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Um thermische Spritzschichten mit erhöhter Haftfestigkeit zu realisieren, erfolgte am Institut für Spanende Fertigung (ISF) der Technischen Universität Dortmund und dem Institut für Werkstoffkunde (IW) der Leibniz Universität Hannover die Entwicklung einer innovativen Verfahrenskombination. Darauf aufbauend wurde untersucht, ob die Schichteigenschaften durch einen zusätzlichen Prozessschritt weiter optimierbar sind, um zukünftig Verschleißschutzbeschichtungen ohne eine Substrataktivierung durch Strahlen zu ermöglichen. To enable thermal sprayed coatings with increased adhesion, the Institute of Machining Technology (ISF) of TU Dortmund University together with the Institute of Materials Science (IW) of Leibniz Universität Hannover developed an innovative process combination. This serves as a basis for investigating whether the coating properties can be further optimized by an additional process step to allow for the production of wear-resistant coatings in the future without substrate activation by blasting.

Conventional thermal spraying processes are almost exclusively carried out in an air atmosphere, resulting in the oxidation of the particle surfaces and interfaces within the coating and between the substrate and coating. Furthermore, the initial process of surface activation conventionally takes place in an air atmosphere, preventing an oxide-free interfacial transition. Consequently, the application of spraying materials with high oxygen affinity represents a major challenge. To overcome these issues, the present study utilized silane-doped inert gases to create an environment in which the oxygen concentration was equivalent to the residual oxygen content in an extreme high vacuum. By transferring the corundum blasting and coating process (wire arc spraying) to this environment, materials with a high oxygen affinity can be applied without oxidation occurring. For industrial use, this is an interesting prospect, e.g., for repair coatings, as the homogeneity of the composite is improved by a non-oxidized coating. Using the example of arc-sprayed copper coatings, the microstructure and mechanical properties of the coatings were analysed. The results showed that the oxide-free, wire arc sprayed copper coatings exhibited an improved wetting behaviour resulting in a significant reduction of the coating porosity. Moreover, the improved wetting behaviour and led to an increase in the bonding rate and apparent Young’s modulus. Contrary to expectations, the residual stresses decrease although relaxation mechanisms should be inhibited, and possible reasons for this are discussed in the paper.

Selective oxidization of tool steel surfaces was studied, as this is a promising approach to realize lubricant-free forming processes. Specifically, the dry sliding wear of a tribosystem consisting of hardened tool steel (1.2379) and DP600 + Z sheet metal was investigated. The tool surfaces were selectively oxidized under an inert gas atmosphere to curtail abrasive wear. It turned out that tool surfaces that contained larger near-surface chromium carbides are not suitable for dry sliding wear, as these promote abrasive wear in the tribosystem. In addition, such carbides pose a challenge for selective oxidation. Optimized heat treatment strategies resulted in smaller chromium carbides, which could be selectively oxidized. To gain further insights into the wear behavior of the oxide layer, a mathematical model was developed. Worn sample surfaces were characterized with high-resolution tools, and the results provided input data for the numerical model.

Selective oxidization of tool steel surfaces was studied, as this is a promising approach to realize lubricant free forming processes. Powder metallurgically manufactured 1.2379 tool steel was employed as substrate. Heat treatment was realized inductively under controlled process atmosphere generating α-Fe2O3 oxide layer systems, which can act as friction reducing separation layers. In this context, it is important to note that hardened 1.2379 tool steel features precipitations of chromium carbides, which also occur in the near surface areas. As coarse carbides will not be covered by the α-Fe2O3 oxide layer, powder metallurgically manufactured tool steel with smaller, finely distributed chromium carbides was used, and better performance was obtained in this respect. Inductive heat treatment with varying frequency and strength of the coil current was employed. The effects on the characteristics of the oxide layer formed were determined by both experiments and simulations.

Specimens of austenitic AISI 304 and ferritic 446 (contains 1.7 wt.% Al) underwent heat treatments up to 1200 °C in ultrahigh vacuum (UHV) in an analysis chamber with a base pressure of 10-10 mbar and high vacuum (HV) in an industrial vacuum brazing furnace with a base pressure of 10-6 mbar. The effect of the two oxygen partial pressure ranges on surface and bulk composition was determined by XPS, AES and EDX, imaging was provided by SEM. For AISI 304, we observe similar behavior for both atmospheres and the removal of the surface oxide layer. For AISI 446, we observe a strong dependency on the atmosphere. Heat treatment in high vacuum yielded a thick aluminum oxide layer on the surface that inhibited wetting with copper silver eutectic. Heat treatments in ultrahigh vacuum did not lead to aluminum oxide formation but rather carbide formation of foreign elements such as titanium was observed on the surface.

Turbine blades must withstand severe loading conditions and damage can occur during operation due to heat, pressure, foreign objects and hot gas corrosion, despite the protective coatings applied onto the turbine blades. Instead of replacing the damaged components, maintenance, repair and overhaul are key to extend the total service life. Besides welding, the repair of turbine blades by brazing is an established repair process in the industry and involves many individual steps that often require a high degree of manual work. In the present study, a hybrid joining and coating technology was developed to shorten the state-of-the-art process chain for repairing turbine blades. With this approach, a repair coating, which consists of a filler metal, a hot gas corrosion protective layer and an aluminum top layer, is applied by atmospheric plasma spraying. The coated turbine blade then undergoes a heat-treatment so that a brazing and aluminizing process is carried out simultaneously. Due to diffusion and segregation processes, pores can occur in the heat-treated coating. In the present study, a full factorial design of experiment was performed to reduce the pores in the coating. The microstructure of the repair coating was investigated by optical- and scanning electron microscopy (SEM), and the impact of the process parameters on the resulting microstructure is discussed.

The application of solid lubricants offers high potential in terms of resource-efficiency in contacts under relative motion. The disadvantage of this lubrication method is the high risk of interface failure after the coating has been worn. In order to predict the service life of coated surfaces, it is necessary to determine the microscopic wear behaviour by analysing the coating hardness and coefficient of friction (CoF). Due to layer thicknesses under 3 µm, these parameters can only be determined using nanoindents and nano-scratch-tests. These tests require a transfer of material properties and tribological parameters from the nano to the micro level. The transfer method to the micro level and validity of the values determined at the nano level is presented while also describing challenges due to different influences of the respective size level. Additionally, oscillating sliding tests are carried out to determine the friction properties during sliding, which correspond to reference tests performed on the micro level.

In addition to using conventional lubricants, such as oil and grease, rolling bearings can also be used with a dry lubricant. For example, the use of dry lubricant systems is necessary when the application of oils or greases is not possible (e.g., at high temperatures or in aerospace applications). The requirements of a solid lubricant are to reduce friction and wear of mechanical contact partners. In this work, a molybdenum-based coating system was applied by means of physical vapor deposition (PVD). The coating system consists of a molybdenum (Mo) reservoir with molybdenum trioxide (MoO3) as the top layer. The MoO3, which is particularly important for the run-in and the lubricating effect, is intended to continuously regenerate from the reservoir via tribo-oxidation. To determine the friction and wear behavior, cylindrical roller thrust bearings were used. Experiments demonstrated that the lubrication system is effective and that the frictional behavior has been improved. On the one hand, the frictional torque of the rolling bearings has been considerably reduced and, on the other, significantly extended operating times have been determined compared to unlubricated reference experiments. Simultaneously, material analyses have been carried out by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The investigations showed that the MoO3 was transferred to uncoated bearing components. This improved the tribological behavior and reduced abrasive and adhesive wear.

As reported in previous studies conducted within the priority program 1676 “Dry forming - Sustainable production through dry machining in metal forming”, selectively oxidized tool surfaces represent a promising approach for the realization of a dry forming process. For the transfer of the findings to an actual industrial deep drawing process, the choice of sheet material is of great importance. The present study reports investigations on hot-dip galvanized dual-phase steel that was deep-drawn with selectively oxidized tools made from hardened 1.2379 tool steel. The data are compared to the deep-drawing behavior of electrolytically galvanized deep-drawing steel, which was used as a reference. The applied oxide system consisted of α-Fe2O3 and was generated in a tailored atmosphere with controlled partial pressure of oxygen. A specially developed modular deep-drawing tool was used for the deep-drawing tests, which allowed carrying out cup-drawing tests in which parts of the drawing ring featured different oxidation conditions. The differently set load cases on the drawing ring were then evaluated and examined with respect to the resulting surface properties of the component produced. The changes in the surface morphology prior to and after deep drawing were analyzed using high resolution analysis and the interactions between tool coating and sheet metal were examined. The results obtained provided information about the deep-drawing properties of the sheet metal materials and confirmed the advantages of α-Fe2O3 tool coating used as a low-friction separating layer for use in dry deep drawing.

In this paper, new technologies for repairing turbine blades are presented, in which the manufacturing processes and materials mechanisms are incorporated. Since the turbine blades taken into consideration here are components of high pressure turbines, the focus of this paper lies on nickel-based alloys. Depending on the size and form of the defects present on the blades, two procedures can be used for repairing turbine blades: brazing and/or cladding. In one approach, a hybrid repair brazing process was developed, in which the filler metal and the hot gas corrosion protective coating were applied by thermal spraying. Subsequently, a combined brazing and aluminizing process was carried out. In a second approach, a laser cladding process for crack repair was developed wherein single crystalline solidification of the cladding material was carried out.

The repair and maintenance of components in the aerospace industry play an increasingly important role due to rising manufacturing costs. Besides welding, vacuum brazing is a well-established repair process for turbine blades made of nickel-based alloys. After the coating of the worn turbine blade has been removed, the manual application of the nickel-based filler metal follows. Subsequently, the hot gas corrosion-protective coating is applied by thermal spraying. The brazed turbine blade is aluminized to increase the hot gas corrosion resistance. The thermal spray technology is used to develop a two-stage hybrid technology that allows shortening the process chain for repair brazing turbine blades and is described in the present paper. In the first step, the coating is applied on the base material. Specifically, the coating system employed here is a layer system consisting of nickel filler metal, NiCoCrAlY and aluminum. The second step represents the combination of brazing and aluminizing of the coating system which is subjected to a heat treatment. The microstructure, which results from the combined brazing and aluminizing process, is characterized and the relevant diffusion processes in the coating system are illustrated. The properties of the coating and the ramifications with respect to actual applications will be discussed.

This paper presents the results and analysis of wear investigations carried out on a wear test bench with high strength sheet metal DP600+Z drawn over selectively oxidised α-Fe 2 O 3 tool steel surfaces. Wear investigations were carried out with several selectively oxidised specimens by varying the pulling and counter force, and thus the surface pressure on the samples. This allowed for a systematic study of the wear behaviour of the oxide layers under different loads. The specimens were characterised using microscopy (light microscopy, scanning electron microscopy and topography analysis) and scratch testing on a nanoindenter. The study shows that it is possible to realise a dry metal forming process while using α-Fe 2 O 3 oxide layers on the tool steel surface. The oxidised surface acts as a friction reducing separation layer and protects the tool against wear. Simultaneously, it was found that the surface of sheet metal drawn over oxidised specimens showed lower zinc abrasion than sheet metal drawn over non-oxidised reference specimens at the same surface pressure. In addition, the oxidised specimens have a reduced Zn pick-up affinity.

A process of removal of surface oxides from stainless steels AISI 304 and 446 that involves reduction by residual carbon, followed by the formation and desorption of carbon monoxide, is studied by investigation of thermal desorption with quadrupole mass spectrometry (QMS) and secondary ion mass spectrometry (SIMS). Carbon monoxide desorption is studied as a function of time and temperature by QMS, and carbon diffusion due to heat treatment is studied with SIMS—twice as much carbon monoxide desorbs from AISI 304 overall and desorption sets in at 900 °C as opposed to 1100 °C for AISI 446. In samples heated to 900 °C, carbon shows surface enrichment in AISI 304 but depletion in AISI 446. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Engineering under protective atmospheres or in vacuum allows the production of materials and components, where the absence of oxygen is an essential requirement for a successful processing. Ideally, joining or coating of (and with) metallic materials needs oxide free material surfaces, in order to achieve durable joints or coatings. Using the established technology of brazing in controlled atmosphere, fundamental physical mechanisms for deoxidation of metal surfaces are presented and the role of oxygen and water residue in the process atmosphere is analyzed. Furthermore, the doping of gases with monosilane for generating virtually oxygen-free process atmospheres is introduced and its advantages for an oxygen-free production are discussed.

Besides welding, high temperature vacuum repair-brazing is already established for nickel-based alloy turbine blades in the aerospace and power plant industries. After the worn turbine blade has been decoated to its substrate material, the filler metal is deposited as a paste, (melt-spin) foil or tape which also consists of a nickel-based alloy. Following this, the hot-gas corrosion protective coating (e.g. NiCoCrAlY) is applied using thermal spraying. The brazed turbine blade is ground or milled to size and subsequently aluminized to further increase its corrosion resistance. Using the current state of technology, a turbine blade can undergo approximately 3 to 4 repair cycles. In the present study, the development of a two-stage hybrid technology for repairing turbine blades is considered which incorporates, on the one hand, a process technology and manufacturing aspects and, on the other hand, considers material-technological mechanisms. During the first stage of this hybrid technology, the filler metal together with the hot-gas corrosion protective coating is applied using thermal spraying. The subsequent second stage combines the brazing and aluminizing processes. The technology developed here brings technical and economic advantages whilst enabling the current state-of-the-art's corresponding process chain for repairing turbine blades to be shortened. DFG/SFB/871

Die Steigerung der Standmenge von Schmiedegesenken stellt eine vielversprechende Moglichkeit zur Verbesserung der Wirtschaftlichkeit von Schmiedeprozessen dar. Hartstoffschichten auf Titanbasis eignen sich fur die Steigerung der Verschleisbestandigkeit belasteter Oberflachen. Im Vergleich zur bestehenden Aufbringung von titanbasierten Beschichtungen mittels CVD-Verfahren, bietet das PVD-Verfahren die Moglichkeit zur Herstellung von Mehrlagenbeschichtungen mit zahlreichen dunnen Einzellagen. In Untersuchungen zum Einsatzverhalten unter industrienahen Bedingungen wurden unterschiedliche Schichtsysteme bewertet. Dazu wurde in verschiedenen Analysen zunachst die Schichtqualitat untersucht. In darauf aufbauenden Serienschmiedeversuchen mit prozessbegleitender Verschleisanalytik wurde der Einfluss verschiedener titanbasierter Schichtsysteme in Kombination mit unterschiedlichen Nitrierungen auf das Verschleisverhalten analysiert. Es wurde festgestellt, dass geometriespezifische Beanspruchungen lokal angepasste optimale verschleismindernde Behandlungsparameter erfordern. Sowohl die Variation der Nitrierung als auch des Schichtsystems zeigen einen deutlichen Einfluss auf das Verschleisverhalten an unterschiedlichen Bereichen der betrachteten Gesenkgeometrie.

In this paper a high loaded hot forging die, which is oriented towards an industrial forging process, is developed with assistance of finite element analysis. In serial hot forging tests and process accompanying analysis methods the designed die geometry gets investigated concerning occurring wear mechanisms. Within the investigation it could be shown, that calculated results of the wear condition are corresponding to the experiments. Abrasion, plastic deformation and mechanical cracking could be detected. The variety of occurring wear mechanisms on the designed die allows a promising approach of further investigations on the valuation of different wear reducing strategies.

Particle in-flight characteristics in atmospheric plasma spraying process are determined by impulse and heat energy transferred between the plasma jet and injected powder particles. One of the important factors for the quality of the plasma-sprayed coatings is thus the distribution of plasma gas temperatures and velocities in plasma jet. Plasma jets generated by conventional single-arc plasma spraying systems and their interaction with powder particles were subject matter of intensive research. However, this does not apply to plasma jets generated by means of multi-arc plasma spraying systems yet. In this study, a numerical model has been developed which is designated to dealing with the flow characteristics of the plasma jet generated by means of a three-cathode spraying system. The upstream flow conditions, which were calculated using a priori conducted plasma generator simulations, have been coupled to the plasma jet simulations. The significances of the relevant numerical assumptions and aspects of the models are analyzed. The focus is placed on to the turbulence and diffusion/demixing modelling. A critical evaluation of the prediction power of the models is conducted by comparing the numerical results to the experimental results determined by means of emission spectroscopic computed tomography. It is evident that the numerical models exhibit a good accuracy for their intended use.

Due to controlled and enhanced tribological conditions in forming technology, oils are generally used. Liquid lubricants, especially mineral oils, are environmentally harmful or even toxic. Within the priority program 1676, researchers are studying technologies to prevent the use of lubricants in forming operations. One approach to avoid lubricants in sheet metal forming is the generation of thin selective oxide layers (thickness of about 300–500 nm) on forming tools. In previous investigations, it was found that these layers (e.g. α-Fe2O3-layers) have the ability to reduce friction between the tool and sheet metal (DP600+Z) as well as zinc pick-up on the tool surface. Furthermore, the wear behavior of those layers was investigated. It could be shown that the oxide layer resists several thousand loads of stroke. Within this study, an overview of the previous work and the future activities of the task will be shown.

For the application in dry lubricated rolling bearings the development of self-regenerative molybdenum oxide layers is necessary. The self-regenerative coatings should provide a hard overcoat in the tribological contact due to oxidative processes. The application of the Mo-layers on the conventional bearing steel (AISI 52100) is carried out by means of physical vapour deposition (PVD). The characterisation of layers in the nanometre range is highly demanding and close the measurable limit. In the development of molybdenum layer systems, the behaviour of the individual layers on the substrate and among each other is of great importance. The coefficients of friction and the elastic and plastic material behaviour were determined by means of nano scratch tests. Moreover, the layer thickness, nano hardness and modulus of elasticity were determined by nanoindents. The measurements were made on a Hysitron TriboIndenter® TI 950. Based on these results the nano investigations are the basis for the further development of the Mo-layer systems.

Thermal spraying is one of the most efficient methods to deposit hard wear resistant coatings. The coatings deposited by High-Velocity-Oxygen-Fuel spraying (HVOF) are characterized by high wear resistance and outstanding tribological characteristics. One of the most challenging tasks for tribologists is to develop surface treatments that allow for both operating the component without lubricants and at the same time minimize wear. WC-based cermets are a group of thermally sprayed coatings known to have high wear resistance under sliding friction conditions. An experimental study on the dry sliding wear behavior of WCCo HVOF-sprayed coating deposited onto a steel substrate is presented in the current paper. A pin-on-disc tribometer was used to carry out the wear tests.

Components in aircraft engines and gas turbines are exposed to extreme conditions in order to increase performance and efficiency of the overall engine, hence there is an increasing need for cost-effective and time-efficient repair strategies. Presented here are two novel approaches to the repair of Nickel-based components. The hybrid brazing process involves the application of a repair coating, a nickel-based filler material, a NiCoCrAlY and an aluminium layer, by thermal spraying followed by a heat treatment and combined brazing-aluminizing process. This significantly shortens the conventional repair brazing process and yields superior results. Single-crystal additive repair by laser cladding is applied for the repair of small or large defects in single-crystal turbine blades by enabling monocrystalline solidification of the cladded material by use of a temperature gradient, thereby allowing for the regeneration of these expensive components. The novel approach that combines layer-wise addition of material and laser melting enables the formation of highly monocrystalline structures.

For the realization of liquid lubricant free forming processes different approaches are conceivable. The priority program 1676 “Dry forming - Sustainable production through dry machining in metal forming” addresses this issue in the context of metal forming processes. The present study reports results from one subproject of the priority program that employs selective oxidization of tool steel surfaces for the implementation of a dry sheet metal deep drawing process. Within the present study, specimen surfaces of the tool steel (1.2379) were heat-treated to optimize their tribological properties with respect to sliding wear behaviour in contact with drawn sheet metal (DP600+Z). The heat treatment was designed to result in the formation of selective oxide layers that can act as friction reducing separation layers. The heating setup employed an inductive heating under protective gas atmosphere. Selective oxidation was realized by controlling the residual oxygen content. Specifically, the specimens were heated in the near-surface region just above the annealing temperature, thus avoiding the degradation of mechanical properties in the bulk. Evaluation of hardness along cross-sections of each specimen revealed suitable initial temperatures for the inductive heat treatment. Oxide layer systems were analyzed regarding their tribological sliding wear behaviour after selective oxidation, as well as their morphology and chemical composition before and after the sliding wear tests.

To realize a dry sheet metal forming process, α-Fe2O3 oxide layers were investigated regarding their friction characteristics, wear behavior and surface energy depending on different surface qualities of the specimens examined. The oxide layers were generated in a new hybrid batch furnace. The layer generation on all specimens of the tool steel 1.2379 used was performed at a target temperature below the annealing temperature (≈ 510 °C). Friction coefficients were examined with plane strip drawing tests. Wear experiments with oxidized wear specimens with variable surface topographies were carried out up to several thousand strokes per surface condition.

The optimum and efficient way of machining is linked to the choice of the most appropriate cutting tool and cutting tool material. High performance cutting ceramics are characterized by excellent hardness properties at elevated temperatures. For this reason, cutting ceramics meet the requirements for machining with high cutting speeds to increase process productivity. Whereas cutting ceramics are widely used in turning and milling operations, their use in drilling processes, using ceramic insert tipped tools, is limited to larger diameter applications due to design restrictions. Beyond small diameters, solid ceramic tools were of negligible interest for industrial applications owing to their excessive tool manufacturing costs. This paper presents a new tool concept which addresses this challenge and permits a more productive machining process for drilling small diameter holes using ceramics as the cutting material. An active brazed compound drill combines the advantageous properties of ceramics and cemented carbide as the cutting tool material and basic holder material, respectively. The investigations presented here describe the manufacturing chain as well as the application of a compound drill, and compares it to a widely used industrial reference tool.

Using two active brazes based on silver–copper, the wetting behaviour was investigated on hard metals and diamond. Here, it was possible both to characterise the in-situ wetting angle as a function of temperature, the surface roughness and the time by using the flow rate and the wetting angle as well as to analyse the underlying mechanisms. A directly proportional relationship between temperature and the formed wetting angle was demonstrated during the tests up to a material dependent limiting value. Moreover, the wetting behaviour was shown to deteriorate with increasing surface roughness values. By means of investigating the diffusion zone between the carbide metal and the braze using scanning electron microscopy, it was possible to establish that the penetration depth of the active element titanium into the carbide metal increased with increasing brazing temperature.

Thermally sprayed coatings and tribological surfaces are a point of interest in many industrial sectors. They are used for better wear resistance of lightweight materials or for oil retention on surfaces. Lightweight materials are often used in the automotive industry as a weight-saving solution in the production of engine blocks. For this, it is necessary to coat the cylinder liners to ensure wear resistance. In most cases, the coating is sprayed directly onto the surface. Previous research has shown that it is possible to transfer these coatings inversely onto other surfaces. This was achieved with plasma sprayed coatings which were transplanted onto pressure-casted surfaces. These transplanted surfaces exhibited better adhesive strength, smoother surfaces, and lower form deviation compared to directly coated surfaces. Additionally, it was shown that even microstructures of a surface coated by plasma spraying can be transferred to pressure-casted surfaces. This paper presents the development and micromilling of different microstructures for transferring thermally sprayed coatings onto pressure-casted surfaces. In the development process, microstructures with different shapes and aspect ratios as well as thin tribological surfaces are designed in order to evaluate the advantages and limitations of the transplantation process. In subsequent experiments, the micromilling process and a simulation of the coating transplantation are presented and analyzed. DFG/Mo 881/9-1 DFG/Bi 498/6-1

Die Eigenschaften thermisch gespritzter Schichten sind von vielen Parametern wie der Spritzwerkstoff- und Substrateigenschaften als auch der Spritzparameter abhangig. In dieser Studie wird der Einfluss zweier variabler Spritzparameter (Spritzabstand und Stromstarke) auf Molybdan-haltige thermische Spritzschichten untersucht. Dabei werden in erster Line werkstoffkundliche und oberflachencharakterisierende Eigenschaften analysiert und deren Abhangigkeit voneinander gepruft. Wichtige Oberflachenparameter stellen hierbei die Porositat und die Porentiefe der Schichten dar. Im Anschluss an die Korrelation zwischen Spritzparametern und Schichteigenschaften wird insbesondere auf den Einfluss der Schichteigenschaften auf deren tribologisches Verhalten im Vergleich zu einer unbeschichteten Stahloberflache eingegangen. The properties of thermally sprayed coatings are dependent on many parameters such as the spraying material, substrate properties, and the injection parameters. In this study, the influence of two variable spray parameters (spraying distance and current) were investigated on molybdenum-containing thermal spray coatings. Particularly, materials and surface characterizing properties were analyzed, and the dependence on each other was examined. The important surface parameters studied in this case are the porosity and the pore depth of the coatings. Following the correlation between spray parameters and coating properties, the influence of surface properties on the tribological behavior will be discussed, in comparison to an uncoated steel surface.

The main aim of this study is to improve the coating properties of three-cathode atmospheric plasma-sprayed coatings with respect to porosity and residual stresses. This was done by means of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in the characteristic threefold symmetrical plasma jet cross section produced by a three-cathode-plasma torch, was developed. The modeling results for the standard injector’s position “0” were calculated and experimentally verified by laser Doppler anemometry. Based on the criteria defined for the concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step, a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification, and deformation due to shrinkage for alumina particles on a rough substrate surface.

A novel method has been developed to process highly filled polymer–particle composites to test samples as braze metal preforms. Polypropylene (PP), low-density polyethylene (LD-PE) and high-density polyethylene (HD-PE) were used as polymer matrices. Two types of nickel-based braze metal microparticles (Ni 102 and EXP 152) were compounded to the polymer matrices at filler contents up to 65 vol %. With enhancing filler content, torque at kneading rotors, and injection molding parameter were significantly affected by increasing viscosity. Injection molded composites show well-distributed spherical microparticles and particle–particle interactions. Polymers decompose residue-free at temperatures above 550°C, even for their composites. Adding particles reduces polymer crystallinity, whereas defined cooling at 5°C/min significantly increases the crystallinity and melt peak temperature of polymers compared to undefined cooling prior injection molding. Storage modulus of polymers increases significantly by adding filler particles. LD-PE + 65 vol % EXP 152 show the most suitable composite performance. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

For high volume manufacturing of many stainless steel components, a continuous brazing process using a shielding gas furnace is usually preferred. Regarding the workability of the nickel-based brazes used here, furnace brazing in reducing or inert shielding gas is subject to considerable constraints: On the one hand, the maximum brazing temperatures are limited by the specific process and, on the other hand, limitations arise caused by the chemical reaction of the materials to be brazed with the process atmosphere. For this reason, the braze materials are an essential starting point for improving the process. In the present contribution, modified nickel-based brazes are introduced which meet the process conditions in continuous shielding gas furnaces and offer the prospect of improving the properties of stainless steel components joined in this way.

This study presents a new approach to enhance the corrosion resistance of tungsten inert gas (TIG) welded AZ31 magnesium alloys by using nanocrystalline magnesium fluoride suspensions in a suspension plasma spray (SPS) process. We have developed a synthesis for the preparation of nanocrystalline magnesium fluoride suspensions, which delivers nearly monodisperse nanoparticles in a gram scale yield. The particles were analyzed with transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Stable suspensions of magnesium fluoride nanoparticles in water were characterized by dynamic light scattering (DLS), zeta-potential, and viscosity measurements. Such suspensions were deposited with an SPS torch onto TIG welded seams of the magnesium alloy AZ31, thus producing a protective magnesium fluoride layer. Magnesium fluoride covered welded seams were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDXS). In order to introduce a simple method for sensoring the deposited magnesium fluoride coatings, the magnesium fluoride nanoparticles can also be fluorescence-labeled by co-doping with cerium(III) and terbium(III), the respective optical properties were characterized by reflection and luminescence spectroscopy. The deposited layers can, thus, be inspected by illumination with an UV lamp, because of their bright green emission. The corrosion properties of the magnesium fluoride layer on the welded seams were studied by means of potentiodynamic potential measurements.

In diesem Beitrag wird die Erzeugung von Oberflachen mit stochastisch verteilten Mikrostrukturen durch thermisches Spritzen mit unterschiedlichen Prozessparametern beschrieben. Diese Oberflachen werden mit einem optischen Rauheitsmessgerat flachenhaft erfasst und weiterverarbeitet, so dass jede einzelne Struktur identifiziert wird. Aus diesen Daten werden Kenngrosen, wie die Flache, das Volumen und die Tiefe der Strukturen, ebenso wie verschiedene Porositaten berechnet. Aus diesen Ergebnissen wird eine Korrelation von Spritzparametern und Oberflachenkennwerten erstellt. Auserdem werden tribologische Experimente mit den Oberflachen durchgefuhrt und der gemessene Reibwert mit der Flache, der Tiefe und der mittleren Rauheit abseits der Strukturen korreliert. In this paper microstructures have been produced with a stochastic distribution by a thermal spraying process with different process parameters. An optical roughness measurement device has been used to measure these microstructures. Afterwards the measurement data is processed in order to identify each microstructure. Using this data several characteristics like the area, the volume and the depth of the microstructures as well as several porosities have been computed. These characteristics could be correlated to the process parameters. Furthermore, tribological experiments have been conducted with these surfaces and additional correlations between the friction value and the characteristics area, depth and the average roughness besides the structures have been performed.

A fast and simple approach for immobilization using copolymers as interlayers is reported. The synthesized copolymers form stable self-assembled layers on implant materials like, e.g., titanium in a simple coating/drying/washing sequence and have functional groups which can bind proteins from an aqueous solution. The copolymer films have been characterized via ellipsometry and contact angle measurements and were tested for biocompatibility. An immunoassay was used to determine the amount of BMP2 and demonstrated an approximately 10-fold increase as compared to previously used self-assembled monolayers. A BMP2-responsive cell line with luciferase detection was used to determine the biological activity of the bound signaling protein.

In recent years, suspension plasma spaying (SPS) has been investigated to provide functional coatings with determined abilities, which could not be reached by conventional thermal spray processes so far. In this paper, the possibilities are discussed to achieve oxide ceramic coatings based on titania by SPS, which show a reduction of their coefficient of friction under dry sliding conditions. Besides the possibility of mixing different feedstock suspensions in the process, the achievable low thickness and homogenous microstructure of suspension-sprayed coatings may allow use of the coatings in applications, where high demands on adhesion, cohesion and near net shape are made. In den vergangenen Jahren wurde das Suspensions-Plasmaspritzen (SPS) hinsichtlich seiner Eignung untersucht, Schichtsysteme mit definierten funktionalen Eigenschaften zu erzielen, die bisher mit konventionellen thermischen Spritzverfahren nicht realisiert werden konnten. In diesem Beitrag werden Moglichkeiten diskutiert, um mittels SPS oxidkeramische Schichtsysteme auf Basis von Titandioxid zu erhalten, die unter trockener Gleitreibung eine signifikante Minderung des Reibungskoeffizienten aufweisen. Neben der Option der Durchmischung verschiedener Werkstoffsuspensionen im Prozess bietet das Verfahren die Moglichkeit, dunne Schichten mit einem sehr homogenen Gefuge einzustellen. Diese Eigenschaften sollen einen Einsatz unter Bedingungen erlauben, die hohe Anforderungen an die Adhasion, die Kohasion und die Endkonturnahe der Schichtsysteme stellen.

Durch Schichtverbundbauweise lassen sich die Eigenschaften von Bauteiloberflachen mit geeigneten Schichtwerkstoffen gezielt an Anforderungen anpassen, die vom vorgesehenen Grundwerkstoff nicht oder nur eingeschrankt erfullt werden konnen. In klassischen Prozessketten hat der thermische Spritzprozess eine unabhangige Stellung in der abschliesenden Bearbeitungsphase der Bauteile. Mit dem vorgestellten Verfahren soll ein thermischer Spritzprozess unmittelbar in die Druckgussfertigung integriert werden. So wird ein Herstellungsverfahren fur beschichtete Gussteile mit weitestgehend nachbearbeitungsfreien Funktionsflachen realisiert. Im Gegensatz zum konventionellen Beschichtungsprozess wird dabei die Schicht nicht auf das Bauteil, sondern auf die Druckgussform oder entsprechende Wechseleinsatze appliziert und bei jedem Abguss als strukturierte Funktionsflache auf das Bauteil ubertragen. Der Transfer einer solchen Schicht von der Werkzeugoberflache auf das Gussteil, auch als Transplantation bezeichnet, bedingt ein enges Prozessfenster bezuglich der Schichthaftung auf der Formoberflache und auf dem Gussteil. Die Schichthaftung an der Werkzeugoberflache muss gros genug sein, um wahrend des Beschichtungsprozesses und der Formfullung nicht abgelost zu werden, jedoch gering genug, um sich beim Entformen vollstandig vom Werkzeug abzulosen und zusammen mit dem Gussteil entnommen werden zu konnen. Im Vordergrund der vorgestellten Untersuchungen steht daher zunachst die gezielte Einstellung der Schichthaftung durch Anpassung der frastechnisch hergestellten Werkzeugoberflache sowie die Anpassung der Schichtsysteme und Spritzparameter des Atmospharischen Plasmaspritzprozesses zur moglichst prazisen Abformung der so eingebrachten Strukturen durch die Spritzschicht. Des Weiteren werden Untersuchungen zur Verbundbildung zwischen Gusswerkstoff und Schichtwerkstoff vorgestellt und Ergebnisse erster Giesversuche zur Transplantation von thermisch gespritzten Schichten im Druckguss auf Aluminium- und Magnesiumlegierungen prasentiert. By manufacturing structured functional-surface coated composites using suitable materials, the properties of components' surfaces can be specifically matched to their requirements which the specified substrate material can not, or can only partially, fulfil. In conventional process chains, thermal spraying and structuring by milling are independent stages in the component's final processing phase. Using the procedure introduced here, thermal spraying is to be directly integrated into the high pressure die casting process. In this way, a manufacturing procedure for coated castings with functionalized surfaces is realised which, as far as possible, eliminates the finishing operation of coated cast parts. In contrast to conventional coating processes, the coating-layer is not applied to the component but to the casting mould or to the corresponding inserts and is transferred to the component as a structured functional-surface during each cast. Transferring this type of coating from the tooling surface to the casting; which is also referred to as transplant coating, requires a narrow process-window with respect to the coating adhesion to both the tooling surface and the cast part. The coating adhesion at the tooling surface must be high enough such that the coating is not detached during either the coating or the mould filling processes, but also low enough to be completely detached from the tooling during mould stripping and be able to be removed together with the cast part. For this reason, the initial priority of the current investigations is to obtain a specific coating adhesion by adapting both the tooling surface, which is manufactured by milling, as well as the coating systems and the atmospheric plasma spray's parameters for the most precise moulding of such plasma sprayed structures. Furthermore, investigations are introduced on forming composites from the cast material and the coating materials and results are presented of preliminary casting tests for transplanting thermally sprayed coatings onto aluminium and magnesium alloys during high pressure die casting.

Continuous process brazing in shielding-gas furnaces is tailor-made for manufacturing mass production components by means of brazing technology. Which brazing tasks can actually be carried out by a shielding-gas furnace, depend on many ancillary conditions. In particular these are, besides the component size and joint geometry, the base materials and brazing filler metals as well as the material specific process parameters which are to be maintained in a continuous furnace in order that a process assured brazing can be guaranteed. In this context, the activation of the component's surfaces play a central role for wetting with the braze melt. Within the scope of this contribution, the variables and ancillary conditions concerning this matter are presented and discussed using the brazing process on stainless steels as an example.

In order to produce optically cooperative surfaces to use triangulation measuring systems, the methods of surface and thin-coating technology are resorted to. Among these number physical methods such as PVD technology, (electro-)chemical treatment as, for example, electrochemical etching and electroless coatings as well as mechanical treatments of the components by means of glass-bead and corundum blasting. The processes mentioned are suitable for producing optically cooperative surfaces since, owing to the corresponding test parameters, a change of the component's surface occurs at the sub-micrometer scale. Currently, no indication in the literature is provided that such processes are employed for manufacturing optically cooperative surfaces vis-à-vis triangulation methods.

In recent years, suspension plasma spraying (SPS) was investigated regarding the spraying of functional coatings with deterministic abilities. In this contribution the possibilities are discussed to achieve oxide ceramic coatings based on titania by SPS, which show a reduction of their coefficient of friction under dry sliding conditions. Besides the possibility to mix different feedstock suspensions in the process, the achievable low thickness and homogenous microstructure of suspension-sprayed coatings shall allow an operation in fields, where high demands towards the adhesion, cohesion and near net shape are made.

Im Rahmen eines Forschungsvorhabens wurden die Spritzwerkstoffe Cr2O3, Cr3C2 NiCr und WCCoCr betrachtet. Es konnten Prozessparameter, welche die Verarbeitung dieser Spritzwerkstoffe in Korngrosen von < 25 μm mit Plasma- bzw. Hochgeschwindigkeitsflammspritzsystemen erlauben, erfolgreich bestimmt, optimiert und validiert werden. Neben verbesserten Schichteigenschaften wie z. B. einem niedrigen Porenanteil und einer erhohten Korrosionsbestandigkeit eroffnet sich damit die Moglichkeit, mashaltige Beschichtungen zu realisieren. Ein dadurch verringerter Nachbearbeitungsaufwand, z. B. aufgrund verkurzter Schleifzeiten, kann zur weiteren Rationalisierung von thermischen Spritzprozessen beitragen. About the application of fine spray materials for improvements in terms of corrosion and wear protection of thermal spray coatings Within a research project the spray materials Cr2O3, Cr3C2 NiCr and WCCoCr were considered. Process parameters were determined, optimized and validated allowing the application of very fine spray material fractions (< 25 μm) with plasma and high velocity oxy-fuel spray systems. In addition to improved coating properties like low porosity and improved corrosion resistance the application of near net shape coatings was enabled. The subsequent reduced effort for after-treatment of the coatings can contribute to further rationalization of thermal spray processes.

Das Fugen von Aluminiumwerkstoffen ist aufgrund der hohen Sauerstoffaffinitat des Aluminiums nach wie vor eine Herausforderung. Insbesondere die Wirtschaftlichkeit des Verfahrens sowie der Verzicht auf Flussmittel beim Schutzgasofenloten stehen derzeit im Vordergrund aktueller Forschungsvorhaben. Die gezielte Zumischung reaktiver Gase zur inerten Prozessgasatmosphare bietet eine geeignete Moglichkeit, Aluminiumwerkstoffe flussmittelfrei zu loten. Unter Einsatz von Chlorwasserstoff soll eine Aktivierung der Lot- und Werkstuckoberflache ermoglicht werden, die eine Zerstorung der Oxidschichten einleiten soll. Des Weiteren kommt das stark reduzierende Gas Silan zum Einsatz, dass die Schutzgasatmosphare gezielt von Sauerstoff- und Wasserruckstanden befreit. Anhand eines geeigneten Ofenlotprozesses sollen sowohl reduzierende als auch aktivierende Zusatze in Kombination eingesetzt und auf ihren Einfluss auf die verwendeten Gasmischungen und Werkstoffe getestet werden [1–5].

Stangelkristallite werden insbesondere beim Loten von Stahlen unterschiedlichen Kohlenstoffgehaltes mit Kupfer beobachtet. Diese aus mit Kupfer durchsetzten Eisen-Kohlenstoff-Mischphasen bestehenden Kristallite wachsen vom kohlenstoffreicheren Fugepartner in das Kupferlotgut. Durchdringen die Stangelkristallite die Lotnaht, so ergibt sich eine grundwerkstoffahnliche Verbindungsfestigkeit. Die festgestellten Wachstumsgeschwindigkeiten der Kristallite hangen in charakteristischer Weise vom Kohlenstoffgehalt, der Lotspaltbreite sowie dem Temperatur-Zeit-Regime des Lotprozesses ab. Obwohl das Phanomen der Stangelkristallitbildung schon seit den 1950er Jahren bekannt ist, konnte der Vorgang bis jetzt noch nicht eindeutig geklart werden. Die in der Literatur beschriebenen, diesbezuglichen Theorien und Hypothesen stutzen sich im Wesentlichen nur auf Vermutungen und Plausibilitatserklarungen, ohne dass die vorgeschlagenen Mechanismen anhand eines schlussigen physikalischen Modells verifiziert wurden. Im Rahmen dieser Arbeit wird ein physikalisches Modell vorgestellt, welches die experimentell beobachteten Abhangigkeiten der Stangelkristallitbildung quantitativ beschreibt. Das am System Eisen-Kohlenstoff-Kupfer verifizierte Modell ist ubertragbar auf andere Lot-Grundwerkstoffsysteme und stellt somit ein Instrument zur systematischen Auswahl und Entwicklung von Loten – insbesondere fur nicht artgleiche Fugeverbindungen – dar. Simulation of columnar crystallite formation in brazed seams of copper-brazed carbon steels When brazing steels of different carbon content with copper filler metal, columnar crystallites form on the carbon-rich iron surface if the width of the brazing gap is smaller than 100 μm. Braze seams with such microstructures were described as early as the 1950ies and it was found out, that the strength of such a joint is significant enhanced, if this crystallites penetrate the entire seam. Extensive experimental investigations in recent years confirm, that the final average length of the crystallite increases superproportionally with decreasing brazing gap width and is almost inversely proportional to the difference in carbon content of the joined steels. Although many attempts to explain this phenomenon are known from literature, the mechanism of columnar structure formation has not been clarified properly until now. The aim of the present work was to develop an appropriate physical model, that describes the growth of crystallites as a function of carbon content in the base materials, the initial brazing gap width and the applied process parameters (temperature, time). The model is an appropriate tool for a general choice and development of filler metal-base material combinations forming columnar crystallites in the braze seam.

Goal of this work is to homogenize properties of three-cathode plasma sprayed coatings on basis of advanced diagnostics and numerical simulations. After a brief description of the general approach to achieve this objective, selected aspects of the current work are presented. Thereby the paper is focused on the self developed suspension injection TriplexII setup, in the special determination of the plasma and in measurement plus presentation of the suspension-particle-behavior. In the suspension plasma diagnostic part (CT-Computed Tomography) the different temperature profiles and geometries are shown and discussed. Additional the suspension-particle-behavior are more explained with the PIV (Particle Image Velocimetry), High-Speed-Camera and AccuraSpray-g3 results, which are calculated and presented with special image processing algorithm. The last part of the paper describes the suspension coating analysis and concludes with an outlook for future measurements of the self developed SPS-TriplexII setup.