The deposition of MCrAlX coatings (where M is Ni, Co, Fe, or a combination of these, and X is Y, Si, Ta, Hf, or a combination of these) via thermal spraying has acquired significant importance in industries such as aerospace, power plants, oil, and gas, etc. Among various thermal spray deposition techniques, high-velocity air fuel (HVAF) has shown a growing potential for the deposition of metallic powders which are sensitive to high-temperature oxidation during spraying. Thus, it is essential to understand the in-flight behavior of these metallic particles in the high-velocity, low-temperature HVAF flame. In this work, a NiCoCrAlY powder was sprayed using two sets of HVAF deposition parameters onto stainless steel substrates. In-flight particle diagnostic tools such as AccuraSpray were employed to understand the behavior of these spray particles. The deposited particles were comprised of partially molten particles and fully deformed splats. Samples with higher powder feed rates showed a primary coating buildup on the substrate surface. EDS plots revealed no traces of inflight particle oxidation but contained carbon residue due to the presence of unburnt hydrocarbons from the fuel-rich HVAF-M3 torch. This study provides a preliminary understanding towards the significance of deposition parameters on the in-flight particle oxidation behavior and splat deformation characteristics by HVAF spraying.

Aircraft gas turbine blades operate in aggressive, generally oxidizing, atmospheres. A solution to mitigate the degradation and improve the performance of such components is the deposition of thermal barrier coatings (TBCs). Specifically for bond coats in aerospace applications, High Velocity Air Fuel (HVAF) is very efficient for coating deposition. However, internal diameter (ID) HVAF has received little attention in the literature and could be a promising alternative to limit oxidation during spraying when compared to conventional methods. The main objective of this study is to analyze how the ID-HVAF process influences the microstructure of NiCoCrAlY coatings. To that end, an i7 ID-HVAF torch is used to deposit NiCoCrAlY splats on a steel substrate with different stand-off distances. The deposited splats showed the presence of craters, and both partially melted and deformed particles at the surface. The particle velocity data was recorded, and the splat deformation and amount of particles deposited was shown to be directly corelated to the stand-off distance. The material composition analyzed and quantified by Energy Dispersive Spectroscopy (EDS) did not reveal any traces of in-flight of particle oxidation, but further investigation is required. This study provided a preliminary understanding towards the importance of stand-off distance on the splat deformation and in-flight oxidation.

High-velocity air-fuel (HVAF) is a combustion process that allows solid-state deposition of metallic particles with minimum oxidation and decomposition. Although HVAF and cold spray are similar in terms of solid-state particle deposition, slightly higher temperature of HVAF may allow further particle softening and in turn more particle deformation upon impact. The present study aims to produce dense Ti-6Al-4V coatings by utilizing an inner-diameter (ID) HVAF gun. The ID gun is considered a scaled-down version of the standard HVAF with a narrower jet, beneficial for near-net-shape manufacturing. To explore the potential of the ID gun in the solid-state deposition process, an investigation was made into the effect of spraying parameters (i.e., spraying distance, fuel pressure, and nozzle length) on the characteristics of in-flight particles and the attributes of the as-fabricated coating such as porosity, oxygen content, and hardness. Using online diagnostics to monitor temperature and velocity of in-flight Ti-6Al-4V particles is challenging due to exothermic oxidation reaction of fine particles, while larger particles are too cold to be detected from their thermal emission. However, DPV diagnostic system was successfully employed to differentiate the non-emitting solid particles from the burning ones. It was found that increasing air and fuel pressure of the ID-HVAF jet led to an increase of the velocity of the in-flight particles, and resulted in improved density and hardness of the as-sprayed samples. However, increasing the spraying distance had a negative effect on the density and hardness of the deposits. It was also observed that the phases of the Ti-6Al-4V deposits were altered by producing vanadium oxide due to the high temperature of the spray jet.

The exceptional properties of Ti-6Al-4V of high strength, lightweight, corrosion resistance and machinability make it one of the most widely used alloys in in the aerospace industry. Significant efforts are underway to establish powder bed additive manufacturing (AM) technologies for Ti-6Al-4V. There are also increasing attempts to use thermal and cold spray to build near net shape parts with buildup rates orders of magnitude higher than powder bed. Thermal spraying, such as HVOF, can oxidize and degrade the alloy due to the high processing temperature. Lowering the flame temperature through inert gas addition in full-size HVOF systems is a possible approach to retain solid state deposition of the feedstock particles, thereby limiting oxidation and detrimental α-case formation, while providing sufficient heat input for particle softening and plastic deformation at impact. Novel miniaturized HVOF systems, with spray jets of only a few millimetre in width, may further offer the possibility to improve the spatial resolution of the buildup for near net shape forming. The process parameter range for solid state deposition of Ti-6A-4V, using the liquid fuelled TAFA Model 825 JPid and the novel hydrogen fuelled Spraywerx ID-NOVA MK-6 with the addition of nitrogen will be discussed. Build-ups at over 80% deposition efficiency generally yield as-sprayed porosities below 3% and hardness above 200 HV100gf. Attainable microstructures and oxygen content as a function of spray parameters are delineated. Recrystallization and beta annealing of selected samples lowered the residual porosity and created equiaxed α and intergranular ß-phases. Ultimate tensile strengths of up to 1100 MPa were attained, however, at limited elongation.

Additive manufacturing processes have been used to produce or repair components in different industry sectors like aerospace, automotive, and biomedical. In these processes, a part can be built by either melted particles as in selective laser melting (SLM) or solid-state particles as in the cold spray process. The cold spray has gained significant attention due to its potential for high deposition rate and nearly zero oxidation. However, the main concern associated with using the cold spray is the level of porosity in as-fabricated samples, altering their mechanical properties. These pores are primarily found in the regions where adiabatic shear instability does not occur. It is worth noting that the deformation of the impacted solid particle plays a vital role in reaching the shear instability. Therefore, for investigating the adiabatic shear instability region, an elastic-plastic simulation approach has been used. For this purpose, it is assumed that an elevated temperature solid Ti6Al4V particle impacts on a stainless-steel substrate surface at high velocity. The results show that increasing particle temperature will significantly enhance particle deformation because of thermal softening. Additionally, they illustrate that a material jet responsible for producing a bonding between particle and substrate by ejecting the broken oxide layer will be formed when the particle has a temperature above 1073 K and substrate remains at room temperature. In the end, it should be noted that increasing particle temperature up to 723 K will not have a significant effect on substrate deformation and final substrate temperature.

In this work, numerical models are developed and used to simulate magneto-hydrodynamic fields inside a dc plasma torch during suspension plasma spraying and their influence on arc attachment. A Reynolds stress model is used to simulate turbulent plasma flow and a discrete phase model simulates the effects of arc fluctuation on suspension droplets in the plasma jet. Submicron yttria-stabilized zirconia particles, suspended in ethanol, are modeled as multicomponent droplets and the KHRT model is used to simulate their breakup. The results show that particles are significantly affected by plasma arc fluctuations and that fine particles near the centerline of the torch are hotter and experience better penetration into the plasma jet.

Water droplet erosion (WDE) is a well-known phenomenon. This type of erosion is due to the impingement of water droplets of several hundred microns to a few millimeters size at velocities of hundreds of meters per second on the edges and surfaces of components. The solution to this problem is in high demand especially for the moving blades of gas turbines’ compressors and those operating at the low-pressure (LP) end of steam turbines. Thermal sprayed tungsten carbide based coatings have been the focus of many studies and are industrially accepted for a multitude of wear and erosion resistance applications. The present work studies the microstructural, phase analysis and mechanical properties and their effects on water droplet erosion resistance of such coatings deposited with high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) processes. The feed nano-agglomerated tungsten carbide-cobalt powders are in either sintered or non-sintered conditions. The WDE tests were performed using 0.4 mm water droplets at 300 m/s impact velocity. The study shows promising results for this cermet (better than the Ti6Al4V bulk material) as WDE resistant coatings when deposited using HVOF or HVAF processes under optimum conditions.

Obtaining a uniform coating on curved mechanical parts such as gas turbine blades is one of the industrial challenges in suspension plasma spraying. Through a three dimensional numerical analysis, this study is aimed at providing a better understanding of the effect of substrate curvature on in-flight particle temperature, velocity and trajectory. The high temperature and high velocity plasma flow is simulated inside the plasma torch using a uniform volumetric heat source in the energy equation. In addition, yttria stabilized zirconia (YSZ) suspension is molded as a multicomponent droplet while catastrophic breakup regime is considered for simulating the secondary break-up when the suspension interacts with the plasma flow. A two-way coupled Eulerian-Lagrangian approach along with a stochastic discrete model was used to track the particle trajectory. Particle size distribution in the vicinity of the substrate at different stand-off distances has been investigated. The results show that sub-micron particles may obtain higher velocity and temperature compared to the larger particles. However, due to the small Stokes number associated with sub-micron particles, they are more sensitive to the change of the gas flow streamlines in the vicinity of a curved substrate

This study investigates the effect of surface morphology on the wetting behavior of atmospheric plasma sprayed (APS) titanium dioxide. It is shown that with appropriate control of particle temperature and velocity and the use of stearic acid treatments, it is possible to achieve TiO 2 surfaces with water contact angles as high as 144°, which is close to a superhydrophobic state. Such coatings were found to exhibit a rough and irregular surface morphology and were obtained by increasing particle velocity while reducing particle temperature.

In this study, a 3D two-way coupled Eulerian-Lagrangian approach is used to model the plasma jet and droplet-particle trajectory, velocity, and temperature achievable by suspension plasma spraying. A Reynolds stress model is used to account for turbulence and the effect of the substrate on the flow field and a Kelvin-Helmholtz Rayleigh-Taylor breakup model is used to predict the secondary breakup of the suspension. The focus of this work is on particle behavior near the substrate. Flat substrates placed at stand-off distances ranging from 40 to 60 mm are modeled to provide detailed information on particle impact behavior.

Intrinsically active nickel electrodes with large porous surface areas have shown promise for producing hydrogen by alkaline water electrolysis. In this study, Ni powder and NiO suspensions were sprayed on Inconel 600 substrates with an atmospheric plasma gun, producing single (Ni, NiO) and double layer (Ni-NiO) coatings. Top surface morphologies were examined, revealing both micro and nano scale features. Based on kinetic parameters obtained from steady-state polarization measurements, APS-SPS coated electrodes are the most catalytically active and thus have the most potential for hydrogen evolution. It is believed that the nanoscale structure increases effective surface area while the microporous structure facilitates mass transport and overcomes hydrogen bubble blockage.

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.

In this study, a downstream injection cold spray nozzle is modeled numerically under various loadings. Instead of micron-sized particles, liquid feedstock as a carrier of nanoparticle suspension is fed into the nozzle through a port located 6 mm downstream of the nozzle throat at the diverging section. Water is used as the liquid carrier with a droplet size distribution of 5-100 µm and liquid-to-gas ratio ranging from 5 to 15%. The radial injection of droplets is simulated by Lagrangian particle tracking which includes the effects of heating and evaporation. The effect of the feedstock on downstream flow is analyzed and the optimum solid-to-liquid fraction in the suspension is determined.

In this study, atmospheric and suspension plasma spraying are used to create nickel-based electrodes with enhanced surface area as required for hydrogen production. Optimal spraying conditions were determined using a Taguchi design-of-experiments approach. Electrochemical double-layer capacitance measurements by cyclic voltammetry show that suspension plasma spray coatings have more surface area than coatings produced by atmospheric plasma spraying. SEM micrographs show that the surface microstructure of the sample with the largest surface area contains high amounts of cauliflower-like aggregates with an average diameter of 10 µm. In general, the combination of melted, semi-melted, and resolidified particles leads to the formation of deposits with high porosity, rougher surfaces, and consequently larger surface areas.

This study compares two methods for modeling the breakup of droplets during suspension plasma spraying. One is based on Taylor analogy breakup, the other on Kelvin-Helmholtz Rayleigh Taylor breakup. A three-dimensional model with two-way coupling is used to simulate flow within the plasma plume and interactions between suspension droplets, and a Reynolds stress model is used to simulate gas field turbulence. After breakup and vaporization, the solid suspended particles are tracked through the domain to determine the characteristics of coating particles. The numerical results are validated against experiments using high-speed imaging.

This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A de Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled equations. Water spray with a droplet size of 10 – 100 μm is modeled using both uniform and Rosin-Rammler size distributions. The two-phase flow of gas-liquid is modeled using an unsteady discrete phase mass source with two-way coupling with the main gas flow. Upon injection, the droplets in the water spray evaporate while travelling through the nozzle due to momentum and energy exchange with the gas flow. The evaporation behavior in presence of water content is modeled and a correlation between the initial diameter and the diameter just before the throat is obtained. As a result, the proper droplet size distribution with a fully evaporative spray can be used as a carrier of nano-particles in cold spray nozzles. Having the results, guide us to substitute the un-evaporated part of the droplet with an equal diameter agglomerate of nano particles and find a minimum fraction of nano particles suspended in the liquid which guarantees fully evaporative liquid spray injection.

In this study, suspension plasma spraying is used to deposit pseudo eutectic alumina-yttria stabilized zirconia as a potential thermal barrier coating. Process variables including feed rate, powder size, and plasma gas composition were altered to determine the influence of spray parameters on the formation of phases in the composite coating. The most significant variable was found to be the auxiliary gas. The gas influences the formation of phases primarily through its effect on in-flight particle velocity.

Suspension Plasma Spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using extra small particles as compared to conventional thermal spraying. Suspension spraying involves, atomization, solvent evaporation and melts consolidation, which can cause substantial complexity in the system. Using feedstock mixtures for composite coatings, such as alumina and zirconia, intricacy of the system increases even more. There is consequently a need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/ 8 wt% yttria stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated by using Taguchi design of experiment (DOE). The microstructural features include microcracks, porosities and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e. temperature and velocity were also measured. The role of the porosity in this multi-component structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity.

The two-phase flow properties of copper particle laden nitrogen are measured and compared to computational fluid dynamic calculations, determining the achievable degree of consistency between model and reality. Two common, commercial nozzles are studied. A two-way coupled Lagrangian scheme along with the RSM turbulence model is used to track the particles and to model the interactions between the gas and the particulate phase. Significant agreement is found for the geometrical gas flow structure, the resulting particle velocities, and the dependence of the two-phase flow on the particulate phase mass loading. The particle velocities decrease with increasing mass loading, even for modest powder feed rates of less than 3 g/s. The velocity drop occurs even when the gas flow rate is kept constant. Adiabatic gas flow models neglecting the energy consumption by the particles are thus inaccurate, except for very dilute suspensions with low technical relevance. For the cases modeled, the experiments evidence the high predictive power of the chosen approach.

A three dimensional model of a Cold Gas Dynamic Spray system with a peripheral non-axisymmetric powder feeder is studied in this work. It is found that the stagnation pressure alternates for different substrate stand-off distances due to the nature of the supersonic flow interaction with the substrate. One can find the optimum substrate location for any given operating condition which results in minimum pressure buildup on the substrate. The three-dimensional analysis shed more light on the complex gas and particle flow fields generated due to the three-dimensional particle injection process. Additionally, the three-dimensional model allows us to further investigate the effect of practical substrate shapes (such as convex and concave) on the flow field and consequently to determine the optimum conditions to deposit coating particles.