Abstract Biofouling has been persisting as a worldwide problem due to the difficulties in finding efficient environment-friendly antifouling coatings for long-term applications. Developing novel coatings with desired antifouling properties has been one of the research goals for surface coating community. Recently hydrogel coating was proposed to serve as antifouling layer, for it offers the advantages of the ease of incorporating green biocides, and resisting attachment of microorganisms by its soft surface. Yet poor adhesion of the hydrogel on steel surfaces is a big concern. In this study, porous matrix aluminum coatings were fabricated by cored wire arc spray, and the sizes of the pores in the aluminum (Al) coatings were controlled by altering the size of the cored powder of sodium chloride. Silicone hydrogel was further deposited on the porous coating. The hydrogel penetrated into the open pores of the porous Al coatings, and the porous Al structure significantly enhanced the adhesion of the hydrogel. In addition, hydrogel coating exhibited very encouraging antifouling properties.

Abstract Twin wire arc is a commonly used thermal spray technology for application of steel coatings to cast iron components. Hardness and adhesion strength are critical properties of such coatings, and significant research is available reporting these properties. However, residual stresses and the anisotropic structure of the coatings leads to significantly different behavior in bending applications than in the purely tensile loading of the standard adhesion test. In addition, microstructural features that are controlled by certain process parameters during deposition of the coating can have a significant effect on these properties. This work seeks to relate the hardness and pull-off adhesion strength to the coating microstructure, and to assess the related bending strength and failure mode. Comparisons between bend tests and pull-off adhesion tests show significant differences to consider when evaluating twin wire arc coatings.

Abstract Assemblies containing fiber-reinforced plastic (FRP) and metal parts are typically fastened together via mechanical joining or adhesive bonding. Mechanical joining processes tend to weaken FRP parts by cutting fibers, while adhesives require long cures and often lead to inseparable material compounds. This paper evaluates a new joining method in which plastic parts are laser treated, then metallized via wire-arc spraying, and finally soldered to mating metal parts using a low-temperature process. Due to the effective increase in interface area resulting from laser structuring, bond strengths of up to 15.5 MPa can be achieved.

Abstract Nickel-aluminum alloys are widely used in harsh environments due to their corrosion resistance, high melting temperature, and thermal conductivity. In this work, Ni-5wt%Al coatings were deposited by twin-wire arc spraying (TWAS) on tool steel using a design of experiments approach to study the effect of process parameters on coating microstructure and performance. Test results presented in the form of process maps show how N2 pressure, stand-off distance, and current affect in-flight particle velocity and temperature as well as coating thickness and oxide content. Using this information, optimized coatings were then deposited on test substrates and subjected, along with uncoated tool steel, to several hours of molten aluminum attack. The coated samples showed no signs of physical or chemical damage, whereas the uncoated substrates experienced oxidation, aluminum infiltration, and formation of Fe-Al intermetallics.

Abstract Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

Abstract One approach for controlling the twin wire arc spray (TWAS) process is to use optical properties of the particle beam like length or brightness of the beam as input parameters for a process control. The idea is that changes in the process like eroded contact nozzles or variations of current, voltage and/or atomizing gas pressure can be detected through observation of optical properties of the particle beam. It can be assumed that if these properties deviate significantly from those obtained from a beam recorded for an optimal coating process the spray particle and so the coating properties change significantly. Thus, the goal is to detect these optical deviations and compensate occurring errors by adjusting appropriate process parameters for the wire arc spray unit. One cost effective method for monitoring optical properties of the particle beam is to apply the process diagnostic system PFI (Particle Flux Imaging): PFI fits an ellipse to an image of a particle beam thereby defining easy to analyze characteristical parameters by relating optical beam properties to ellipse parameters. Using artificial neural networks (ANN) mathematical relations between ellipse and process parameters can be defined. Thus in the case of a process disturbance through the use of an ANN-based control new process parameters can be computed to compensate particle beam deviations. In this paper, it will be shown that different process parameters can lead to particle beams with the same PFI parameters.

Abstract A real-time measuring system has been developed to record and analyze the fluctuations of burner voltage and current. The objective is to identify the occurrence of stripping unmolten wire pieces (cold shuts) and to detect corresponding spraying conditions. To identify voltage and current fluctuations causing cold shuts, high speed camera shots are synchronized with electrical measurements to analyze spraying parameters which were assumed leading to cold shuts. As a result, different cold shut conditions could be identified. The electrical cold shut identification then triggers a camera system, to capture online the size and velocity of cold shuts. Based on electrical measurements it is intended to detect wear of spraying equipment which leads to enhanced cold shuts occurrence during industrial spraying process.

Abstract The application of aluminum coatings onto steel for corrosion mitigation is governed by standards specifying surface cleanliness and roughness prior to coating, and minimum coating bond strength. Controlling the surface preparation and spray parameters to achieve the specified surface condition and coating bond strength is challenging, particularly for manual on-site work. In this research, the process parameters were varied and the effect on surface quality and coating adhesion determined. It was found that blasting at angles as low as 30° from the surface, and varying stand-off distances up to 100 mm from the optimum, produced an acceptable surface; whilst spray angles of 60° to 90° and stand-off distances up to 50 mm from the optimum produced acceptable coatings with adhesion above 20 MPa. Adhesion appeared unaffected by a limited amount of remaining mill scale, but was reduced to ≈15 MPa when the surface chloride content was increased from 2 to 20 μg cm-2.

Abstract Thermally sprayed coatings are mechanically bonded to the substrate and present porosities and a lamellar microstructure that make them less attractive in applications requiring high coating toughness and impermeability to gas and liquids, these properties been obtained with more technically advanced overlaying processes. This paper presents the research work carried out to increase the erosion resistance of arc-sprayed coatings containing hard Fe2B crystals dispersed in mild and alloyed steel-based matrices. These arc-sprayed coatings were a) heat-treated in furnace up to 1000°C and b) fused with an oxy-acetylene torch. The sprayed specimens were tested in a particle erosion device at the impact angles of 25° and 90°. The evolution of microstructure was done by SEM and wear damage by Time- Domain Optical Coherence Tomography. It was shown that both the heat treatment and fusing processes considerably enhanced the erosion resistance of coatings particularly at the impact angle of 90°. This increase in erosion resistance is attributed to the disappearance of stringers between sprayed lamellae. Liquid phase sintering is the mechanism responsible for the homogenization of arc-sprayed coatings containing Fe2B. Grain growth observed in arc-sprayed coatings heat-treated up to 1000°C or fused with an oxyacetylene torch does not have a detrimental effect on erosion resistance.

Abstract For coating increasingly complex geometries that require a high accuracy with respect to a near net-shape coating distribution, the use of computer-aided path planning algorithms is mandatory to minimize the need for expensive prototyping experiments. During the planning process, coating simulations are typically evaluated frequently, thus both fast and accurate simulations greatly enhance the potential to find optimal path solutions. In this contribution, an efficient approach for computing the coating thickness on complex workpieces is presented, which makes use of the computational capabilities of mainstream graphics hardware to achieve simulation times well within the single-digit range of seconds for average-sized workpieces. Using a semi-automatic, measurement-based calibration routine, this simulation can easily be adapted to different processes. The simulation has been coupled with a path-optimization approach and was successfully utilized to enhance the deposition accuracy in the context of applying wear-resistant coatings to deep-drawing tools by means of a TWAS process.

Abstract Internal coating of cylinders has always been a challenge for cars engineers. Driven for more than two decades now by the ecological and economical constrains applied to the automotive industry, it constitutes a dynamic way of research and development for industrial applications. One of the most economical processes for this kind of coatings is the rotating twin-wire arc spray (TWAS) system. Meanwhile the actual quality and the performances of the corresponding coatings still leave place for some improvements. Therefore, in the work presented here, attention was paid to the second atomization phenomena in a TWAS system considering the influence of the gas flow parameters on the particles’ morphology and deposition behavior. Numerical modeling of the plume and comparisons between several designs of the second atomizing units were also considered.

Abstract In order to improve the wire arc spray process an inverter power source (PS) from gas metal arc welding (GMAW) process was used to evaluate the influence of current modulation on the formation of coating particles. Using the inverter PS allowed the application of high current pulses with varying amplitude and frequency. It was shown that particle formation can be limited to the high current phases, and that a strong interaction with the gas flow can be observed. The investigations suggest that using this technology new parameters may be introduced to control the wire arc spray process.

Abstract Comprehensive study of the wire arc thermal spray technology will allow for better design and optimization of guns. In wire arc spray, a feed of two electrically-charged wires are melted using an arc. This bath of molten metal goes through an atomization process with a high pressure air being blown upon it. Flow of air will then carry the generated molten drops and deposits them on the substrate. The focus of this study is on the numerical simulation of wire arc sprays using ANSYS FLUENT software. Effects of geometrical paramers on resulting flow conditions and flow circulations inside the gun are studied. Simulation results help in better parameter selection for effective wire arc coating.

Abstract The computational fluid dynamic approach is adopted in this work, using L16-Taguchi matrix, to study the effect of different secondary atomization gas outlet configurations on the gas velocity, jet divergence, and pressure distribution at cap outlet. The spraying process variables that are integrated in this study are primary and secondary atomization gas pressure, PG and SG respectively. In addition, the geometrical variables of the SG air-cap like the position, the number and the angle of the outlet holes for SG are a part of the L16- taguchi matrix. The effect of the process variables and geometrical design variations are analyzed on the obtained gas flow characteristics. Increasing the number of the SG outlet holes leads to a higher gas velocity at the cap outlet. The amount and the angle of the SG outlet holes have a direct effect on the plume divergence. The SG outlet angle determines the distance between the flow intersection point (PG-flow and SG-flow) and the air-cap outlet. Increasing the SG outlet angle leads to a reduction of the gas velocity. The use of Design of Experiment (DoE) in the optimization of the air-cap design by implementing CFD-simulation was proved to be a very useful and efficient tool to design high performance air-caps of twin-wire arc-spraying.

Abstract In the paper, a newly composite TiB 2 /Al 2 O 3 coating prepared by High Velocity Electric Wire Arc Spraying and cored wire with ceramic powders composed of refractory Al 2 O 3 and TiB 2 filling inside was introduced. The microstructure and composition of the TiB 2 /Al 2 O 3 coating were investigated by metallgraphic microscope, scanning electron microscope and X-rayed diffraction, Bonding strength and hardness and mechanics properties were tested. The experimental results show the TiB 2 /Al 2 O 3 composite coating possessed high hardness and higher bonding strength with substrate and excellent comprehensive mechanical properties. It can be used for improving shoot-proof properties in the surface of arms.

Abstract Two cored wires L1 and L2, have been developed to produce arc-spraying coatings for treating high temperature erosion wear of four tubes in the power station, grit erosion experiments under various conditions such as temperature, incident angle, grit weight and shocking velocity have been taken, results show that coating L1 prepared by the new cored wire is more erosion resistance than the comparative materials 20g steel, coating L2 has different behavior varies with the distinct state, from the basis of surface structure and erosion performance, it is also concluded that coating L1 and L2 show mainly plastic erosion wear behavior during the grit erosion process.

Abstract Thermal sprayed coatings are widely used for industrial applications. One of the main characteristics that have to be evaluated in a coating is its microstructure that finally determines the coating performance. Several techniques and processes are available for coatings deposition and new materials have been incorporated in the long list of the available ones. Therefore, since microstructure is a key factor to be evaluated, its preparation has to follow some rules in order to inhibit incorrect statements that can arise from wrong interpretation of an incorrect sample preparation. In this work, a series of distinctive materials are thermally sprayed onto low carbon steel substrates. The metallographic preparation of samples for the different coatings is presented and the effects of correct or wrong sample preparation are discussed and correlated to coating microstructure and process characteristics. The coatings were applied by Air Plasma Spray (APS) and Electric Arc Spray.

Abstract The laser-flash method is used to determine the thermal diffusivity of HVOF sprayed WC-Co(Cr) and Cr 3 C 2 -Ni20Cr as well as APS sprayed Cr 2 O 3 and electroplated hard chromium coatings in the temperature range between RT and 600°C. Additionally bond and/or corrosion protective coatings like Ni5Al, Ni20Cr and 316L are characterized taking into account the different manufacturing methods twin wire arc spraying, HVCW and HVOF. With respect to the application example of drying rollers in paper industries the Taber-Abraser wear test is applied to evaluate the wear resistance. Finally the coatings are characterized concerning their corrosion resistance by salt fog test and by exposure to humid SO 2 environment. For WC-CoCr feedstock the effect of carbide size and micro hardness on thermal, wear and corrosion properties are studied. WC-CoCr coatings with maximum micro hardness and fine carbides show the best thermal conductivity. The use of coarse carbide feedstock permits manufacturing of coatings with the highest resistance against dry abrasive wear, but the protective function depends severely on the processing conditions.

Abstract Low running costs, high spray rates and efficiency make electric arc spraying a good tool for coating large areas with high production rates. The main applications are in the field of corrosion and wear protection of large structures, e. g. parts for bridges or offshore industry. New applications are expected for high quality coatings produced by cored wires. Disadvantages of the arc spraying process are that only electrically conductive wires can be processed and the lower particle velocity in comparison to other thermal spray processes like HVOF or APS. Depending on the process parameters the oxidation of particles has a negative effect on the mechanical and the electrochemical properties of the coating, too. In this paper some investigations with new and flexible power supply systems for arc spraying are presented. The particle size and the morphology of the coating can be optimised, due to the possibility of changing the current generator characteristic and modulating the power by pulsing up to 500 Hz. The oxidation of particles can be reduced by a lower heat input based on lower spray voltage. For a higher quality of the coating microstructure investigations with dynamic generators were performed for Zn, Al, ZnAl, 110MnCrTi8 and Al. An enhancement of the process stability was achieved, too.

Abstract An experimental study was conducted to study the effect of impact velocity and substrate temperature on splat morphology using a wire arc spraying system. Aluminum was sprayed onto polished AISI304L substrates held at various temperatures. In-flight particle parameters (size, velocity, temperature) were measured using a DPV 2000 system. Individual splats deposited on the substrate were photographed. Comparisons between the in-flight particle size distribution and the final splat size distribution showed that increasing substrate temperature results in increased mean spread factor. A transition temperature from splash to disk splats was determined. Coatings were also produced at various substrate temperatures and porosity levels measured. The effect of substrate temperature on deposition efficiency and coating adhesion was measured. Increasing substrate temperature was found to improve both deposition efficiency and adhesion strength.