid Films. 517 (2009) 1858–1867.
https://doi.org/10.1016/j.tsf.2008.07.014.
[47] G. Abadias, E. Chason, J. Keckes, M. Sebastiani, G.B. Thompson, E. Barthel, G.L. Doll, C.E. Murray, C.H. Stoessel, L. Martinu, Review Article: Stress in thin films and coatings: Current status, challenges, and prospects, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 36 (2018) 020801. https://doi.org/10.1116/1.5011790.
[48] H.N. Southworth, Scanning electron microscopy and microanalysis, in: Physicochem. Methods Miner. Anal., Springer US, Boston, MA, 1975: pp. 421–450. https://doi.org/10.1007/978-1-4684-2046-3_11.
[49] J. C. Vickerman, I. S. Gilmore, Surface analysis – The principal techniques 2nd edition, 2009. http://doi.wiley.com/10.1002/9780470721582.
[50] D. Briggs, X-ray photoelectron spectroscopy (XPS), Handb. Adhes. Second Ed. (2005) 621–622. https://doi.org/10.1002/0470014229.ch22.
[51] G. Greczynski, L. Hultman, Compromising science by ignorant instrument calibration—need to revisit half a century of published XPS data, Angew. Chemie - Int. Ed. 59 (2020) 5002–5006. https://doi.org/10.1002/anie.201916000.
[52] G. Greczynski, L. Hultman, X-ray photoelectron spectroscopy: Towards reliable binding energy referencing, Prog. Mater. Sci. 107 (2020) 100591. https://doi.org/10.1016/j.pmatsci.2019.100591. [53] K. Arstila, J. Julin, M.I. Laitinen, J. Aalto, T. Konu, S. Kärkkäinen, S. Rahkonen, M. Raunio, J.
Itkonen, J.P. Santanen, T. Tuovinen, T. Sajavaara, Potku - New analysis software for heavy ion elastic recoil detection analysis, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 331 (2014) 34–41. https://doi.org/10.1016/j.nimb.2014.02.016.
[54] F.M. Smits, Measurement of sheet resistivities with the four‐point probe, Bell Syst. Tech. J. 37 (1958) 711–718. https://doi.org/10.1002/j.1538-7305.1958.tb03883.x.
[55] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology, J. Mater. Res. 19 (2004) 3–20. https://doi.org/10.1557/jmr.2004.19.1.3.
28
Magnetically induced crystal structure and phase stability in Fe 1c Co c, 1996.
[57] B.O. Mukhamedov, A. V. Ponomareva, I.A. Abrikosov, Spinodal decomposition in ternary Fe-Cr-Co system, J. Alloys Compd. 695 (2017) 250–256. https://doi.org/10.1016/j.jallcom.2016.10.185. [58] H.S. Oh, D. Ma, G.P. Leyson, B. Grabowski, E.S. Park, F. Kormann, D. Raabe, Lattice distortions in
the FeCoNiCrMn high entropy alloy studied by theory and experiment, Entropy. 18 (2016). https://doi.org/10.3390/e18090321.
[59] Z. An, H. Jia, Y. Wu, P.D. Rack, A.D. Patchen, Y. Liu, Y. Ren, N. Li, P.K. Liaw, Solid-solution CrCoCuFeNi high-entropy alloy thin films synthesized by sputter deposition, Mater. Res. Lett. 3 (2015) 203–209. https://doi.org/10.1080/21663831.2015.1048904.
[60] C. Linder, S.G. Rao, A. le Febvrier, G. Greczynski, R. Sjövall, S. Munktell, P. Eklund, E.M. Björk, Cobalt thin films as water-recombination electrocatalysts, Surf. Coatings Technol. 404 (2020) 126643. https://doi.org/10.1016/j.surfcoat.2020.126643.
[61] C. Dai, H. Luo, J. Li, C. Du, Z. Liu, J. Yao, X-ray photoelectron spectroscopy and electrochemical investigation of the passive behavior of high-entropy FeCoCrNiMox alloys in sulfuric acid, Appl. Surf. Sci. 499 (2020) 143903. https://doi.org/10.1016/j.apsusc.2019.143903.
[62] A.A. Rodriguez, J.H. Tylczak, M.C. Gao, P.D. Jablonski, M. Detrois, M. Ziomek-Moroz, J.A. Hawk, Effect of molybdenum on the corrosion behavior of high-entropy alloys CoCrFeNi2 and
CoCrFeNi2Mo0.25 under sodium chloride aqueous conditions, Adv. Mater. Sci. Eng. 2018 (2018) 1– 11. https://doi.org/10.1155/2018/3016304.
Papers
The papers associated with this thesis have been removed for
copyright reasons. For more details about these see:
Phase formation in
multicomponent
films based on 3d
transition metals
Linköping studies in Science and TechnologyLicentiate Thesis No. 1904
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FACULTY OF SCIENCE AND ENGINEERING
Linköping studies in science and technology, Licentiate Thesis No. 1904, 2021 Department of Physics, Chemistry and Biology (IFM)
Linköping University
SE-581 83 Linköping, Sweden
