Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract Refractory borides are suitable for wear protection due to their excellent material properties. In addition to their high hardness, they have excellent corrosion resistance and very good wetting behavior of metals. In addition, the density of many borides, such as titanium diboride, is significantly lower than that of conventionally used toilets. In this paper, the investigations show, that mechanical alloying permits the production of nanostructured titanium diboride composite feedstock for thermal spray application. In order to prevent the formation of undesired secondary borides, a suitable Ni-based alloy is selected for production. A liquid fuel-operated HVOF system is used to prevent the fine microstructure of the composite powders from degrading during thermal spraying. The sprayed layers are characterized in terms of layer morphology and wear behavior and compared with conventional cermets. Paper includes a German-language abstract.

Abstract This paper uses three types of gases (standard Air, nitrogen, and Ar) as atomization gas, and the arc spraying is carried out in air atmosphere. It examines the microstructure change according to the type of atomization gas and the influence of gas elements on crystalline structure of the titanium coatings by X-ray diffraction and quantitative analysis of nitrogen and oxygen contents in the coatings. The paper aims to research the influence of atomization gas on nitriding and oxidizing process of the titanium. Furthermore, with regard to Ar atomized coating, it investigates the change of crystalline structure with increase of spraying distance in order to research the effect of inert gas, Ar. An influence on the nitrogen content is determined. It turns out that TiN0.3 is formed due to the effect of alpha-titanium as a nitrogen-stabilizing element. Paper includes a German-language abstract.

Abstract The different HVOF processes have established themselves alongside the conventional thermal spray processes for the production of wear and corrosion protection layers. The increasing demands on functional component surfaces with regard to mechanical, chemical, and thermal loads lead to the development of novel material systems for spray application. Due to the synergy of excellent material properties, SiC is of great interest for thermal spraying. High-energy grinding is an interesting and economical alternative to the production of SiC-containing composite powders. In this paper, mechanically alloyed SiC composite powders with different metallic matrices are produced. These SiC composite powders are processed using HVOF. The sprayed layers are characterized in terms of layer morphology and wear behavior and compared with conventional cermets. Paper includes a German-language abstract.

Abstract Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Abstract In this paper, the production of NiCr-TiC powder by SHS, suitable for HVOF spraying, is discussed together with results on the microstructure and coating properties. Compacts for SHS were prepared by mixing elemental Ti and C with pre-alloyed Ni-20wt.% Cr powder to give an overall composition of 35wt.% NiCr and 65wt.% TiC. These were then ignited and a self-sustaining reaction proceeded to completion. Reacted compacts were crushed, sieved, and classified to give feedstock powders in size ranges of 10-45 µm and 45-75 µm. All powder was characterized prior to spraying based on particle size distribution, x-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS). Thermal spraying was performed using both H2 and C3H6 as fuel gases in a UTP/Miller Thermal HVOF system. The resulting coatings were characterized by SEM and XRD analysis, and the microstructures correlated with powder size and spray conditions. Abrasive wear was determined by a modified 'dry sand rubber wheel' (DSRW) test and wear rates were measured. It has been found that wear rates comparable to those of HVOF sprayed WC-17wt% Co coatings can be achieved.

Abstract High Velocity Oxygen Fuel (HVOF) spraying established itself as an effective method in addition to the conventional thermal spray processes within a very short period. Self fluxing nickel alloys, cermets (e.g. WC-Co / Cr3C2-NiCr) as well as oxide ceramic coatings have proved themselves suitable for wear protection applications. Weight reduction, the care of resources and the increase of efficiency for structural components leads to the substitution of customary hard particles. Titanium carbide (TiC) characterizes itself on account of the material features such as the high hardness, the high melting point, the high strength and the low density for the substitution of conventional carbides. The Self Propagating High Temperature Synthesis (SHS) is a suitable process for the production of composite powders. The powders produced by SHS show a high carbide content, which is finely distributed with an almost stoichiometric composition of the TiC inside the powder particles. The carbides are protected against dissociation and oxidation during the thermal spray process by a complete velum of matrix alloy. The current investigations deal with the wear resistance of TiC-composite coatings produced by HVOF compared to conventional wear resistant coatings. The investigations contain the analysis of the microstructure by optical and scanning electron microscope (SEM) and the measurement of the microhardness of the deposited coatings. Special attention is drawn to the interface between the hard particles and the matrix alloy. The optimized coatings are tested with different wear tests, such as Taber-Abraser test, sliding and oscillating wear test and are compared with common wear resistant coatings in order to underline the high potential for different wear applications. Moreover an additional corrosion test (salt fog test) is carried out with regard to the corrosion resistance of the different matrix alloys.

Abstract Molybdenum silicides have the potential as protective coatings for high-temperature applications because of their high melting point and their high-temperature oxidation resistance. Reinforcing MoSi2 with SiC shows an improvement of its low toughness at room temperature and low creep resistance at temperatures above the brittle-ductile transition temperature of approximately 700-1000 °C. A new kind of powder processing was used to produce MoSi2 and MoSi2-SiC as a feedstock for thermal spraying. Mixtures of the elemental powders, molybdenum and silicon, were prepared by milling and subsequent heat treatment to get highly dispersed, pre-reacted powders. As high-energy milling equipment, a planetary ball mill was used to prepare the powders. In the case of reinforcement, SiC was mixed to the pre-reacted MoSi2 at the end of the milling process, that means before heat treatment. On these as-milled powders, X-ray diffraction characterization (XRD), scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and determination of the oxygen level were carried out. Vacuum plasma spraying has been used to deposit the powders onto a carbon steel substrate. Evaluated coating characteristics were the microstructure (SEM), phases (XRD), EPMA, oxygen content, microhardness and surface roughness. Tests at high temperatures will be considered in future work.

Abstract The plasma-spray process is specified by the associated processing parameters, where these influence the properties of the resultant deposits. This article describes the preparation and processing of composite powders for use in thermal spraying by mixing high purity zircon and alumina powders. The spheroidized powder were obtained by high energy ball milling and rapid solidification from the molten state during plasma spraying. The article discusses the processes involved in spray drying and plasma spheroidization, describing thermal analysis and mullitization kinetics in the spheroidized alumina/zircon mixtures.

Abstract An experimental study of the spheroidization efficiency of induction plasma processes was completed. The main objective being to obtain models which could be subsequently used for the prediction of the spheroidization efficiency for various powders and plasma operating conditions. Silica, alumina, chromium oxide and zirconia powders were treated during the experimentation. For the plasma treatment of the powders the installation used had a maximum available power of 50 kW with an operating frequency of 3 MHz. Operating conditions were varied such to minimize side reactions and the evaporation of powders. The resulting powders did show the presence of cavities and a slight change in the mean diameters. The maximum energy efficiency based semi-empirical model did predict the spheroidization efficiency of the particles beyond a defined critical point known as the maximum energy efficiency point. For the model, the maximum energy efficiency is distinct for the individual powders but remain within a defined range which is reflected in the small variations in the Z constant.

Abstract The growing need for new materials and material combinations with superior properties for severe service applications has led to the development of near net-shape forming techniques for certain materials, such as superalloys, refractory metals (Ta, W, and Mo) and highly reactive metals (Ti and its alloys). Vacuum plasma spray (VPS) was used to produce dense Ti-6Al-4V deposits for mechanical properties evaluation. Spherical Ti-6Al-4V powder, produced by Plasma Atomization (PA), a novel patented powder fabrication technique, was used as the starting powder. Plasma atomized Ti-6Al-4V powder characteristics include: high purity, tight particle size range, highly spherical with no attached satellites, and excellent flowability. The resulting as-sprayed Ti-6Al-4V deposits were dense and low in oxygen content. Thermal treatment was conducted after spraying in order to improve the structure and the properties of the spray formed material. The mechanical properties of the material, including tensile strength, elongation and hardness, in both the as-sprayed and the heat treated conditions were compared. The mechanical properties of these preliminary VPS Ti-6Al-4V specimens indicate that the combination of high purity starting powder and controlled environment deposition can be used to produce dense spray formed Ti-6Al-4V structures with properties comparable to those of cast or sintered powder metallurgy parts.

Abstract This study was aimed at the production of SiC-MoSi2 composite powders through a high-temperature plasma reaction route. The addition of SiC appears to be the best second phase reinforcement for improving the mechanical properties of MoSi2 material for high-temperature structural application. The in-flight carbonization of MoSi2 powders was carried out in an Ar-H2-CH4 induction plasma process. Using methane served as both the powder carrier gas and the "precursor" to react with the MoSi2 powders forming the SiC phase in-situ . Under the experimental conditions employed in this investigation, up to about 8.0 wt. % of carbon was incorporated into the MoSi2 powder particles. The chemical composition, phase content and the microstructure of the composite powder products were examined by XRD, SEM, EDS etc. analysis methods. The reaction mechanisms are discussed in terms of the calculated thermodynamic equilibria.

Abstract A new family of spherical powders produced by the spray drying route has been developed. This paper describes as an example the manufacturing method of an Y2O3-coated aluminum powder. Atmospheric Plasma Spraying (APS) was used to test the corresponding coatings. Morphology and phases of powders and coatings were investigated by optical and scanning electron microscopy while the level of porosity was evaluated using image analysis. Results show that homogenous composite coatings can be obtained from cladded spray dried powders.

Abstract Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce Inconel 718 coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled Inconel 718 powders, as well as the HVOF Inconel 718 coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth as evidenced by a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good correspondence with the experimental results.

Abstract A new plasma atomization process was developed for the production of spherical metal powders. This process, unique in its ability to produce large quantities of fine powders (< 75 µm), is especially interesting for value added products such as refractory or reactive metals/alloys powders. The excellent feeding characteristics of spherical powders make them ideal for applications such as injection molding and vacuum plasma spray (VPS) deposition. Powders are known to have a significant influence on the final characteristics of VPS deposited coatings. A variety of factors will control the coating quality, including: powder chemistry, morphology, microstructure, and feeding behaviour. This article includes a review of titanium coatings, and new results obtained using plasma atomized spherical powders.

Abstract Thermal-sprayed titanium coatings were investigated as anodes for impressed current cathodic protection systems for steel reinforced concrete structures. The coatings were applied by twin-wire thermal-spraying using air and nitrogen as atomizing gases. The coatings were non-homogeneous due to oxidation and nitridation of the molten titanium with the atmospheric gases oxygen and nitrogen. The primary coating constituents were α-Ti (containing interstitial nitrogen and oxygen), γ-TiO and TiN. Nitrogen atomization produced coatings with less cracking, more uniform chemistry, and lower resistivity than air atomization.