The applications of Wire Arc Spraying (WAS) include large-area corrosion protection coatings e.g. the protection of off-shore wind power plants. While WAS is cost efficient and well-known, the inherent instabilities can lead to coating defects and subsequent vulnerabilities in the corrosion protection coating. The occurrence of these process-related fluctuations cannot be predicted by deterministic models. However, these fluctuations can be monitored in situ, analyzed and finally minimized. A sensor unit is set up on the free jet of a WAS process using ZnAl15 wire. Voltage, amperage, noise and wire feed rate are measured in situ at a sampling rate of 80 MHz. Following a design of experiments approach, 64 different parameter settings are run and measured. For that purpose, voltage, atomizing gas and wire feed rate of the free gas jet have been varied. A generalized linear model (GLM) is trained on the dataset. A Fast Fourier Transformation (FFT) in conjunction with smoothing filters is conducted. Adopting the GLM enabled the calculation of parameters that minimize process fluctuations. Plots in the form of response surfaces depict the influence of the varied parameters on the process stability. A signal analysis using FFT revealed major periodic changes of the voltage in the range of 0.5-1 kHz next to process control-related frequencies at 20 kHz. The mounting and structuring of the data as well as the calculation of key figures is fully automated. Due to the high degree of automation, large quantities of data can be processed. In the future, a simplified version of the adopted sensor unit may be adopted to optimize parameters in an autonomous way. This can ensure not only the minimization of process fluctuations for any chosen feed wire, but also indicate irregularities in the process. The high-resolution recording and automated analysis of the data allows the determination of optimized parameters as well as major underlying frequencies.

In cold spray, optimum process conditions to accelerate particles vary with different densities and melting temperatures of the materials. Therefore, material-specific nozzle designs are required. In the present study, a nozzle geometry optimization concept based on 3D-CFD simulations was developed to provide a specific nozzle design for a given material. Al6061 and pure copper with mean particle diameters of 40 μm were taken as examples. Together with a design of experiments (DoE) approach, the model seeks for the optimal nozzle geometry. In order to reach the highest particle velocity prior to impact upon the substrate, different geometry parameters were varied, such as the nozzle throat cross section, the aspect ratio, and the nozzle divergent section length. The process gas was nitrogen with set stagnation pressure and temperature of 50 bars and 500 °C. For both materials, the simulation identified nozzle divergent section length as the most influential parameter, followed by the throat cross-section. The aspect ratio must be tuned to avoid over expansion of the gas in the nozzle.

In the semiconductor industry, plasma etching processes are widely used. Process chamber parts that are located in the plasma etching system are also exposed to the harsh environmental conditions. Thus, parts located close to the process area are typically coated with yttria to increase service life, and thus process performance. However, such yttria coatings are usually porous, and thus can be attacked by fluorine containing plasma. In order to increase the lifetime of the components in the plasma etching system, this research project aimed to improve the protective yttria layer by reducing the porosity of the protective layer. Specifically, a design of experiment was employed in which the porosity was the target value. The main effects of the coating parameters and their interactions including the surface treatment before the coating process were determined. Furthermore, the bonding of the protective coating to the component to be protected, as well as the element distribution and the coating morphology were investigated. The results and their ramifications with respect to the envisaged application will be discussed.

This study investigates the solid particle erosion performance of cold sprayed tungsten carbide-nickel coatings using alumina particles as erodent material. After coating fabrication, specimens were annealed in an electric furnace at a temperature of 600 °C for 1 hour. The coatings were examined in terms of microhardness and microstructure in the as-sprayed (AS) and annealed (AN) conditions. Subsequently, the erosion tests were carried out using a General Full Factorial Design with two control factors and two replicates for each experimental run. The effect of the annealing on the erosion behavior of the coating was investigated at the two levels (AS and AN conditions), along with the impact angle of the erodents at three levels (30°, 60°, 90°). Finally, two regression models that relate the impact angle to the mass loss were separately obtained for the two cold spray coatings.

Acquisition of a new LVPS and APS coating system at Delta Air Lines necessitated optimization of the coating parameters on both systems, especially for application of bond coat (LVPS) and top coat (APS) for a TBC coating system. To expedite the coating optimization, it was determined that a design of experiments (DOE) approach would best enable the establishment of the operating window for the two systems. Samples prepared were primarily evaluated for their performance while exposed to a cyclic oxidation cycle. Samples were also evaluated for the microstructure and composition using energy dispersive spectroscopy (EDS) analysis. Samples from the ceramic coating DOE were also evaluated for their erosion characteristics. Results indicate a low correlation between the individual bond coat parameters evaluated to the furnace cycle life. However, the top coat spray parameters were found to have a greater correlation to furnace cycle life and erosion performance.

Internal diameter (ID) coating by means of thermal spraying for the wear and corrosion protection of components is currently experiencing growing interest in science and industry. While high-kinetic spray processes (such as HVOF, HVAF or warm spraying) in combination with cermet materials (e.g. WC-Co or Cr3C2-NiCr) are well established for this purpose in traditional coating of external OD (outer diameter) surfaces, they have hardly been used in the ID (internal diameter) area so far. Even though a few special ID spray guns with compact design and low combustion energy are by now available on the market, only little is known about the effects and interactions of the spray parameters on the particle behavior and the coating properties. Due to the mentioned gun specifications and the usually required short spray distances for ID coating, fine spray powders < 15 μm must be used to ensure sufficient melting and acceleration of the particles. In this study warm spraying of fine WC-12Co powders (-10 + 2 μm) using a novel spray gun “ID RED” (Thermico, Germany) was investigated. Statistical design of experiments (DoE) was employed to analyze and to model the influence of varying spray parameter settings on the in-flight particle behavior and the corresponding coating properties.

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.

This study investigates the effect of preheating on the dynamic flowability of HVOF powders, including conventional WC-Co, nano WC-Co, WC-FeCrAl, and Cr 3 C 2 -NiCr. The results show that the flowability of WC-Co powders can be significantly improved with a two-hour preheat at 200 °C. One explanation for the improvement is that moisture absorbed by the powder is released during pretreatment, but further study is required as it was found that dynamic density influences flow behavior as well.

Thermal barrier coatings (TBCs) with high thermal strain tolerance and erosion resistance are commonly applied onto the inner and outer diameters of hot sections of gas turbine engine components. In this work, strain tolerant, segmented TBCs with a variety of crack densities and porosities were developed using the SinplexPro cascaded torch. Design of experiments were carried out to study the effect of process variables such as plasma power, powder feeding rate, spraying distance and surface speed on the coating microstructure and properties. Optimized process parameters for the segmented coating microstructures at shorter spray distance (<75mm) and longer spray distance (>114mm) are achieved, which are targeted for spraying inner diameter and outer diameter engine components, respectively. The plasma torch hardware life was evaluated by torch cycle duration runs. Examples of highly strain tolerant TBCs onto the ID and OD engine components were demonstrated, highlighting the wide versatility and process range of the SinplexPro.

Due to good performance in abrasive and sliding wear and enhanced oxidation behavior, coatings based on Co-Cr-W alloys are widely used in industrial applications, where the material is exposed to high temperature. Within the scope of this study, a Co-based alloy similar to commercial Stellite 6, which additionally contains 20.6 wt.% of vanadium, was deposited by Twin Wire Arc Spraying (TWAS). Multi-criteria optimization using statistical design of experiments (DoE) have been carried out in order to produce adequate coatings. The produced coatings have been analyzed with respect to their tribological behavior at elevated temperatures. Dry sliding experiments were performed in the temperature range between 25°C and 750°C. Oxide phases were identified in the investigated temperature range by X-ray diffraction (XRD) using synchrotron radiation. The V-doped Stellite-based coating possesses a reduced coefficient of friction (COF) of about 0.37 at elevated temperatures (above 650°C), which was significant lower when compared to conventional Stellite 6 coating that serves as reference. In contrast, both produced coatings feature a similar COF under room temperature. X-ray diffraction reveals the formation of cobalt vanadate and vanadium oxides above 650°C. The formation of vanadium oxides exhibits the ability of self-lubricating behavior, thus leading to enhanced tribological properties.

The high wear resistance of Cr 3 C 2 -NiCr coatings is reliant on the formation of a dense coating containing a high percentage of carbide grains, with minimal carbide degradation. Such coating characteristics are typically achieved through the use of high velocity oxygen fuel (HVOF) spraying. The propane fuelled, manually operated HIPOJET 2700 HVOF system is one of a suite of smaller sized commercial HVOF systems recommended for smaller job shops. However, few works have characterised the properties of carbide composite coatings produced with this system. In this work a full factorial design of experiment analysis was used to assess the effect of key operating parameters on the quality of Cr 3 C 2 -NiCr coatings. The combustion parameters (fuel and oxygen flows) were fixed at the manufacturers recommended settings in order to focus on the effect of nozzle length, powder feed rate and, spray distance. The effect of these variables on the porosity/oxide content, carbide content, microhardness, coating thickness, and relative deposit efficiency is discussed.

In this work, a design-of-experiments approach is used to map the main parameters of a high-velocity airfuel (HVAF) spraying process. Chromium carbide based material with a NiFeCrSi matrix was sprayed with varying gas flows and nozzle designs while monitoring their effect on particle temperature, velocity, and coating build-up. It was found that sufficient heating is critical to abrasive wear resistance and that particle temperatures are primarily controlled by fuel flow rates. Nozzle geometry, on the other hand, had the biggest effect on particle velocity, which was found to increase nearly 100 m/s by switching from a cylindrical to a convergent-divergent design.

In this work, process models combining statistics, materials science, and mechanics were developed to optimize and qualify HVOF sprayed coatings as if for aerospace application. In an attempt to correlate process variables with typical coating properties, a preliminary microstructure model was developed using statistical data obtained from 34 design-of-experiments runs. A second model, based on fatigue testing, was also developed. The fatigue model was subsequently used to guide the selection of variables for a final model that is shown to aid in both process qualification and the optimization of process-operating parameters.

Non-destructive eddy current evaluation is a practical and well-established tool in aerospace and other industries, used to find or identify material defects not otherwise detectable. It can also be employed to measure the thickness of various coatings, although it is not yet fully optimized for multi-layer thermal spray systems, such as thermal barrier coatings (TBCs). The first part of this paper aims to look at the underlying mechanisms of the eddy current thickness measurement technique and uses a Design of Experiment (DoE) study to identify key characteristics related to thickness measurement of thermal barrier coatings. The second part of the paper is a case study on the application of the findings into general production, showing the achieved improvements in accuracy and repeatability of thickness measurements.

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.

One of the economical and fast solutions for failure against erosive wear in oil and gas industries is the deposition of cermets using HVOF thermal spray. Recently, especially with the new development of bimodal feedstock powders, the composition percentages of the mixed powders have played a key factor in the final coating performance. In the present study, a design of experiment (DOE) software was implemented to study the influence of different powder percentages on the coating performance. The coating mechanical properties and its performance were investigated via dry solid particle erosion tests, hardness measurement and SEM respectively. The results showed that both the hardness and erosion resistance of the coating increases as the composition percentage of the nanostructured WC-12Co increased due to the strong adhesion of WC nano size grains at the substrate/coating interface as a result of improved mechanical interlocking.

In this work, CFD simulations are used to evaluate air cap configurations for twin wire arc spraying (TWAS). Investigators employed a design of experiments (DoE) approach to identify air cap parameters with the greatest impact on gas velocity, jet convergence, and pressure distribution. The ones selected for study are the convergence angle, the length and diameter of the throat, and the distance between the air cap outlet and the point where the wires intersect. In all configurations studied, the spray wires deflected the flow of the primary gas and narrowed the cross-section of the plume along one axis. The effects of each air cap parameter are discussed in the paper along with possible design improvements.

In this study, a design of experiment (DOE) approach is used to investigate the influence of powder size and mixing ratio on the microstructure and properties of thermal spray coatings. Nano and micro sized WC-12Co powders combined in different proportions with Inconel 625 were deposited on carbon steel substrates by HVOF spraying. The resulting coatings are examined and the effect of different powder combinations on hardness and yield strength is assessed. Spraying procedures and test methods are described and the findings are discussed. Of the various coatings produced, one shows great promise for wear protection, particularly in oil and gas applications.

This study evaluates the wetting behavior of TiO2 coatings deposited by atmospheric and suspension plasma spraying. A design-of-experiments method is used to investigate the effect of different spray parameters on the water contact angle (WCA) of the coatings. Despite the hydrophilic nature of TiO2, coatings with WCAs as high as 140° were achieved by controlling various spray parameters. SEM imaging shows that these coatings have a cauliflower-like surface morphology that repels and sheds water.

Bioactive coatings are proven to enhance bone regeneration, implant integration and act as drug-delivery systems following bone replacement surgeries. Polycaprolactone (PCL) was used in this study as coating material due to its superior biocompatibility and biodegradability. Polymethyl-methacrylate (PMMA) was used as an additive in order to improve the flowability of the PCL powder. The processing technique used to obtain polymeric coatings was oxy-acetylene flame spraying. Seeing that biodegradable polymers were not thoroughly investigated in the past, a Design of Experiments (DoE) analysis was necessary in order to understand the effects of spraying parameters on coating characteristics (thickness, roughness, adhesion, wettability) and to be able to optimize the coating properties for specific requirements. The polymer matrix was sprayed onto titanium substrates. The statistical analysis was followed by FTIR spectroscopy, which showed that the coatings underwent little chemical degradation. Finally, biocompatibility tests showed that cells proliferated well on the flame sprayed polymer coatings, which confirms that the coating technique used did not affect the biological performance of the material.