Shrouding a thermal spray process to reduce the oxidation of particle stream is not a new concept. Nevertheless the actual use of this idea is not very widely used in practice, due to some problems arising. In this paper the results of the study performed to optimize two shrouding devices for thermal spray are presented. Gas flows inside the device and in between its outlet and the target were modeled by Computational Fluid Dynamics both for an arc-wire and a plasma system. To validate the modeling methodology measurements were performed of the oxygen content using a gas sampling probe and a paramagnetic O 2 analyzer. A good agreement between calculated and measured O 2 values was obtained. Modeling results in different conditions could suggest the best shroud configuration and shielding gas flows necessary to reduce the oxygen content around the particle stream at very low values. Coatings with very low defect contents can now be produced.

An important aspect of the APS plasma spray process is the turbulent mixing of the spray jet with the surrounding air. The air mixing into the jet causes undesirable oxidation of the sprayed coating. In this work the air mixing in the plasma jet was determined by direct measurement of the oxygen content. The measuring method is based on electrochemical determination of the oxygen potential using a solid electrolyte cell. The partial pressure of oxygen along the centreline of the plasma jet was measured with the hydrogen/argon and helium/argon ratio, the gas flow rate and the stand-off distance as experimental parameters. The oxygen content of the plasma tail flame was found to vary between 13.6 to 19.3 % depending on the hydrogen to argon ratio and the stand-off distance. Such high oxygen contents are far too high to avoid serious oxidation of metal coatings. The plasma spraying tests were carried out with WC-Co 17 coating powder. The plasma gases were Ar/H 2 and Ar/He. The respective oxygen contents by air mixing were measured to be 18.9 and 17.3 %. The WC-Co was observed to be decarburised more in the Ar/H 2 mixture than in the Ar/He mixture, which was attributed to the higher oxygen concentration, higher particle temperature and longer flight time in the plasma jet. Solid electrolyte cell technique was applied to this type of measurements and it proved to be a convenient way to determine the oxygen mixing in the plasma jet.

The adhesion mechanisms involved in the cold spray coatings are not still well elucidated. The quality of the deposit does depend mainly on particles and dynamic characteristics (which result from nozzle type, nozzle-substrate distance, etc.). The present work is based on the study of particle-substrate and particle-particle interfaces in the tantalum-copper coating-substrate system. The content focuses on the influence of the oxygen content in the starting powder on interface features, consequently on coating properties. Tantalum powders with different oxygen levels were studied using SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis). Laser shock spallation of cold-sprayed Ta coatings was developed as a reliable and flexible process to achieve Ta spalls to be deposited at a high-velocity onto Cu targets. The velocity due to the laser shock could be controlled to be similar to that of particles in conventional cold spray. This results in Ta-Cu interfaces, the study of which was carried out to go into interface phenomena involved in cold spray, using TEM (Transmission Electron Microscopy) in particular. Results were compared to those obtained from laser shock spallation of Ta bulk specimens (i.e. made of a conventional Ta sheet). The role of powder oxidation on interface soundness was exhibited. Adhesion was shown to be all the lower as powder oxygen content was higher, using LASAT (“ Laser Shock Adhesion Test”) in addition to direct observation of interfaces. Results were exploited to discuss properties of the corresponding Ta coatings onto Cu, i.e. which were cold sprayed using powders with different oxygen contents.

Oxidation of HVOF sprayed 316L stainless steel coatings was studied experimentally. Oxygen content in the sprayed coatings was analyzed and its dependence on several spray parameters such as spraying distance, mixture ratio of fuel to oxygen, and composition of atmospheric gas on the substrate was studied. The oxygen content in the original powder was about 0.03 wt%, which typically increased to 0.3 % in the HVOF sprayed coatings under the standard spraying conditions. Reduction of spray distance significantly increased the oxygen level due to the excessive heating of substrates by the flame. The sprayed deposits were analyzed by XRD and the oxides within the coatings were identified as magnetite Fe 3 O 4 or chromite FeCr 2 O 4 . By using a nitrogen-gas shield attached to the substrate, it was revealed that the oxidation during flight is around 0.2 wt%. Control of oxidation by attaching a gas shroud to the HVOF nozzle has been attempted and oxygen content below 0.15 % has been achieved so far while maintaining deposition efficiency over 73 %.

Atmospheric plasma sprayed (APS) CuNiIn coatings have been widely used for fretting wear protection in many important areas such as aircraft engines for decades. The oxides in CuNiIn coating prepared by APS hinder splat bonding formation and thus degrade the coating fretting performance. In this study, CuNiIn powders of different boron contents were designed to realize the self-oxide-cleaning effect for in-flight molten droplets and thus deposit the dense CuNiIn coating with high fretting performance. Scanning electron microscope was used to characterize the microstructure. The oxygen content in the coating was measured by the inert gas fusion technique. Fretting test was performed for three coatings under different loadings. The results show that CuNiIn2B and CuNiIn4B coatings presented the oxide content of 0.40wt% and 0.38wt%, which are lower than 1.6wt% of the CuNiIn coating. The oxygen content in the CuNiIn4B coating decreased with the increase of spray distance while the oxygen content in CuNiIn coating increased with the increase of the spray distance. Such results clearly reveal the boron in-situ deoxidizing effect of inflight molten droplets. As a result, the dense CuNiIn2B and CuNiIn4B coatings were deposited with oxide-free molten droplets. The test results showed that the fretting wear performance of B-alloyed CuNiIn coatings were increased by a factor over three comparing with conventional CuNiIn coating.

Thermal spraying of dense titanium coatings in the air atmosphere was achieved by using a two-stage high velocity oxy-fuel process (HVOF) called the Warm Spray Process. In the process nitrogen gas is mixed with the combustion gas to lower the gas temperature. Gas dynamics modeling of the flow field of the gas in the spray apparatus as well as the acceleration and heating of titanium powder injected from the powder feed ports were conducted. Based on the obtained temperature history of a titanium powder particle, its oxidation during flight was also predicted by using a Wagner-type oxidation model. These results were compared with measured velocity and temperature of sprayed particles by DPV2000 and the properties of deposited coatings. Significant discrepancy in the temperature of sprayed particles was found between the calculation and measurement whereas the measured velocity was closer to the model calculation. The model prediction of oxygen content was in a good agreement with the analysis of actual coatings. Furthermore, properties of the sprayed coatings such as porosity, oxygen content were correlated with the particle velocities and temperatures. Nitrogen gas was highly effective in lowering the oxygen content, but excessive nitrogen addition caused the coating porosity to increase due to insufficient particle temperatures.

Critical velocity is an important parameter in cold spraying. It determines the deposition efficiency under a given spray condition. It depends not only on material types, but also particle temperature and oxidation conditions. In this present work, three types of materials including copper, 316L stainless steel, and Monel alloy were used to deposit coatings by cold spraying. The critical velocities of spray materials were determined using a novel measurement method. Oxygen content in three powders was changed by isothermal oxidation at ambient atmosphere. The effect of oxygen content on the critical velocity was examined. It was found that critical velocity was significantly influenced by particle oxidation besides material properties. The critical velocity of Cu particles increased from about 300 m/s to over 610 m/s with a change in oxygen content in the powder. The results suggest that with a severely oxidized powder, critical velocity tends to be dominated by the oxide on the powder rather than material properties.

The product quality of selective laser melting (SLM) is closely related to the alloy powder characteristics, including the size distribution and the oxygen content. In this work, the 316L stainless steel powder was prepared by a vacuum atomization furnace and sieved into a normal-sized distribution range from 15 to 53 μm with a median diameter of 37.4 μm, and a fine-sized distribution range from 10 to 38 μm with a median diameter of 18.9 μm. Then they were mixed with each other in different proportions. The results show that, under the condition of the same SLM parameters, the SLM part, with adding a large amount of fine-sized powder, has a lower density and strength, as well as more holes and spheroidized particles, compared with the SLM part with adding a small amount of finer-sized powder. Furthermore, the 316L stainless steel powder with a high oxygen content was prepared by a non-vacuum atomization furnace. Although the 316L stainless steel powder with a high oxygen content can be evenly spread in the SLM process, the surface layer of the powder is easy to form an oxide film during the cooling and solidification of powder inside the molten pool. Under the action of thermal stress, the small crack forms and expands along the oxide film, eventually leading to large cracks inside the melt channel.

Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

Since its commercial introduction in the early 1980s, it has been known that HVOF spraying can produce layers with low oxide levels, high toughness, and excellent adhesion strength. Unlike low-pressure and vacuum plasma spraying, the HVOF process does not require a controlled atmosphere. As a result, the oxygen content of the wettable powder plays a significant role in layer quality. In this paper, the oxygen content of normal MCrAlY powder is varied and its influence on coating quality is examined. It is shown that, by limiting the initial oxygen content in spray powder, higher density HVOF layers with lower oxide proportions can be achieved. Paper includes a German-language abstract.

A conventional GTV K2 kerosene fuel HVOF spraying system has been modified with the aim to achieve process conditions comparable to cold gas spraying concerning the average particle velocities and surface temperatures in the spray distance. The employed measurements include the use of expansion nozzles that have been optimized for supersonic conditions up to a Mach number of 2.5 and the use of combustion chambers with reduced critical diameters that provide increased combustion chamber pressures up to 1600 kPa. Copper powders with different size fractions and oxygen content are sprayed with the novel HVOF technology. Coatings are analysed concerning their microstructure, oxygen content and electrical conductivity. In-flight particle velocities and surface temperatures are determined by the GTV NIR Sensor. Results are compared with those obtained for cold gas spraying using identical powders. The new HVOF technology permits the production of copper coatings that show similar levels of porosity, oxygen content and electrical conductivity like cold gas sprayed coatings. Also aluminium powder has been sprayed successfully with the novel technology. In-flight particle velocities can be almost as high as in modern cold gas spraying systems. Coatings are analysed and show a microstructure comparable to cold gas sprayed coatings.

In the course of this investigation, thermal spraying with different fuel and shroud gas combinations was investigated in terms of its effect on the in-flight particle properties (temperature, velocity) and on the final coating properties (coating thickness, porosity, oxygen content and corrosion behaviour). Independent on the shroud gas, the particle in-flight temperature and velocity were highest when using ethylene as fuel gas and lowest when using propane. Methylene resulted in intermediate properties. The change in the shroud gas from air to nitrogen generally resulted in lower in-flight particle temperatures and also lower velocity. The coating properties in terms of porosity and oxygen content directly correlated to the particle in-flight properties. With decreasing velocity and increasing temperature, the porosity and the oxygen content increased, respectively. The corrosion behaviour of the nickel coatings was studied in 0.5 M sulfuric acid media by means of potentiodynamic polarization curves. Good corrosion properties were observed when methylene and air served as fuel gas and shroud gas, respectively.

The effect of particle size range on oxidation behavior was investigated according to exposure time in isothermal oxidation condition. Emphasis was placed upon oxygen content, porosity, and oxide scale formation. Commercially available CoNi- and CoCrAlY powders of several different particle size ranges were vacuum-plasma sprayed on a nickel alloy substrate. The results show that the isothermal degradation of coatings is considerably influenced by the particle size distribution. It can be clearly observed that a remarkable increase in the oxygen content in the as-sprayed coating occurred with a decrease in the mean particle size. But after thermal exposure, the difference of the oxygen contents between the smaller and larger particle coatings is decreased. The distribution of particle size plays the important role of porosity than only the mean particle size. The powder which has the widest range and sample variance leads to make good porosity inside coatings during the deposition process.

Recent experimental investigations of reactive spray deposition of aluminum alloys have indicated that oxides could not be detected for atomization gas oxygen contents lower than 10%. In order to elucidate this behavior, an analysis of the oxidation kinetics during reactive spray deposition based on the Mott-Cabrera theory of oxidation is proposed herein. A linear growth law is obtained that indicates that the oxide growth rate decreases with decreasing temperature or oxygen pressure. Furthermore, the oxide growth rate is found to decrease faster at low oxygen pressure with decreasing temperature as well as at low temperature with decreasing oxygen pressure. Calculations of the width of oxide stringers as a function of oxygen content and superheat temperature based on this analysis are in good agreement with the experimental observations.

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.

This is the second paper of a two part series based on an integrated study carried out at the State University of New York at Stony Brook and Sandia National Laboratories. The goal of the study is the fundamental understanding of the plasma-particle interaction, droplet/substrate interaction, deposit formation dynamics and microstructure development as well as the deposit properties. The outcome is science-based relationships, which can be used to link processing to performance. Molybdenum splats and coatings produced at three plasma conditions and three substrate temperatures were characterized. It was found that there is a strong mechanical /thermal interaction between droplet and substrate, which builds up the coating/substrate adhesion. Hardness, thermal conductivity increase, oxygen content and porosity decreases with increase of particle velocity. Increasing deposition temperature resulted in dramatic improvement in coating thermal conductivity and hardness as well as increase in coating oxygen content. Indentation reveals improved fracture resistance for the coatings prepared at higher deposition temperature. Residual stress was significantly affected by deposition temperature, although not to a great extent by particle conditions within the investigated parameter range. Coatings prepared at high deposition temperature with high-energy particles suffered considerably less damage in a wear test. The mechanism behind these changes is discussed within the context relational maps which is under development.

Warm Spray has demonstrated that it could fabricate comparatively dense metal coatings keeping with high purity during the atmospheric process. Its key technology is the control of the temperature of the supersonic combustion jet prior to supplying feedstock. So far, even titanium (Ti), known as one of materials difficult for the atmospheric process, could be deposited with less oxidation and higher density of the resulting coatings. For instance, the porosity and oxygen content of two coatings obtained were 2.3 vol% and 0.28mass%, and 1.1vol% and 0.92mass%, respectively. Further densification of Ti coatings was achieved by bi-modal size distribution of feedstock powder upon Warm Spraying in this study. When bigger Ti particles were mixed with the usual feedstock powder under 45 µm, the coating porosity was decreased to 0.8vol% simultaneously with the low oxygen content of 0.26mass%, which was comparable to the level of feedstock powder. This densification is caused by the balance of the enhancement of the peening effect by big particles and of optimization of the filling rate of the big and small particles.

Previous studies have shown that gas shrouding is an effective means for controlling oxidation during HVOF spraying. In this present work, the authors attach a gas shroud to an oxyfuel torch with a longer barrel to further investigate the correlation between the state of HVOF sprayed particles and the density and oxygen content of the resulting layers. It is shown that with gas shielding, extended barrel length, and optimized spraying parameters, it is possible to accelerate powder particles to a velocity of over 750 m/sec with maintaining a high molten fraction, thereby producing very dense (zero porosity) stainless steel layers with oxygen contents less than 0.2% by weight. Paper includes a German-language abstract.

Reactive plasma spraying (RPS) with hydrocarbon (HC) gas has been studied as a way to improve the mechanical properties of Ni-Cr alloys and reduce the oxygen content of MCrAlY coatings. A conventional dc plasma torch has been modified by attaching a conical graphite tube (reactor) onto the end of the gun and spraying parameters were adjusted accordingly. Significant improvements have been achieved in terms of both objectives. As test results indicate, the hardness of Ni-Cr alloys has been doubled and the oxygen content in MCrAlY coatings has been reduced by an order of magnitude.

Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.