In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.

The quality of feedstock powder is a critical factor in determining the properties of coatings deposited using cold spray (CS). However, most commercially available powders are not designed or optimized for CS applications, making it challenging to tailor powders to desired quality. In this research, we investigated and compared the cold-sprayability of four different Cu powders produced by electrolysis and gasatomization methods. We assessed the powders' microstructure, particle morphology and size distribution to understand the effect of manufacturing methods on Cu powder characteristics. We also studied the flowability of the powders using the shear cell method and evaluated their mechanical properties and deformability for CS using nano-indentation. Our results showed that gas-atomized powders with equiaxed grains exhibited promising flowability and deformability for CS applications, outperforming the other powders tested. Specifically, the spherical morphology of gas-atomized powders provided less surface area than the irregular-shaped electrolytic powder, reducing the interaction of surface forces and contributing to smooth powder flow. Additionally, the gasatomized powder with small dendrites in the microstructure exhibited the highest nano-hardness value (HIT= 1.6±0.1 GPa), while the porous electrolytic Cu powder had the lowest value (HIT= 0.7±0.2 GPa). In conclusion, we found that gas-atomized Cu powders with equiaxed grains may hold promise as the optimised feedstock for CS application, considering both effective metrics of flowability and deformability.

Material’s tensile strength can be improved by the presence of a body-centered cubic (BCC) phase, which is essential in highstrength applications and highly corrosive environments. Thus, synthesizing a BCC single-phase, equiatomic AlCoCrFeNi high-entropy alloy (HEA) feedstock particle using a highenergy mechanical alloying (HE-MA) method was investigated. The transient alloy particles were developed using a planetary mill at a constant rotational speed of 580 rpm employing milling times in the range of 4 to 24 hours. During the process, stearic acid of 3 wt.% of the precursor composition was used as a process-controlling agent (PCA). Two HE-MA manufacturing regimes were utilized: i) conventional (milling constituent elements simultaneously), and ii) sequential (progressive milling while adding elements in a certain order). In addition to the conventional method, a sequential regime was employed to develop FeNiCoCrAl, wherein individual elements were added every 4 hours to the starting/milled Fe + Ni mixture. Based on the results, the HE-MA FeNiCoCrAl showed a BCC single-phase formation after 24 hours, with no intermetallic or contamination traceability. Finally, a nanoindentation hardness measurement was carried out to support the observed phase transformation before and after the HE-MA process.

The widespread use of additive manufacturing and modern powder-based technologies (thermal spraying, hardfacing, sintering) encourages the search for alternative routes enhancing the development of metal and metal alloy powders. The state-of-the-art powder production processes, like gas, water or plasma atomization, are dedicated to mass production, which limits the availability of new powder compositions with desired characteristics. In this study, stainless steel powders were investigated. The powders were atomized by an in-house developed ultrasonic (UT) atomization set-up, called ULTRAMIZER. In this system, the atomization of melt is possible by using a high-power ultrasonic field. The atomized powders were characterized in terms of morphology and particle size distribution (PSD). The powder features were then correlated with operating parameters of: (i) UT atomization system, mainly frequency and root mean square power (RMS), and (ii) the orientation of the atomization plate against the melting system, by means of distance and tilting angle. The study shows that the ultrasonic atomization allows producing nearly spherical, defect-free powder particles, with a very narrow and controllable size distribution. These are important advantages over other metal powder production methods, especially when it comes to the development of new types of powder.

The use of metal powders is dynamically increasing in many different fields of research and industry. The development of additive manufacturing technology or innovative thermal spray processes still enhance the range of possible applications. This means that there is a big demand for high quality metal powder materials. Currently, the atomization methods, like water or gas atomization, seem to be most established technologies for metal powder production. This work concerns the development of an innovative technology of metal powder manufacturing, namely ultrasonic atomization. First, the general idea of ultrasonic atomization is discussed. The designing of sonotrode, in order to generate appropriate ultrasonic field responsible for metal stream atomization, is discussed. Then, the ultrasonic set-up was verified by using non-contact fiber optic displacement sensor. Finally, the developed system was preliminary tested under water loading and confirmed positively in terms of ultrasonic atomization capabilities.

The addition of refractory metals represents a promising development approach for future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic alloys with their homogeneous property profile are of particular interest. In the present work, two eutectic HEAs (EHEAs) were developed for the starting Al 0.3 CoCrFeNi using electric arc furnace. Following mechanical and microstructural characterization, the two alloys Al 0.3 CoCrFeNiMo 0.75 and Al 0.3 CoCrFeNiNb 0.5 were identified. For thermal spray processing, powders were prepared by inert gas atomization. The coatings produced by high velocity oxy-fuel (HVOF) spraying were characterized and evaluated comparatively to the castings, allowing process-structure-property relationships to be derived. Based on the results, statements on possible application potential can be made.

In this work, Inconel 718 gas-atomized powder was successfully heat treated over the range of 700-900°C. As-atomized and as-heat treated powders were cold sprayed with both nitrogen and helium gasses. Cold spray of high strength materials is still challenging due to their resistance to particle deformation affecting the resulting deposit properties. Powder heat treatment to modify its deformation behavior has recently been developed for aluminum alloy powders, however, there is no literature reported for Inconel 718 powders. The microstructural evolution of the powder induced by the heat treatment was studied and correlated with their deformation behavior during the cold spray deposition. Deposits sprayed with heat-treated powders at 800 and 900 °C and nitrogen showed less particle deformation and higher porosity as compared to as-atomized deposit associated to the presence of delta phase in the powders precipitated by the heat treatment. In contrast, deposits sprayed with helium using both powder conditions, as-atomized and as heat-treated powders, showed high particle deformation and low porosity indicating that the type of gas has a greater effect on the particle deformation than the delta phase precipitated in the heat-treated powders. These results contribute to understanding the role of powder microstructure evolution induced by heat treatment on the cold spray deposits properties.

Ni-Al intermetallics have excellent corrosion and oxidation resistance, but their use in thermal spraying has been limited due to issues with in-flight oxidation. In this study, a novel approach is proposed to remove oxide from Ni-Al droplets in-flight by adding a deoxidizer (diamond) to the feedstock powder. A mixture of nickel, aluminum, and diamond powders was mechanically alloyed using a combination of cryogenic and planetary ball milling. The resulting Ni/Al/diamond composite powder was then plasma sprayed via the APS process, forming Ni-Al coatings on Inconel 738 substrates. Phase composition, microstructure, porosity, and microhardness of the coatings were characterized by X-ray diffraction, scanning electron microscopy, image analysis, and hardness testing, respectively. Oxygen content measurements showed that the coatings contained significantly less oxygen than coatings made from ordinary Ni/Al powders. In-flight particle temperatures were also measured and found to be higher than 2300 °C. The low oxygen content in the coatings is attributed to the in-situ deoxidizing effect of ultrahigh temperature droplets which are also oxide-free.

A novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotary tube furnace has been investigated as a means to improve the cold sprayability of H13 tool steel powder. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature, cooling rate, and atmosphere), cold spray of H13 powder went from virtually no deposition to the production of thick dense deposits with a deposition efficiency of 70%. Powder agglomeration, surface state, microstructure evolution, and softening are identified as key factors determining powder deposition efficiency and resulting deposit microstructure.

This paper addresses a need for information on nanoparticle emissions and related issues such as worker exposure, filtration efficiency, and dustiness. A survey has been conducted on the working conditions and safety measures used in thermal spray companies and the results compared to scientific literature and previous surveys. Responses to questions on matters of health and safety reveal a lack of information and awareness of the risks posed by the emissions of ultrafine particles generated by thermal spraying processes.

Wire atomization processes used to make refractory and high temperature alloy powders are relatively expensive due to the cost of feedstock, energy, and gas. A new process based on Transferred Arc Wire Atomization technology, however, has the potential to overcome these problems. This paper introduces the innovative process which, in combination with hydrogen generation, presents new opportunities for several alloys that can be more easily processed by plasma wire atomization. The new approach shows promise to reduce both fixed and variable costs for certain refractory and high temperature materials.

In this work, a 2D axisymmetric model of gas atomization at unsteady state that accounts for break-up and solidification is used to simulate laser melting of gas atomized powder. With an optimal nozzle width of 0.6-1 mm and a nozzle angle of 30-32°, the yield of fine 15-45 μm stainless steel powder, suitable for selective laser melting, is shown to increase from 20% to 35%. The effect of laser power on the melting channel width, microstructure, and mechanical properties of the sample is also investigated.

The ongoing development of new HVOF spray guns for internal diameters is driving demand for finer spray powders. Fine spray powders (< 20 µm) are necessary to achieve short spray distances, but they also create new challenges. The first steps in the thermal spray process chain are powder preparation and feeding. If these steps are not stable, no sufficient coating quality can be obtained. This present work compares volumetric and fluidization powder feeding methods and investigates the feeding behavior of agglomerated and sintered WC-Co(Cr) powders with particle fractions of -5+15 µm, -20+5 µm, and -10+2 µm. Particle size fraction was measured ex situ by laser diffraction and particle outflow from the injectors was recorded in-situ by means of particle image velocimetry.

Cold gas dynamic spray has significant potential for load-bearing repairs of high-value metallic components, as it is capable of producing pore and oxide-free deposits of significant thickness and with good levels of adhesion and mechanical strength. However, recently published research has shown that the rapid solidification experienced by gas atomised powders during manufacture can lead to a non-equilibrium powder microstructure, including clusters of dislocations as well as significant localised segregation of alloying elements within each particle. This paper reports on an investigation into the solution heat treatment of a precipitation hardenable aluminium alloy powder. The objective was to create a consistent and homogeneous powder phase composition and microstructure before cold spraying, with the expectation that this would also result in a more favorable heat treatment response of the cold spray deposits. Aluminium alloy 7075 gas atomized powders were solution heat treated at 450 °C for 5 hours in a sealed glass vial under vacuum and quenched in water. The powder particle microstructures were investigated using scanning electron microscopy with electron back scatter diffraction (SEM/BSE) and optical microscopy. The dendritic microstructure and solute segregation in the gas atomized powders was altered, with the heat-treated powder particles exhibiting a homogeneous distribution of solute atoms. The influence on the mechanical properties of the powder particles was studied using micro-indentation. The heat-treated powders exhibited a hardness decrease of nearly 25% compared to the as-received powders. This paper relates the behavior and the deformation of both as-received and heat-treated powders during spraying (single particle impacts), comparing the measured hardness with the deformation effect and the material jetting occurring upon impact.

This paper summarizes the results of a decade-long study on nanoparticle reconstitution and its role in thermal spraying. The effect of the reconstitution process on coating nanostructure was investigated for different materials and applications, a number of which are covered in this report, including Al 2 O 3 -TiO 2 , SiC-Al 2 O 3 -ZrO 2 , and zirconia-based TBCs.

In this work, the aim is to develop a cost-effective coating to protect cast iron and carbon steel from corrosion and wear. An alloy with a composition of FeCr25Mn10BC was designed that could be readily converted to powder form by gas atomization. Different sized powders were produced, characterized, and subsequently sprayed using a three-cathode air plasma generator. It was found that fine powders with fractions of -25 +10 μm and -10 μm had a much higher affinity to oxidation than coarser ones. Nevertheless, using suitable parameters, dense coatings with low oxide content could be realized even with the finest powder. The results show that full utilization of the powder is achievable due to the wide parameter window of three-cathode plasma spraying and that the average deposition efficiency is more than 70%. In addition to savings in material and processing costs, the new alloy system provides greater wear resistance than stainless steel coatings and exhibits significantly higher corrosion resistance than unprotected cast iron and carbon steel.

In this study, yttria films with high thermal shock resistance were synthesized from a metal-EDTA complex by means of combustion flame spraying. A rotating stage and various cooling agents were used to control substrate temperature during deposition. Although thermally extreme environments were employed during synthesis, the obtained films showed only a few cracks and some minor peeling in their microstructures. In the case of a Y 2 O 3 film synthesized using substrate rotation and water atomization, the porosity was found to be 22.8% and the temperature of the film immediately after deposition was 453 °C, owing to a high heat of evaporation in the cooling water.

In this investigation, spherical Cr 7 C 3 -(Ni,Cr) 3 (Al,Cr) powder with different compositions was prepared by vacuum melting and gas atomization. During the solidification process, Cr 7 C 3 is in-situ synthesized and uniformly distributed in the Cr-doped Ni 3 Al phase. Coatings produced from the powders by HVOF spraying were characterized based on composition, microstructure, hardness, and tribological properties. The results show that the coatings compare well with commercial Cr 3 C 2 -NiCr coatings used on piston rings in heavy duty diesel engines. Optimization routes for further improvement are also discussed.

A new nanoparticle plasma spray process has been developed that uses conventional powder feeders and injectors to produce fine ceramic coatings at high deposition rates. This paper explains how powder feedstocks are prepared and how the resulting coatings compare with coatings by other methods. The feedstock used in the demonstration was produced by adding YSZ nanoparticles to an acrylic liquid resin, which was then solidified, crushed, and screened. SEM images show that the nanoparticles are well dispersed throughout the resin fragments. In the plasma flame, the resin fragments burn down as the nanoparticles are heated and accelerated toward the substrate, producing fine zirconia layers free of microcracks and pores as observed by SEM. The presence of carbon deriving from the resin material is dealt with by post-process heating at different temperatures, the effect of which is assessed by means of thermogravimetry. Vickers hardness of the YSZ phase was measured to estimate the sintered density.

The aim of this work is to fabricate nanostructured ceramic coatings using unsintered agglomerated powder and to characterize differences in microstructure, especially those at the nanoscale, due to spraying conditions. Feedstock powders were prepared from commercial YSZ nanoparticles that were reconstituted into solid spheres (70-100 μm) by spray drying. The surface morphology of sintered and unsintered agglomerates was examined by FE-SEM prior to deposition by atmospheric plasma spraying using two gun configurations, one with a lengthened barrel and one with water cooling. YSZ coating cross-sections were examined by optical and electron microscopy, revealing details at the micro and nano scale. The results show that the unsintered agglomerates, which were successfully deposited using both spray guns, are favorable for developing bimodal coating structures with fine grains and porous nano zones.