A study is carried out of the spheroidization of ceramic and metallic powders using induction plasma technology. The process is based on the central injection of the powder in the plasma discharge followed by the in-flight cooling and solidification of the molten droplets prior to their collection at the bottom of a stainless-steel water cooled chamber. The degree of spheroidization is evaluated using image analysis. The results are correlated as a function of the powder feed rate, the plasma operating conditions and the thermophysical properties of the powders treated. The model's fit to the obtained experimental data is very good. The results show that the technology can be successfully used for the spheroidization and densification of a wide range of materials.

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.

Spheroidization of powder particles is one of the successful commercial applications of induction plasma technology. A review is presented of case studies in which powder densification and spheroidization using induction plasma technology has played a key role in substantial improvement of powder quality and fluidity. Results are given for both metallic and ceramic powders at the pilot plant and industrial scale production. The presentation will cover both technical and economic features of the process. A detailed economic analysis of the process is presented for a production capacity of 15 and 30 kg per hour of tungsten carbide powder.

Plasma spheroidization utilizing a plasma spray gun has been demonstrated for water-atomized stainless steel SUS316L powder. The angular particles were successfully spheroidized, and the D50 size of particles were decreased from 35 μm to 24 μm with 9 kW of processing power and to 15 μm with 17 kW of processing power. It was found that the high processing power of 17 kW generates a significant number of fine particles with the size of under 1 μm. By contrast, the powder formed on low processing power of 9 kW has better flow-ability and low cohesiveness, suggesting that an appropriate processing power exists to form the spherical powder suited for additive manufacturing

Inductively coupled radio frequency (RF) plasma spraying, powered by high-frequency oscillating electrical current, performed an important role in fine powder manufacture. It was used in the present study to prepare fine spherical bioceramic powders of hydroxyapatite (HA) whose chemical composition similar to those of natural bone. The as-sprayed powders consisted of both micron-sized spherical particles and nano-sized particles. In addition to the spheroidization effect, rf plasma treatment led to the decomposition of HA into secondary calcium phosphate phases including tri-calcium phosphate (TCP), tetra-calcium phosphate (TTCP) and calcium oxide (CaO). The microstructure investigation showed that the spheroidized particles were either fully dense or hollow structure with a shell. The reason for the formation of hollow spheres was contributed to the higher density of the solidifying surface layer compared with the molten phase during solidification.

Plasma spraying of large-scale components (such as paper mill rolls, rocket exit nozzles, parts of the land-based turbines for power generation, thermally sprayed free-standing tubes, etc.) and also powder spheroidization) are just several examples of industrial applications where the processing time and costs are critical. To ensure productivity and cost-effectivity of the process, it is beneficial to use plasma torches which can process high powder throughputs, i.e. with feed rates in tens of kilograms per hour. Such torches must have sufficiently high enthalpy and plasma temperature, to ensure homogeneous particle treatment and high deposition efficiency. Hybrid water-stabilized plasma (WSP-H) system with its enthalpy of more than 140 MJ/kg and plasma temperature of 25 000 K is able to process up to 40 kg of powders per hour. Since the WSP-H torch consumes typically only 15 slm of Ar at full power of 180 kW, it is a very powerful and also economical tool that meets all the prerequisites for large-scale plasma spraying applications. Spraying of three representative ceramics: alumina, zircon, and yttria-stabilized-zirconia is presented in this study. It reveals only limited influence of increasing powder throughput on particle velocity, temperature and coating microstructure. Deposition efficiency of the processes is discussed and the deposited coatings are analyzed by SEM and XRD.

Fine (median size 6 μm and 0.3 μm) cobalt spinel (Co 3 O 4 ) powders were processed suspended in a suitable liquid phase. Suspensions exceeding 50 wt.% solid phase content were successfully injected into an inductively coupled plasma. Spheroidized powders with large particle size (up to 80 μm) were prepared, and cobalt oxide coatings were produced by this novel RF-SPS method. The microstructural features of the coatings can be controlled by parameter optimization similarly to plasma spraying of dry powders. Numerous variations of the physical and chemical conditions of the process were performed in an attempt to overcome the main disadvantage of the process, i.e. the decomposition of the spinel phase to CoO. So far, the spinel phase could be reestablished only by a post-treatment of the deposited coatings with atomic oxygen in the RF plasma.

DC plasma spraying with its products has gained a high technical importance. With the availability of technically reliable high-frequency plasma torches whose basic development can be traced back to about 40 years ago, some of the disadvantages of the DC spray method are no longer existing or can be avoided to a great extent. This paper describes the principle, construction, and function of high-frequency plasma torches in which the plasma is generated by induction and metallic electrodes are not required (as is the case with conventional DC plasma torches). Typical examples of HF plasma spray application are discussed. Paper includes a German-language abstract.

This study focuses on RF-Plasma Technology (RFPT) to enhance the physical properties of powdered materials (metals, carbides, ceramics, inter-metallics, and mixtures). The efficiency and flexibility of RFPT allows for the economically viable production of powders with a high degree of densification, spheroidization and purity. RFPT is based on the used of RF inductive power used to create a plasma at atmospheric pressure. A complex model based on theoretical calculations and empirical data has been developed to describe changes in particles exposed to the plasma stream for a variety of process parameters. A specific combination of parameters is referred to as a scheme. Advanced schemes have been developed to increase the coefficient of heat transfer from the plasma stream to particles by up to 35%. RFPT is especially useful for the production of powders that have been difficult or impossible to create with traditional methods. Some of the materials processed include: ZrO 2 , W 2 C, WC and WC-Co combinations. Particle size distribution (PSD) can be tightly controlled, and can vary from 5 to 600 microns.

Suspensions of cobalt spinel (Co3O4) powders were rf plasma sprayed to form electrocatalytically active anode layers. Stable cobalt oxide suspensions of low viscosity exceeding 50 wt% solid phase have been processed. A spheroidization study revealed the formation of large spherical powder particles (- 30 + 80 µm). Cobalt oxide coatings were produced by rf suspension plasma spraying. The porosity was controlled by optimizing spray distance and reactor pressure. The main disadvantage of the thermal plasma processing of cobalt spinel is that the decomposition of the spinel phase into CoO could not be prevented, not even with the application of an 80% oxygen plasma. However, with a relatively low power oxygen plasma post-treatment, the deposited CoO layers can be oxidized to Co3O4, greatly improving the electrochemical performance of the anode layers.

A study on induction plasma shape forming with yttria stabilized zirconia (YSZ) was conducted as part of an effort to develop a new method for producing nuclear fuel. YSZ was selected because its melting point is similar to that of UO2. Nuclear fuel pellets were made using a large (70 mm) induction plasma flame that sprays more than 100 pellets simultaneously and a small (10 mm) supersonic plasma flame that produces one pellet at a time. Process optimization for the large induction plasma flame was done based on chamber pressure, plasma plate power, powder spraying distance, sheath gas composition, probe position, and particle size. The best results were 97.11% theoretical density (TD) for 5-mm thick pellets. For the single pellet approach, densities as high as 99% TD have been obtained in 12-mm thick free-standing pellets.

The aim of this paper is to study the behavior of two different sets of as-sprayed coatings obtained from two different starting HA powders, in a defined simulated body fluid (SBF) for a period of 28 days. Spray-dried hydroxyapatite (SDHA) and spheroidized hydroxyapatite (SHA) were the two selected powders. A Controlled Atmosphere Plasma Spraying system (CAPS) was use for the production of the coatings. Effect of pressure and surrounding atmosphere during the spraying of the coatings, were also evaluated. During the in-vitro test, dissolution of the coatings and precipitation of a poorly crystallized apatite in a preferential crystallographic orientation ([001] direction) was observed for the two sets of coatings. Dissolution of the coatings was measured by: 1) weighing the specimen before and after soaking and 2) by measuring the calcium ion concentration in the SBF solution with the Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). Scanning Electron Microscopy (SEM) exposed the morphology of the coatings and X-Ray diffraction (XRD) and Fourier Transform Infrared Spectrometer (FTIR) revealed their structure and composition.

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.

Thermal spray coatings of surfaces with metal, alloy and ceramic materials for protection against corrosion, erosion and wear is an intense field of research. The technique involves injection of the powder into a plasma flame, melting, acceleration of the powder particles, impact and bonding with the substrate. Feedstock powders of thermal spray applications have to meet several requirements. Particle shape, size and its distribution, powder flow characteristics and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. For smooth and uniform feeding of powders into the plasma jet, the powder particles have to be spherical in shape. In the present investigation powder particles are fed into the thermal plasma and spheroidized. The plasma was generated in a dc atmospheric plasma spray torch operating in non-transferred mode. Plasma processed powders were allowed to cool in atmospheric air inside a plasma reactor. The particle while free fall and cooling got spheroidized by surface tension forces. The feedstock powders were in the size range from 40 to 100 microns. The processed powders were analyzed through XRD, SEM/ Optical microscopy and irregularity parameter (IP) and roundness factor were determined. The same parameters were determined through theoretical methods and compared with experimental results.

Zirconia can induce enhanced fracture toughness to a number of ceramics when introduced as a reinforcement either in the form of particulates, dispersed phase or whiskers because of its unique tetragonal-monoclinic transformation. This paper presents the preparation of ZrO2 reinforced mullite by plasma spraying a mixture of zircon and alumina. The dissociation of zircon into zirconia and silica in a plasma flame is well-known. Pre-mixed powders of zircon and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon and alumina. It was found that fine, mostly amorphous and chemically homogeneous composite powders can be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The amount of amorphous phases was much higher in plasma sprayed coatings than in spheroidized powders, and the relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders.

Zirconia is effective in improving the fracture toughness to a number of ceramics when introduced as a reinforcement either in the form of particulate, dispersed phase, or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO 2 reinforced Mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon (ZrSiO 4 ) and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The dissociation of zircon in thermal plasma is a well-established fact. The present investigation aims to utilize the plasma dissociation of zircon to produced ZrO 2 -toughened mullite from a zircon + alumina mixture. The results indicate that the plasma sprayed powders consist of zirconia, amorphous SiO 2 , zircon, and alumina. It was found that ball milling and plasma spraying could yield fine grained, even amorphous and chemically homogeneous, composite powders. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000 °C in the sprayed coatings.

Reactive plasma spraying (RPS) has been considered as a promising technique for in situ formation of aluminum nitride (AlN) based thick coatings. This study investigated the reactive plasma spraying of AlN coating with using Al 2 O 3 powder and N 2 /H 2 plasma. It was possible to fabricate a cubic- AlN (c-AlN) based coating. The phase composition of the coating consists of c-AlN, α-Al 2 O 3 , Al 5 O 6 N and γ-Al 2 O 3 . Understanding the nitriding process during coating deposition is essential to control the process and improve the coating quality. The nitriding process was performed by spraying, collecting the particles into a water bath (to maintain its particle features) and observing their microstructures and cross sections. During the coating process, the sprayed particles were melted, spheroidized and nitrided in the N 2 /H 2 plasma to form the cubic aluminum oxynitride (Al 5 O 6 N). The particles collided, flattened, and rapidly solidified on the substrate surface. The Al 5 O 6 N is easily transformed to c-AlN phase (same cubic symmetry) by continuous reaction through plasma environment. Improving the specific surface area by using smaller particle sizes enhances the surface nitriding reaction and improves the nitriding conversion. Furthermore, using AlN additives enhances the nitride content in the coatings. It was possible to fabricate thick and uniform coatings with high AlN content by spraying fine Al 2 O 3 /AlN mixture. Furthermore, the N 2 gas flow rate improved the nitriding conversion and the coating thickness.

Pre-alloyed and plasma spheroidized composite powders were used as the feed-stock in the plasma spraying of functionally graded ZrO 2 /NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing FGM coatings were optimized in order to obtain the best performance for the thermal barrier coatings. Microstructure, physical, mechanical and thermal properties of ZrO 2 /NiCoCrAlY FGM coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using prealloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of ZrO 2 and NiCoCrAlY phases in the coating and no clear interface was found between two adjacent different layers. Oxidation of elements in Ni alloy occurred during plasma spray and the so-formed Al 2 O 3 and Cr 2 O 3 combined with ZrO 2 in a wide range of proportions. The bond strength of FGM coatings was about twice than that of the duplex coatings. Results also indicate that the thermal cycling resistance of FGM coating was much better than that of the duplex coating.

In this study, pure spherical Ta powders made by induced plasma sphero technique were used in the laser cladding process. The powders were sent into high-energy laser zone and were melted at the surface of a steel substrate to create a Ta layer. The microstructure development in the Ta layer was investigated in a scanning electron microscope. The results showed that the layer was basically dense with some pore/crack defects. In the layer, typical dendritic crystalline structures were formed. With the help of an energy dispersive spectroscope, Fe was detected in the Ta layer. The top surface had about 5% Fe while at the bottom of the cladded layer 15% Fe was detected. So, the diffusion of Fe upwards occurred. With the participant of Fe, the microstructure of the Ta layer was changed. Thermocalc software was used to simulate the phase constitution at different Ta-Fe compositions. The results by the simulation basically agreed with the experimental observations.

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.