Die Entwicklung und Herstellung neuer Materialsysteme gehört seit jeher zu den Triebkräften des technischen Fortschritts. Eine neue Klasse im Bereich der Legierungen, welche als Hochentropielegierungen (HEA) bezeichnet wird, gewinnt dabei seit etwa zehn Jahren zunehmend an Interesse. Konventionelle Legierungen besitzen einen Hauptbestandteil, welcher üblicherweise den Großteil der molaren Zusammensetzung ausmacht, wie z.B. Eisen im Stahl. Eine HEA hingegen zeichnet sich dadurch aus, dass mindestens vier verschiedene Elemente in ungefähr gleichen Volumenanteilen vorliegen. In diesem Fall kommt es zu einer deutlichen Zunahme der namensgebenden Mischungsentropie gegenüber der Enthalpie, was wiederum zur präferierten Bildung einphasiger, fester Lösungen anstelle von geordneten Metallphasen führt. Durch die nahezu unbegrenzte Anzahl möglicher Element- und Anteilskombinationen zur Bildung von HEAs ergibt sich ein mindestens ebenso großer Raum an potentiellen Anwendungen. Auf dem Gebiet der Materialwissenschaften wurden beispielsweise bereits HEAs mit herausragenden Verschleiß- und Korrosionseigenschaften demonstriert. Auch hinsichtlich Härte, Zug- und Umformfestigkeit oder Strahlungsbeständigkeit existieren vielversprechende Arbeiten, welche die hohe Anwendungsrelevanz von HEAs untermauern. Zahlreiche Herstellungsmethoden konnten dabei bereits erfolgreich auf HEAs angewandt werden, wobei alle Dimensionen, von wenige nm dünnen Schichten bis zu gegossenen oder gesinterten Volumenkörpern im cm-Bereich, abgedeckt werden. Der Einsatz moderner 3D-Drucktechnik erlaubt zudem die Kombination der herausragenden Materialeigenschaften von HEAs in komplex geformten Bauteilen. Trotz des hohen und noch immer rapide zunehmenden Forschungsinteresses an HEAs gibt es nur eine sehr überschaubare Anzahl an Arbeiten, die sich mit der gezielten Untersuchung struktureller, chemischer oder elektronischer Aspekte kristalliner HEA-Oberflächen auseinandersetzen. Ein Grund dafür ist die unzureichende Verfügbarkeit solcher Systeme, denn die Herstellung von großen Kristallen mit hoher Ordnung bedarf in der Regel sehr langsamer und wohl kontrollierter Züchtungsverfahren. Eine Möglichkeit, diesen Engpass zu überwinden, besteht im Wachstum epitaktischer Kristallfilme auf geeigneten Substraten. Deren Herstellung und Charakterisierung mittels Magnetronsputtern stellt einen der Schwerpunkte in dieser Arbeit dar, wobei die Legierung CoCrFeNi als Demonstrationsmaterial gewählt wurde. Zunächst werden zwei aus unterschiedlichen Ausgangsmaterialien gesinterte Arten der benötigten Sputtertargets analysiert und die davon abgeschiedenen Schichten untersucht. Die sich einstellenden Kristallphasen können mittels Röntgenbeugung, die chemische Zusammensetzung und Elementverteilung des Volumenmaterials durch energiedispersive Röntgenspektroskopie bestimmt werden. Eine zwei-Schritt-Präparation der Probenoberfläche, bestehend aus Ar+-Ionenbeschuss und anschließendem Heizen erlaubt die Analyse sauberer Oberflächen. An diesen kann mittels Photoelektronenspektroskopie die Oberflächenzusammensetzung und durch niederenergetische Elektronenbeugung eine etwaige Kristallstruktur untersucht werden. Durch die Zugabe weiterer Elemente soll exemplarisch die gezielte Modifikation einzelner Eigenschaften der gesputterten Dünnschichten untersucht werden. Hierzu wird durch co- Sputtern von CoCrFeNi und WC eine Serie gemischter Schichten hergestellt und der Einfluss auf die Schichthärte mittels Nanoindentation untersucht. Letztendlich wird das epitaktische Wachstum von CoCrFeNi auf den einkristallinen Substraten MgO(100) sowie Al2O3(0001) demonstriert. Hierbei konnte nicht nur erstmalig die Ausbildung einer elektronischen Bandstruktur in HEAs experimentell nachgewiesen, sondern auch erste direkte Zusammenhänge mit der Struktur der CoCrFeNi(100)-Oberfläche aufgezeigt werden. Durch die Verwendung von Substraten verschiedener Struktur und Orientierung ist ein Kristallwachstum der HEA-Schicht entlang unterschiedlicher Richtungen mit hoher Reproduzierbarkeit stimulierbar. Dies stellt eine potentielle Grundlage für weiterführende oberflächenphysikalische und -chemische Experimente, sowie für die Übertragung der in dieser Arbeit beschriebene Herstellungsroute auf weitere HEAs und Substrate dar. / The development and fabrication of new material systems has always been one of the driving forces of technical progress. A new class in the field of alloys, known as high entropy alloys (HEA), has been gaining a rising interest since about ten years. Conventional alloys have one main component, which usually makes up the majority of the molar composition, such as iron in steel. A HEA, on the other hand, is characterized by the fact that at least four different elements are present in approximately equal volume proportions. In this case, there is a significant increase in the eponymous mixing entropy compared to the enthalpy, which in turn leads to the preferential formation of single-phase solid solutions instead of well ordered metallic phases. The almost unlimited number of possible element and proportion combinations to form HEAs results in at least an equally large range of potential applications. In the field of materials science, for example, HEAs with outstanding wear and corrosion properties have already been demonstrated. There is also promising work with regard to hardness, tensile and yield strength or irradiation resistance, which underlines the high applicational relevance of HEAs. Numerous manufacturing methods have already been successfully applied to HEAs, covering all dimensions, from layers as thin as a few nm to cast or sintered solids in the cm range. The use of modern 3D printing technology also allows the combination of the outstanding material properties of HEAs with complex shaped components. Despite the high and still increasing research interest in HEAs, there is only a very limited number of works that deal with the investigation of structural, chemical or electronic aspects of crystalline HEA surfaces. One reason for this is the insufficient availability of such systems, as the production of large crystals with high order usually requires very slow and well-controlled growth processes. One way to overcome this bottleneck is the deposition of epitaxial crystal films on suitable substrates. Their production and characterization using magnetron sputtering is one of the main focuses of this work, with the CoCrFeNi alloy being chosen as the demonstration material. First, two types of sputtering targets sintered from different starting materials are analyzed and the layers deposited from them are examined. The resulting crystal phases can be determined using X-ray diffraction, and the chemical composition and element distribution of the bulk material can be investigated using energy-dispersive X-ray spectroscopy. A twostep preparation of the sample surface, consisting of Ar+ ion bombardment and subsequent heating, allows the analysis of clean surfaces. The surface composition can be examined using X-ray photoelectron spectroscopy and the crystal structure is available by means of low-energy electron diffraction. By adding further elements to the alloy, the systematic modification of individual properties of the sputtered thin films will be addresses. For this purpose, a series of mixed layers is produced by cosputtering CoCrFeNi and WC and the influence on the layer hardness is examined using nanoindentation. Ultimately, the epitaxial growth of CoCrFeNi on the substrates MgO(100) and Al2O3(0001) is demonstrated. Not only was the formation of an electronic band structure in HEAs experimentally proven for the first time, but also direct connections with the structure of the CoCrFeNi(100) surface could be made. By using substrates of different structures and orientations, crystal growth of the HEA layer can be stimulated along different directions with high reproducibility. This represents a potential basis for further surface physics and chemistry experiments, as well as for the transfer of the manufacturing route described in this work to other HEAs and substrates.

Alloying aluminum offers the possibility of creating low-density high-entropy alloys (HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination. The effect of aluminum on the phase composition and microstructure of the compositionally complex alloy (CCA) system AlxCoCrFeNiTi was studied with variation in aluminum content (molar ratios x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the different domains in the microstructure. The crystal structure was determined using X-ray diffraction (XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with associated orientation distribution was performed using electron backscatter diffraction. This made it possible to correlate the chemical and structural conditions of the phases. The phase formation strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum amounts. The hard η-(Ni, Co)3Ti phase in the x = 0.2 alloy was detected via metallographic investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc) with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural and chemical properties as well as microstructure formation contribute to a better understanding of the alloying effects concerning the aluminum content in CCAs. Especially in the context of current developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach to the development of new alloy systems.

The novel alloying concept of high-entropy alloys (HEAs) has been the focus of many recent investigations revealing an interesting combination of properties. Alloying with aluminium and titanium showed strong influence on microstructure and phase composition. However, detailed investigations on the influence of titanium are lacking. In this study, the influence of titanium in the alloy system AlCoCrFeNiTix was studied in a wide range (molar ratios x = 0.0; 0.2; 0.5; 0.8; 1.0; 1.5). Detailed studies investigating the microstructure, chemical composition, phase composition, solidification behaviour, and wear behaviour were carried out. Alloying with titanium showed strong influence on the resulting microstructure and lead to an increase of microstructural heterogeneity. Phase analyses revealed the formation of one body-centred cubic (bcc) phase for the alloy without titanium, whereas alloying with titanium caused the formation of two different bcc phases as main phases. Additional phases were detected for alloys with increased titanium content. For x ≥ 0.5, a minor phase with face-centred cubic (fcc) structure was formed. Further addition of titanium led to the formation of complex phases. Investigation of wear behaviour revealed a superior wear resistance of the alloy AlCoCrFeNiTi0.5 as compared to a bearing steel sample.

The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which makes this special alloy interesting for surface engineering and particularly for thermal spray technology. In this study, the suitability of inert gas-atomised HEA powder for high-velocity-oxygen-fuel (HVOF) thermal spray is investigated. This process allows for high particle velocities and comparatively low process temperatures, resulting in dense coatings with a low oxidation. The microstructure and phase composition of the atomised powder and the HVOF coating were investigated, as well as the wear behaviour under various conditions. A multiphase microstructure was revealed for the powder and coating, whereas a chemically ordered bcc phase occurred as the main phase. The thermal spray process resulted in a slightly changed lattice parameter of the main phase and an additional phase. In comparison with a hard chrome-plated sample, an increase in wear resistance was achieved. Furthermore, no brittle behaviour occurred under abrasive load in the scratch test. The investigation of wear tracks showed only minor cracking and spallation under maximum load.

Alloying aluminum offers the possibility of creating low-density high-entropy alloys (HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination. The effect of aluminum on the phase composition and microstructure of the compositionally complex alloy (CCA) system AlxCoCrFeNiTi was studied with variation in aluminum content (molar ratios x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the different domains in the microstructure. The crystal structure was determined using X-ray diffraction (XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with associated orientation distribution was performed using electron backscatter diffraction. This made it possible to correlate the chemical and structural conditions of the phases. The phase formation strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum amounts. The hard η-(Ni, Co)3Ti phase in the x = 0.2 alloy was detected via metallographic investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc) with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural and chemical properties as well as microstructure formation contribute to a better understanding of the alloying effects concerning the aluminum content in CCAs. Especially in the context of current developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach to the development of new alloy systems.

The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which makes this special alloy interesting for surface engineering and particularly for thermal spray technology. In this study, the suitability of inert gas-atomised HEA powder for high-velocity-oxygen-fuel (HVOF) thermal spray is investigated. This process allows for high particle velocities and comparatively low process temperatures, resulting in dense coatings with a low oxidation. The microstructure and phase composition of the atomised powder and the HVOF coating were investigated, as well as the wear behaviour under various conditions. A multiphase microstructure was revealed for the powder and coating, whereas a chemically ordered bcc phase occurred as the main phase. The thermal spray process resulted in a slightly changed lattice parameter of the main phase and an additional phase. In comparison with a hard chrome-plated sample, an increase in wear resistance was achieved. Furthermore, no brittle behaviour occurred under abrasive load in the scratch test. The investigation of wear tracks showed only minor cracking and spallation under maximum load.

Hochentropielegierungen stellen einen neuen Entwicklungsansatz metallischer Werkstoffe ohne ein eigenschaftsbestimmendes Hauptelement dar. Die zielgerichtete Übertragung der bisher überwiegend an Massivwerkstoffen ermittelten Eigenschaften in die Beschichtungstechnik erfordert die Kenntnis der bestimmenden Einflussfaktoren. Für die Schichtherstellung werden die Verfahren des thermischen Spritzens betrachtet. Hierfür wird eine geeignete Prozessroute ermittelt. Die detaillierten Untersuchungen zu den Prozess-Struktur-Eigenschaftsbeziehungen erfolgen an Legierungen mit variabler Struktur. Diese werden anhand von thermodynamischen Parametern sowie Untersuchungen an schmelzmetallurgisch hergestellten Massivwerkstoffen ausgewählt. Zur Bewertung des Einflusses der Größe der Strukturmerkmale, der Heterogenität und möglicher Ungleichgewichtszustände werden schmelz- und pulvermetallurgisch hergestellte Massivwerkstoffe als Referenz betrachtet. Die geplanten Forschungsarbeiten tragen zu einem Verständnis der Prozess-Struktur-Eigenschaftsbeziehung von Hochentropielegierungen bei. Weiterhin wird eine geeignete Prozessroute für die pulvermetallurgische Verarbeitung sowie für Anwendungen in der Oberflächentechnik ermittelt.

High hardness and good wear resistance have been revealed for the high-entropy alloy (HEA) system AlCoCrFeNiTi, confirming the potential for surface protection applications. Detailed studies to investigate the microstructure and phase formation have been carried out using different production routes. Powder metallurgical technologies allow for much higher flexibility in the customisation of materials compared to casting processes. Particularly, spark plasma sintering (SPS) enables the fast processing of the feedstock, the suppression of grain coarsening and the production of samples with a low porosity. Furthermore, solid lubricants can be incorporated for the improvement of wear resistance and the reduction of the coefficient of friction (COF). This study focuses on the production of AlCoCrFeNiTi composites comprising solid lubricants. Bulk materials with a MoS2 content of up to 15 wt % were produced. The wear resistance and COF were investigated in detail under sliding wear conditions in ball-on-disk tests at room temperature and elevated temperature. At least 10 wt % of MoS2 was required to improve the wear behaviour in both test conditions. Furthermore, the effects of the production route and the content of solid lubricant on microstructure formation and phase composition were investigated. Two major body-centred cubic (bcc) phases were detected in accordance with the feedstock. The formation of additional phases indicated the decomposition of MoS2.