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.

A multi-layered thermal-sprayed coating system, developed as a resistive heating system, was deposited on a carbon steel pipe. The feasibility of using a 50Cr-50Ni coating as a heating element on top of a conductive substrate was studied. Alumina was deposited to serve as an electrically insulating layer between the metal coating and the substrate to restrict the flow of electrons from the metal alloy heating element to the steel substrate. Continuity, homogeneity, and adhesion of the coating were qualitatively analyzed by studying scanning electron microscope images. The performance of the heating system was determined by measuring the ice temperature and the times required to heat and melt the solid ice that was formed within the pipe. It was found that the coating system was able to generate the heat required to melt the ice in the pipe, thus avoiding the detrimental effects on the pipe of internal liquid freezing. This suggests that the proposed novel resistive heating system can be used on an industrial scale to mitigate or avoid the detrimental effects of ice accumulation in steel and other metallic pipes.

This study investigates the relationship between in-flight particle characteristics, splat formation, and microstructure of CrMoBW-Fe base nanocomposite coatings produced by wire arc spraying. Focused ion beam milling was used to prepare transversal cross-sections of in-flight particles and splats for SE-SEM, EDS, and XRD analysis. The results show that particle size and splat morphology greatly influence the microstructure and properties of the coatings. In-flight particles were found to have a spherical shape with a broad size distribution ranging from 0.8 to 115 μm. The particles were well flattened on impact, resulting in flower-shaped splats with an average diameter of 100 μm. Splats were very thin with fine lamella structure, implying that the particles were fully melted at the time of impact.

Chromium containing metals are an industry staple due to unparalleled corrosion resistance and durability. Unfortunately, many if not all Cr-containing alloys can produce Hexavalent Chromium (Cr 6+ ), a known carcinogen. Studies conducted by the California Air Resources Board (CARB) showed that 30-60 percent of chromium emissions produced in thermal spray processing can contain Cr 6+ . In addition, several independent studies found that Cr 6+ emissions produced from Twin Wire Arc Spray (TWAS) can be up to 3,000 times greater than the legal limits established by the CARB. This study details efforts to develop the next generation of high performance thermal spray alloys which are Chromium free, thereby resulting in zero Cr 6+ emissions. In order to meet these objectives high throughput computational metallurgy was employed. The initial results have shown that next generation alloys can be developed to meet or exceed the performance of incumbent Cr-bearing alloys currently in service, and that the future of Cr-free alloys is on the horizon.

This work assesses the feasibility of using a high-velocity airfuel (HVAF) gun both to grit blast and spray substrate surfaces. A design of experiments (DoE) approach was used to establish relationships between grit blasting variables, substrate surface conditions, and coating properties. Alumina was selected as the abrasive media, the substrates were HSLA steel, and CrC-NiCr and Fe-based powders were used to form the coatings. Uncoated and as-sprayed substrates were characterized based on hardness, residue levels, surface roughness profiles, and adhesion strength, which are correlated with mesh size, feed rate, offset angle, and standoff distance.

In this study, FeCr coatings with different percentages of chromium were deposited on Grade 91 steel substrates by HVOF spraying, followed by aluminum enrichment via pack cementation. Overaluminizing parameters were optimized to prevent changes in the substrate microstructure, making it possible to determine the role of Cr and the effect of Al enrichment on anticorrosion behavior. The combination of HVOF spraying and pack cementation are shown to be adequate techniques for depositing intermediate and top layers in protective coating systems.

This study evaluates the friction and wear behavior of iron-base coatings produced by arc spraying using experimental cored wires. Coating microstructure was analyzed and various wear tests were performed. The results show that the tribological properties of the ferrous coating materials are greatly affected by porosity, oxide inclusions, particle shape, and microhardness.

This paper presents the results of a study on arc-sprayed coatings made from powder-cored wire. The wires used consist of a ferrochromium core with additions of boron, aluminum, and carbon compacted in a steel sheath. The coatings are sprayed using a modified burner operated at 34 V and 140 A with an air jet pressure of 0.6 MPa. During spraying, specimens are rotated at a speed of 60 rpm. XRD analysis is used to examine the initial phase composition of the layers as well as friction-induced changes in the subsurface resulting from block-on-ring wear tests. Cross-sectional examination shows that the coatings have low porosity (∼10%) and small grain size (50 to 150 µm). Based on these findings, it is concluded that the wear resistance of wire arc sprayed coatings is largely determined by powder wire composition, initial coating structure, and the structural stability of the subsurface layer. Paper includes a German-language abstract.

Thermal spray processes are widely used to deposit high-chromium nickel-chromium coatings to improve high temperature oxidation and corrosion behaviour. However, in spite of the efforts made to improve the present spraying techniques, such as HVOF and plasma spraying, these coatings may still exhibit certain defects such as unmelted particles, oxide layers at splat boundaries, porosity and cracks, which are detrimental to corrosion performance in severe operation conditions. Due to low process temperature only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3%Cr- 42.5%Ni - 2.5%Mo - 0.5%B). The coatings were prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power fiber coupled continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of scanning speed and beam width (10 mm and 20 mm). Coating remelted with the highest traverse speed tended to suffer cracking during rapid solidification inherent to laser processing. However, choosing appropriate laser parameters, non-porous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy (OPM, SEM). Dilution between coating and substrate was studied with EDS. The properties of the laser remelted coatings were directly compared with properties of as-sprayed HVOF coatings.

Usually small samples are used to test corrosion resistance of a coating, it is sometimes difficult to make a coating on the small samples. When a cylindrical sample is used and the coating is formed on whole of the surface, three or four times coating processes are required. Thus an abacus bead type sample is proposed in this paper. In this sample, only two times coating processes are required to make coating on whole of the surface. In this paper, the high-temperature corrosion resistance of a novel coating made of Cr-based alloys for use in waste incineration plants is tested and compared with the coating made of a Ni-based alloy. Paper includes a German-language abstract.

CrNiAlTi, NiCrBSi and WC-Ni coatings have been thermal and plasma sprayed projected over a stainless steel surface in order to protect it against heat and erosion actions encountered in power plant boilers. Their microstructure, porosity and microhardness have been measured. High temperature oxidation under an atmosphere similar to service conditions in power plants and thermal fatigue tests have also been performed in our experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer has been evaluated by means of tensile tests. The obtained results have been discussed paying especial attention to the microstructural materials evolution due to thermal effects and coating projection methods.

The central components for solid oxide fuel cells (SOFC) are the electrodes-electrolyte multilayer arrangement (PEN) and the separating bipolar plates. The PEN (Positive electrode- Electrolyte-Negative electrode) assembly consists of a dense gastight yttria-stabilized zirconia (YSZ) electrolyte and porous electrodes for which usually Ni-YSZ cermet anode and Sr-doped LaMnO 3 cathode layers are used. The various PEN units are connected in a cell stack by bipolar plates which are either metallic or ceramic ones. Furthermore, a protective layer on the metallic bipolar plates consisting of a chromium alloy is required to prevent chromium evaporation leading to a rapid and strong degradation of the SOFC performance. At the DLR Stuttgart both the DC and the RF vacuum plasma spraying technique have been further developed and adapted to meet the requirements for the manufacture of the different SOFC components. The DCVPS process using specially developed Laval-like nozzles is especially appropriate to the production of thin and dense coatings as required for the electrolyte and the protective layers. However, applying special spray parameters and nozzles it is also possible to deposit porous electrode layers. The production of the entire PEN arrangement in one consecutive DC-VPS process is the objective of the actual development. On the other hand, the RF plasma spray technique is suitable for the near net-shape production of bulk components such as the metallic bipolar plate. The development of the deposition processes for the production of SOFC components using DC and RF plasma spray methods and the results obtained concerning PEN fabrication, deposition of protective layers and the near net-shape production of metallic bipolar plates are presented in the paper.

A chromium alloy as used for the metallic bipolar plate of a solid oxide fuel cell was processed by RF-plasma spraying to dense free-standing parts. The plasma spray parameters were successfully adapted for two different types of powder. The layer properties, particularly the porosity and the splat shape were investigated in dependence on the spray angles. All the coatings produced with off-normal spray angles show higher porosity increasing from spray angles of 60° to 30°. The splat orientation changes from parallel to the inclined surfaces to almost perpendicular to the plasma jet axis with shallower angles.