This research establishes a foundation for enhancing suspension plasma spraying stability and efficiency. Building on previous work, we evaluated three gas mixtures using the Axial III torch with a consistent nozzle throat diameter. Our analysis focuses on in-flight particle measurements and coating performance for each gas mixture, while accounting for the plasma transport coefficients. Additionally, we recorded voltage and current measurements across all three cathode-anode units.

Numerical studies directly quantifying particle-substrate adhesion under different spray process parameters during impact remain scarce, primarily due to the lack of consideration for adhesion models. This study addresses this gap by investigating bonding behavior under varying spray conditions using a single-particle model with the PD numerical method, incorporating adhesion forces.

In this study, two particle-tracking velocimetry systems, the Oseir HiWatch HR2 and the Oseir HiWatch CS2 were tested. The particle-sizing velocimetry results were then compared with corresponding particle characteristics obtained using a Tecnar Cold Spray Meter and a Tecnar Accuraspray 4.0 unit. In addition, particle size distributions measured by laser diffraction were included for validation.

The objective of this study is to optimize thin electric and mechanical functional copper coatings using atmospheric plasma spray by determining the impact of particle temperature and velocity on coating properties. A particular focus is placed on the formation of real contact between the particles within the coating, which is crucial for electrical conductivity while the contact at the interface is essential for adhesive strength.

The aim of this study is to develop a solid shroud to minimize in-flight oxidation using the particle swarm optimization (PSO) algorithm. Additionally, the shroud is specially designed to use air as a cooling medium instead of water and is made from common stainless steel to help reduce equipment and process costs.

This research proposes an experimental methodology towards visually observing high strain rate polymer deformation characteristics at scales relevant to cold spray particle impacts. Macro-scale (~ 3 mm) polymer impact testing via a light gas gun has shown evidence of cold spray indicative features at certain (material, particle/substrate temperature, velocity, etc.) conditions.

In this research work, MoNbZrTiV and AlNbTaTiV refractory high-entropy alloy (RHEA) material combinations were investigated as potential candidates for feedstock materials for cold spray additive manufacturing. The two RHEA materials as precursors for developing micron-sized particles were alloyed mechanically through high-energy ball milling following a rigorous material design-of-experiments curriculum on account of elemental melting point differences. Detailed particle characterization techniques were employed to gain insights into the RHEA particle properties.

This study focuses on a novel online cold spray process monitoring system that uses in-situ diagnostics during cold spraying and cold spray additive manufacturing. It combines the in-flight behavior of particles with the expected properties of the resulting coatings.

In this study, axisymmetric two-dimensional numerical analysis was performed to clarify the particle behavior in supersonic hybrid aerosol deposition (HAD). The predicted result showed that the small particles, which can deposit via HAD, impact the substrate on a wide region while the large particles, which can abrade the surface of film and substrate, impact around the center.

This study investigates the effects of varying particle size and particle size distribution of stoichiometric Yb 2 Si 2 O 7 feedstock powder on SiO volatilization, Yb 2 SiO 5 secondary phase formation, and crack behavior in Yb 2 Si 2 O 7 EBCs.

Thick deposits were produced from pure Al powder of three different sieve sizes using cold spraying at the same process parameters. The in-plane mechanical and fracture properties of the deposits were investigated using bending of small specimens in four specimen orientations. It was shown that increasing the Al particle size by approximately 50% and 100% leads to small, but statistically significant differences of yield strength. Further, the increase in the powder particle size led to higher fracture toughness K IC but lower fatigue crack growth threshold ΔK thr . This can be attributed to two different fracture mechanisms in the cold sprayed deposits. A trans-particular fracture in the near-threshold fatigue regime is controlled by the microstructure and work hardening of the particles. At higher cyclic loads and in quasi-static regime, the particle decohesion and the resulting crack path determine the fracture behavior instead. However, the observed effect of particle size was rather small, much smaller than the effect of spray process parameters observed in the previous research.

The deformation behavior of particles plays a significant role in achieving adhesion during cold spray. The deformation behavior of the particles is associated with the fracture of the oxide layer and recrystallization, which are the key elements of the quality of cold spray. Studies of particle compression have been made to understand the deformation behavior of a particle. However, the deformation behavior of particle under controlled load and precise and high strain rate is yet to be studied. Here, we show the oxide layer fracture pattern and recrystallization regime under controlled load with a precise and high strain rate. We found that the cracks in the oxide layer initially appeared on the equator of the particle and propagated towards the edge of the top surface. Meanwhile, on the top surface, the circumferential crack was developed. On the other hand, the nanoindentation result showed that the compressed particle under a high strain rate has an unusual load-displacement behavior. Our results demonstrate that the oxide layer fracture behavior corresponds to the adhesion mechanism suggested by previous studies. Our study also revealed that recrystallization takes place within the particle under a high strain rate. We anticipate this finding to give a general insight into the deformation behavior of particles during cold spray. For instance, since the recrystallization behavior at a given strain rate can be predicted through this study, the resultant grain size and shape, which is associated with mechanical properties, can also be predicted. Furthermore, the amount of strain and strain rate to form optimal adhesion can be evaluated.

In plasma spraying, H2 or N2 is commonly added to the primary Ar plasma which may increase the specific enthalpy, thermal conductivity and thus improve the process efficiency. The objective of this study is to provide a process characterization of a three-cathode plasma torch with various binary gas compositions. Several process diagnostics are used to characterize the impact of binary plasma gas mixtures in plasma spraying. High-speed video analysis is utilized to capture the jet fluctuations of the studied process parameters. In addition, current and voltage measurements are performed to further complement the plasma diagnostics. The impact of the binary plasma gas mixtures is determined using particle diagnostic system DPV-2000 by measuring the particle in-flight properties of Al 2 O 3 feedstock. Furthermore, the deposition efficiency (DE) of the investigated process parameters is determined. The results show that at the identical volumetric flow rate and current, the addition of H2 yields the highest particle temperatures, followed by Ar/N2 mixtures and pure Ar plasma. In reverse order, pure Ar plasma results in the highest particle velocities. In addition, the increased DE of plasma spraying with binary gas mixtures for Al 2 O 3 coatings offers the potential to increase the deposition rate of other ceramic materials. This study provides a comprehensive correlation between plasma and particle diagnostics and the deposition efficiency of binary plasma gas mixtures.

Oxygen present in the High Velocity Air-Fuel (HVAF) process can react with the in-flight metallic particles and cause their oxidation. A grown brittle oxide shell on metallic microsize particles can reduce their deposition efficiency and impair the coating final deposited properties/microstructure. In the current study, the oxide growth of MCrAlY particles, where M stands for Ni and Co, during their flight in the HVAF process has been modeled using the particle tracking scheme. A comprehensive theoretical oxide layer growth background is presented and used to track the particle oxidation process. The oxidation development includes the Mott-Cabrera theory for very thin films, which is function of the particle surrounding temperature and oxygen partial pressure. The very thin film regime, applicable under a limiting thickness, is defined based on the electric field present across the growing oxide layer. As the electric field decreases with oxide thickness, the oxidation rate is determined by thermal diffusion. The obtained results provide a correlation between HVAF system design and surface oxidation phenomena while offering a clear description of different oxidation stages.

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.

The feasibility of processing various polymers by cold spray has been exemplified by depositions with low porosity and properties comparable to the bulk material. However, cold sprayed polymers are generally deposited with low deposition efficiency compared to more extensively studied metal sprays. Low efficiencies in polymer sprays are attributed to characteristic differences in material properties between metals and polymers. Notably, the thermophysical properties of polymers limit heat transfer and promote intra-particle thermal gradients that develop during cold spray processing. These properties (e.g., thermal conductivity, heat capacity, density) and low deposition efficiencies demand alterations to the cold spray process equipment outside typical metal powder spray conditions. Herein, a modified powder feed tube is used to pre-heat powder to temperatures (~84 °C) below the powder melting point, or cool it (~-55 °C) below room temperature before contacting the high velocity carrier gas in the nozzle of a CSM 108 cold spray system. Numerical simulation demonstrated that pre-heating/cooling the powder feedstock is a viable means of adjusting particle temperature upon impact with the substrate; however, this technique has generally not been deliberately utilized in the cold spray of polymers. In the present work, no significant increase in deposition efficiency (~65% for all sprays) was found by increasing the pre-heat temperature. However, pre-heated particles had a mechanical strength 28% higher than particles injected at room temperature and -55 °C. Despite this, scanning electron microscope images indicated no notable differences between the deposit microstructures. Future works are planned to study the effect of pre-heat at higher particle impact velocities and degrees of pre-heat to improve powder consolidation.

In this work, the possibility of controlling the thermally sprayed TBC microstructure is in order to improve the overall TBC system performance. The control is possible primarily by metallic bond coat surface microtexturization prior to ceramic top coat spraying. Such pretreated bond coat was modeled to investigate the influence of the substrate topography on the plasma stream behavior as well as the feedstock particle thermophysical properties and trajectories in the substrate closest proximity. The microscale computational domain was considered here. It was extracted from entire spraying domain and located in the microtextured substrate boundary layer. Then, advanced flow models were introduced to the governing equations to define heat flux to the substrate, turbulent flow, and plasma jet / feedstock droplets interaction. Feedstock discrete phase was defined by the means of Discrete Phase Model (DPM) including particle drag laws and DPM source modelling. The motivation for this study was to model and investigate the influence of the bond coat microtexturization on the behavior of the feedstock particles in the substrate boundary layer. This opens the possibility of better understanding the TBC build-up mechanism and strictly controlling the microstructure of such TBCs.

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.

Thermal spray is a widespread technology on the way to large scale production. Driven by industrial importance, companies of all sizes employ this technology, with research mainly driven to improve quality and reduce cost. In order to produce high-quality coatings, efforts have been made to provide on-line process control. To find out which parameters affect coating formation and thus which of these parameters should be monitored high level cost intensive diagnostic systems have to be used. This work introduces selected diagnostic systems for the on-line process control under the condition of serial production: shadow imaging for the particles (PSI), a system to measure particle velocities (PVI) and an integral working system using a qualitatively image algorithm (PFI).