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The addition of refractory metals represents a promising approach for development of future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable additives for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic high-entropy alloys (EHEAs) with their homogeneous property profile are of specific interest. In the present work, two EHEAs were developed starting from the alloy Al0.3CoCrFeNi using electric arc melting. Following mechanical and microstructural characterization, the two alloys were found to have the compositions Al0.3CoCrFeNiMo0.75 and Al0.3CoCrFeNiNb0.5. For thermal spray processing, powders of the above alloys were prepared by inert gas atomization. The coatings produced by high-velocity oxy-fuel spraying (HVOF) were characterized and evaluated compared with castings, allowing process–structure–property relationships to be derived. Based on the results, statements on possible application potential can be made.

High-entropy alloys (HEAs) represent a relatively new group of multicomponent alloys that have shown great potential for applications requiring tribological and oxidation resistant properties. Consequently, thermally sprayed coatings of different HEA chemistries have received increasing research attention. In this paper, atomized equimolar CrFeCoNi and AlCrFeCoNi feedstocks were used for high velocity air-fuel spraying (HVAF) to produce overlay coatings using two different nozzle configurations. The microstructure, phase constitution and hardness of the coatings were analyzed along with the primary aim of testing the coatings for their oxidation behavior. The performance of the two HEA chemistries was compared with two commercial MCrAlY coatings that are well-established bond coat materials for thermal barrier coatings (TBCs). An investigation was conducted to test the coatings’ performance as bond coats by applying suspension plasma sprayed yttria-stabilized zirconia top coats and evaluating the thermal cycling behavior of the TBCs. The AlCrFeCoNi-coating was found to demonstrate a lower oxidation rate than the CrFeCoNi-coating. However, the AlCrFeCoNi-coating was found to form more rapid oxide scales compared with the commercial bond coat material that also contained reactive elements.

The adaptation of medium-entropy alloys (MEAs) by minor alloying constituents allows a targeted modification of the property profile of this material class for surface protection applications. In the present work, the potential of BSiC additions in the MEA system CrFeNi as base for adapted feedstock materials for thermal spraying is investigated. The alloy development was carried out in an electric arc furnace. Compared with the initial alloy, a significant increase in the wear resistance of the castings was demonstrated for the adapted alloy composition. Subsequently, powder was produced and characterized by inert gas atomization, followed by processing via high velocity oxy-fuel (HVOF) spraying. The tribological behavior was evaluated comparatively for all manufacturing variants considered. A good agreement in the property profile was determined, confirming the basic alloy development approach based on metallurgical processes. The evaluation of the process-structure property relationships confirms the great potential of adapted alloy systems for complex alloys in the field of surface engineering.

The development of thermal spray processes usually requires an analysis of the complex coating microstructure. In order to inspect critical areas of a coating, destructive testing methods such as the preparation of cross-sections are commonly used. In this work, the suitability of largely non-destructively measured polarisation curves for the quality assessment of thermal sprayed AISI 316L coatings is investigated. Therefore, a 3.5 % NaCl gel electrolyte was developed to prevent the corrosive medium from infiltrating the porous and micro-cracked microstructure characteristic for thermal sprayed coatings. In addition, a measuring cell based on the 3-electrode arrangement was designed to simplify the setup, reduce the measurement time and enable mobile measurements directly on the component surface at a later stage of development. The effects of process-related differences in the microstructure of HVAF and APS AISI 316L coatings on the polarisation curve was investigated by determining the corrosion current density. The microstructure of the AISI 316L coatings was analysed by optical microscope, SEM and EDS, focussing on the porosity and oxide content. The results clearly show that the potentiodynamic polarisation curves of the AISI 316L coatings differ significantly depending on the spray process used and microstructure created. Even small changes in the oxide content within a coating can be detected. Therefore, electrochemical measurement methods using gel electrolyte offer an interesting opportunity to evaluate the quality of thermal sprayed AISI 316L coatings in a largely non-destructive manner.

Laser-clad AlCrFeCoNi high entropy alloy (HEA) coatings, produced on the surface of AISI 316 L stainless steel, were pack-borided at 1000 °C for 4 h in open air, high-purity Ar and vacuum environments. The HEA, which had an initial hardness of 6.14 ± 2.06 GPa, formed a complex boride layer consisting of (CoFe)B2, (CrFe)B2 and Cr2Ni3B6 phases on its surface, with hardness ranging from 15.95 ± 0.7 to 20.15 ± 4.50 GPa as a result of the boriding process. While the greatest boride layer thickness was obtained in the sample borided in vacuum, the highest surface hardness was obtained in the sample borided in air. The borided coatings showed improved wear resistance and lower friction values compared to the untreated control samples, both at 25 °C and 650 °C. The borided coatings also showed reduced coefficients of friction at 650 °C. The wear losses at 650 °C significantly exceeded those at 25 °C.

Selective laser melting (SLM) is one of the promising techniques for producing metallic glass components with unlimited geometries and dimensions. In the case of iron-based metallic glasses, the appearance of cracks remains a problem. In this work, two alloys Fe48Mo14Cr15Y2C15B6 and (Fe0.9Co0.1)76Mo4(P0.45C0.2B0.2Si0.15)20, differing in their plasticity, were printed with a double stage scanning strategy. Both alloys were characterized by a fully amorphous structure and a crack grid that coincided with the hatch distance in the first scan. Segregations of metalloids were observed in the vicinity of the cracks. Fe48Mo14Cr15Y2C15B6 samples were characterized by a high compression strength of 1298 ± 11 MPa and zero plasticity. The compression strength of the (Fe0.9Co0.1)76Mo4(P0.45C0.2B0.2Si0.15)20 samples was 142 ± 22 MPa. The results obtained suggest that further development of scanning strategies and research on the influence of alloying elements is needed.

The plasma electrolytic oxidation is an innovative method for the surface treatment of titanium and its alloys. This review provides an overview of the historical development of the process and summarizes the current state of the art. The chemical as well as the electro- and plasma-chemical basics of the layer forming mechanisms, which comprises the substrate/electrolyte interface before discharge initiation and the different types and stages of plasma electrolytic discharge phenomena are explained within the context of titanium-based materials. How these phenomena can be influenced by the use of suitable electrolytes and controlled by the electrical regime is described. Subsequently, the microstructures and composition of the layers are described in detail, and the properties for specific applications are then discussed. The resistance of a PEO coating to corrosive environments, tribological factors, and alternating mechanical stress is viewed critically, and the extensive functional properties such as physiological compatibility, photocatalytic activity, and decorative properties are revealed. Finally, examples of various practical applications in the medical engineering, aviation, automotive, and environmental technology fields, as well as other branches of industry, are presented., M. Aliofkhazraei acknowledges comments received from Prof. A. Yerokhin during intial steps of preparation of this review paper. D.D. Macdonald gratefully acknowledges the support of the FUTURE (Fundamental Understanding of Transport Under Reactor Extremes), an Energy Frontier Research Center funded by the U.S. Department of En- ergy (DOE), Office of Science, Basic Energy Sciences (BES). E. Matykina would like to thank the support of the RTI2018-096391-B-C33 (MCIU/ AEI/FEDER, UE) and S2018/NMT-4411 (Regional Government of Madrid and EU Structural and Social Funds). S.V. Gnedenkov ac- knowledges the support of the Russian Science Foundation (No. 20-13- 00130).

In this investigation, thermally sprayed cylindrical specimens are machined by turning with different cutting speeds. To ensure that process-induced shearing loads do not cause delamination, a fine helical dovetail structure is cut into the substrate before it is coated with FeCrNi alloy by air plasma spraying. Dovetail structures with different geometries were produced and their effectiveness is compared. The finish-machined surfaces of the FeCrNi coatings were examined and characterized with respect to feed marks, cracks, open pores, pull outs, and residual stresses. It is shown that surface roughness and the number of pull outs decrease with increasing cutting speed while residual stresses remain relatively unchanged, except for the orientation of the first principal stress.

The use of electrochemical methods allows fast and inexpensive corrosion measurements of bulk materials with high significance. In the case of thermal spray coatings, electrolyte penetration into open pores up to the substrate material can cause undesired mixed potentials. Furthermore, the implementation of complex geometries or rough surfaces remains a problem. Preconditioning of the surface or the use of the electrochemical cell is required to eliminate leakage. Therefore, reliably measuring corrosion is still a challenging task. This undermines fast monitoring of corrosion performance as a part of the production process. Gelling agents are investigated to increase the viscosity of many electrolytes. A procedure has been developed to determine the concentration level and the mixing conditions. Passivation and pitting-corrosion testing are performed on thermal spray AISI 316L coatings. The electrochemical potential curves as well as the corroded surface layers were studied in comparison to a liquid electrolyte. The suitability of the test on rough surfaces in the sprayed condition was investigated. The results prove the novel approach as an alternative to established electrochemical test methods with extended application range.

Increasing demands on component performance and efficiency require continuous development efforts in existing material systems and concepts. In addition to the specific material properties, economic aspects have to be taken into account. Thermochemical treatments of iron-and nickel-base alloys allow for a distinct improvement of hardness and wear resistance. The process of boriding enables the highest hardness values and the formation of thermally stable precipitates. Especially nickel-base alloys are suitable for high temperature applications. An economic application of these alloys can be achieved by applying coating technologies and limiting the material usage to the surface. High-velocity oxygen fuel thermal spraying of the nickel-base alloy Inconel 718 and subsequent powder-pack boriding is conducted. Furthermore, the influence of a solution annealing step prior to the boriding process is investigated with the motivation to achieve a homogenisation of the coating. A successful diffusion enrichment and the formation of a precipitation layer could be achieved. The investigation of the resulting properties revealed a distinct increase of hardness and an improvement of wear resistance tested under reciprocating conditions in a wide temperature range.

The embedding of ceramic particles of a certain size and concentration in a ductile matrix is a strategy for producing wear-resistant coatings. In wire arc spraying, flux-cored wires with a metallic sheath and particle-filled core can be used for this purpose. The influence on wear properties of wire diameter and particle fraction has hardly been investigated so far. Due to the high kinetics of arc and gas flow, there are considerable challenges with regard to reproducibility, homogeneity and material efficiency. As a basis for a significantly improved understanding of the process, the relationships between the wire diameter, the particle fraction used and the wear resistance of the produced coatings have been investigated. The objects of investigation were flux-cored wires with an iron sheath and a particle filling of Al2O3/ZrO2. By using the ball-on-disc test and rubber wheel test, the wear resistance of the respective coatings was investigated.

The surface area of the test sample depicted in figure 2 was calculated wrong (without the contribution of the lateral area). That’s why the current densities in Table 3, Figure 4 and Figure 5 were wrong also. We adjusted the mentioned article elements as well as the corresponding statement within the abstract: As a result of this, it was possible to obtain alumina layers of 80 micrometers in thickness, with a high corundum content of approximately 50 to 90%, after 37 minutes of treatment time, at a current density below 10 (instead of 25) A/dm2 on C8C-steel.

The interlaminar strength of mechanically interlocked polymer-metal-interfaces strongly depends on the metal surface topography. Hence, this contribution assesses standardized surface roughness parameters as well as fractal dimension for interlaminar strength prediction of EN AW-6082/polyamide 6 polymer-metal-hybrids. Seven different metal surface topographies generated by mechanical blasting, alkaline etching, thermal spraying as well as laser structuring are investigated. Interlaminar strength testing is carried out with regard to tensile and shear strength using butt-bonded hollow cylinder specimens. The surface roughness profiles are recorded with a tactile stylus profiler. The fractal dimension of the metal surface is derived from cross-sectional images. The measured interlaminar strength values show good correlation to the fractal dimension, moderate correlation to the root mean square slope of the surface roughness profile and no correlation to surface roughness height parameters.

Within this work, aluminum is oxidized via plasma-electrolytic oxidation (PEO) in the presence of organic substances, including reactive monomers. The aim of this approach is to generate polymers and simultaneously bind them to the surface of the freshly generated oxide layers. For this purpose, sheets of aluminum were immersed into electrolytes that comprised either 4 % of sodium acrylate or 10 % of sodium benzoate. The aluminum sheets were oxidized by anodic pulse current at 0.25 A/cm2 for 30–90 s. By this process, predominantly oxidic conversion layers were produced. The presence of the monomers in the electrolyte influenced the passivation and discharge behavior and finally the microstructure of the layers. It further gave rise to organic material in the layer. In particular, infrared attenuated total reflection spectroscopy (ATR-IR) and X-ray photoemission spectroscopy (XPS) show that layers which were generated in the presence of acrylate or benzoate comprised -C-C-, -C=C-, -C-O, and C=O bonds.

The simulation of large-scale industrial electrodeposition of zinc is of major importance, since through the simulation, important data about the positioning of electrodes, thickness of layers, etc. can be generated. But the models used in practical applications indirectly assume small anodes and cathodes for which the electrical potential or the charge-transfer current are constant over the cathode, resp. anode. In this paper, a new type of model for the dissolution of zinc anodes is described. Furthermore, a numerical scheme to treat the model will be described and verified. The new model will be compared to a commonly known model. The paper gives an alternative model for the calculation of the current density on anodes and formulates a FEM for the approximation of Poisson equations on multiple domains with Robin-interfaces. Besides the application to the dissolution of zinc-anodes during electro-plating, the basic model and FEM can be applied to the dissolution of anodes made of a different material and to the modelling of corrosion processes.

In the section Results and Discussion of our published paper the units for the heating energy and the toughness contain a mistake.

In the present study, 35vol% SiCp/AlSi7Mg0,6 composites were prepared using field assisted sintering technique in order to investigate the effect of different particle fractions and size distributions of the AlSi7Mg matrix powder on the tensile properties of the produced composite material. In most usecases the size of the reinforcement phase is given by the application and is only variable within narrow limits (< 20pm particle size in this work). On the other hand, there is potential for optimization of the matrix powder. In this investigation, fine (d50 = 25 µm), coarse (d50 = 52 µm), bimodal (50wt% of fine + 50wt% of coarse, d50 = 36 µm) and as received (d50 = 40 µm) aluminum powder was used as the matrix powder. Using fine matrix powder has improved yield strength by 5 % and ultimate tensile strength by 7 % compared to the as received condition. This is largely due to the lower porosity of the composite produced under the use of the fine matrix powder ((0.07 ± 0.04) %) in contrast to the composite using the as received aluminum powder ((0.62 ± 0.35) %). At the same time, the consumed heating energy of the composite was decreased by almost a third when using the fine matrix powder in comparison to the use of the as-received matrix powder. This paper presents results of an optimization approach for mechanical properties of aluminum matrix composites without any changes of the sintering parameters.

Direct powder rolling (DPR) can be used to create metallic sheet at a reduced cost compared to traditional methods. The process is particularly relevant for metals that are beneficiated to a powder form. The DPR process, however, is not yet optimised and the material properties corresponding to DPR sheet do not compare well with sheet produced from rolled metallic billet. This study explores the effect of DPR input parameters on DPR aluminium sheet material properties. Roll speed, roll gap, sintering time and sintering temperature are shown to have an impact on the density, microstructure, hardness and ductility of DPR aluminium strip. It is shown that a low roll speed and a small roll gap produced the most dense strip. A roll gap of 0.7 mm and roll speed of 0.366 rpm produced green strip with a density of 90% of cast aluminium while a roll gap of 0.8 mm and roll speed of 0.73 rpm produced green strip with a density of 79.6% of cast aluminium. In addition, a Vickers hardness analysis and a three-point bending test show that increasing the sintering temperature of the green DPR strip will produce softer aluminium strips with higher ductility, as expected.

This paper presents a novel approach to enhance the corrosion resistance of cylinder-bore coatings for internal combustion engines by changing surface characteristics and microstructures of the coating and utilizing alternative materials. The approach of this study refers to the knowledge of the complex gas-flow conditions during the internal-diameter coating of cylinder-bores using a twin-wire arc-spray process. Selective influencing of the gas flow enables the preparation of sample coatings with inhomogeneous and more homogeneous microstructures and surfaces. The various coatings were exposed to corrosive environments using the Kesternich testing method and an alternating immersion test. Significant improvements in corrosion resistance have been demonstrated by up to 50 % through altering the microstructure of the coating and almost 100 % by additional changing the spraying material. The study reveals an increased protection of cylinder-bore coatings against corrosive attacks through a homogeneous microstructure prepared in oxygen-poor coating conditions.

Different manufacturing approaches have been investigated regarding their suitability to process high-entropy alloys (HEAs). However, comprehensive investigations on the influence of the production route on the microstructure, phase formation and properties have not been conducted yet. For the current study the alloy AlCoCrFeNiTi0.5 is considered. Previous investigations have proven the formation of phases with predominantly body centred cubic structure for this alloy. Castings are produced by arc-melting. Feedstock material for coating deposition and powder metallurgical processing is produced by inert gas atomisation. For the processing high-velocity-oxygen-fuel (HVOF) thermal spraying and spark plasma sintering (SPS) are applied. Due to the significantly differing process conditions and temperature-time profiles, differences of microstructure, phase formation and resulting properties can be observed. Wear investigations under various conditions have been conducted. Especially under sliding and reciprocating wear conditions the structural defects formed for the thermally sprayed coating cause a reduction of wear resistance. The formation of structural defects could be avoided by SPS. However, the additional tetragonal phase causes a reduction of the wear resistance. The current study contributes to a better understanding of the interaction between process, microstructure and properties.

Coordination complexes [M(O

In this investigation, an air plasma sprayed TBC system consisting of a CoNiCrAlY bond coat (BC) and a YSZ topcoat (TC) is produced with a PVD AlOx interlayer in order to study its effect on thermally grown oxides. For comparative purposes, a reference TBC without the AlOx interlayer was also prepared and studied. A cyclic thermal load was applied to both systems and the coatings were examined after 6, 12, 24, 40, and 80 cycles. Crack lengths were measured in the YSZ layer and TGO thicknesses were assessed at the BC-TC interface. An examination of coating microstructures revealed the expected mixed-mode failure in both TBCs. In comparison to the reference TBC, the system with the AlOx interlayer showed reduced crack formation in the TC and slowed TGO formation at the BC-TC interface both during and after thermal treatment.

TThe physical characteristics and volume growth of TGOs in TBC systems lead to TBC failure. It is proved that enriching the BC/TC interface with α-Al2O3 is beneficial to an extended operational time by prolonging the steady-state growth stage of the TGO. The corresponding phase reactions in TBC systems with heightened Al activity, however, are not studied yet. In this work, the stage formation of TGO layers of TBC systems with PVD-Al interlayers is described. The study uses thermal cyclic loading with dwell time at maximum high temperature of 1,150 °C. The crack formation in the ceramic top coat and the TGOs thickness at the interface are investigated by SEM/EDS after 1, 6, 12, 24, 40 and 80 thermal cycles. The results plot the interface change and crack formation as a function of the thermal cycle number. The corresponding failure mode is discussed.

Die Untersuchung des Agglomerationsverhaltens von nanoskaligen Partikeln in Abhangigkeit der Konzentration der Einzelbestandteile eines Watts-Nickelelektrolyten und der mechanischen Konvektion ist Gegenstand dieses Artikels. Ausgehend von der Beschreibung der Messprinzipien werden die Ergebnisse der Zetapotentialmessungen und Bestimmung der Agglomeratgrosen dargestellt, wobei ebenfalls auf die auftretenden Schwierigkeiten bei den Messungen eingegangen wird. Nickelsulfat zeigte die groste Wirkung zur Bildung groser Agglomerate wahrend die Borsaure darauf keinen Einfluss aufwies. Characteristics of nano-scaled Aluminium oxide- and Titanium dioxide-particles in nickel electrolytes The analysis of the agglomeration behaviour of nano-scaled particles as a function of concentration of the single constituents of the Watts-Nickel-Electrolyte and the mechanic convection is topic of this article. Beginning with the description of the measuring principles, the article demonstrates the results of zetapotential measurements and size measurements, and goes into the occurring measurement problems. Nickel sulphate lead to heavy agglomeration, boric acid in contrast did not show any influence on particle size.

A main problem in the field of sheet metal characterization is the inhomogeneous plastic deformation in the gauge regions of specimens which causes the analytically calculated stresses to differ from the sought state of stress acting in the middle of the gauge region. To overcome this problem, application of X-Ray diffraction is analyzed. For that purpose a mobile X-ray diffractometer and an optical strain measurement system are mounted on a universal tensile testing machine. This enables the recording of the whole strain and stress history of a material point. The method is applied to uniaxial tension tests, plane strain tension tests and shear tests to characterize the interstitial free steel alloy DC06. The applicability of the concepts of stress factors is verified by uniaxial tension tests. The experimentally obtained values are compared with the theoretical values calculated with crystal elasticity models utilizing the orientation distribution functions (ODF). The relaxation problem is addressed whi...

Suspension plasma spraying is a process which enables production of finely grained nanometric or submicrometric coatings. The suspensions were formulated with the use of fine powder of ceramic particles of yttria stabilized zirconia in water with alcohol. The present paper focuses on the theoretical analysis of the formation process of sintering of fine solids impacting the growing coating’s surface. The heat flux input to the coatings was estimated and their surface temperature at spraying was measured. The theoretical analysis of sintering during the coating’s growth was carried out. The different models of sintering were analyzed and adapted to the suspension plasma spraying conditions. The model of surface diffusion was found to be the most appropriate to describe the sintering during suspension plasma spraying. The formation of the necks having the relative size equal to 10 % of the particle diameter was found to be possible during the coatings deposition.

Cermets like WC/Co or Cr3C2/Ni20Cr are well-established materials for thermally sprayed wear protection coatings. However, their high price and the adverse health effects of nickel and cobalt cause the motivation for the development of novel materials with excellent wear resistance. Within the AiF/DFG research cluster “Thermal Spraying”, a multi-institutional cooperation of various German research centres, the focus is put on particle-reinforced iron-based composite alloys. High-alloyed steels serve as matrix materials into which hard CrB2 particles are incorporated by means of high-energy ball milling (HEM). By adjusting appropriate milling parameters, the microstructure of the powder and its level of amorphisation can be influenced effectively. The high-velocity oxygen fuel process (HVOF) allows a transfer of the desired nanocrystalline structure from the particles to the coatings. The deposited coatings exhibit low porosity and high microhardness values of more than 1000 HV0.3. The wear resistance of the coatings was determined by means of Miller test (ASTM G75-01) and compared with conventional wear protection materials and coatings produced with agglomerated and sintered powders. The obtained outstanding results qualify particle-reinforced iron-based materials as a promising alternative for a wide range of applications.

Thermally sprayed alumina coatings are widely used in a range of industrial applications to improve wear and erosion resistance, corrosion protection and thermal insulation of metallic surfaces. These properties are required for many components for production processes in the paper and printing industry. By means of efficient and adjustable coating processes, long-term use of the refined surfaces is obtained. It can be seen that cost-efficient arc-sprayed Al coatings post-treated by plasma-electrolytic oxidation (PEO) form Al2O3-layers with outstanding hardness, bonding strength, abrasion and corrosion resistance as well as extended service time. These coatings are designed to partially replace hard chromium.