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Plasma Spraying and Plasma Transferred Arc Processes
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Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 155-159, June 2–4, 2008,
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Although the history of using the thermal inductively coupled RF-plasma (ICP) for spraying processes has been started in the early sixties, up to now all but no industrial applications are known. ICP-spraying of coatings has been investigated in various labs for interesting applications like coatings for medical implants and electrodes for SOFC´s. All the processes are VPS-applications. This on one hand of course is caused by the oxygen affinity of the used materials, on the other hand the current view in thermal spraying is, that very dense and excellent adherent coatings can be sprayed only by increasing the particles velocity. In contrast to this mind this contribution will try to show, that also under atmospheric and low vacuum conditions, i.e. using a laminar flowing plasma with nearly no acceleration of the axially injected particles, it becomes possible to spray coatings with comparable values of porosity and bond strength but special features that can not be produced with common technologies. This can be explained by the changed condition of heating, deformation and cooling down of the considerably larger particles. Actual examples are given for various spraying materials like ceramics and hard magnetic materials.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 160-164, June 2–4, 2008,
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For different applications, it is on interest to understand the relationship between the microstructure and the transverse permeability of a plasma sprayed coating. In the SOFC field, for example, the electrodes are very often characterized in term of porosity, particularly when they are elaborated by sintering. This can’t be applied to thermally sprayed coatings because of their very singular microstructure, lamellar and isotropic, depending strongly on the initial material and plasma conditions. In this study, coatings were manufactured by plasma spraying using different spraying parameters, many proportion of a porous agent, metallic or ceramic powders and two particles size in order to obtain various microporous structure. Measurements of their permeation with the pressure drop method and their open porosity just as the observation of the morphology and the structure by optical microscopy were achieved. The different data show that the evolution of the transverse permeability with the open porosity follows the Kozeny-Carman equation. This result correlated with the microstructural observation highlights the relationship between the permeability and the physical properties of porous plasma sprayed layers.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 165-170, June 2–4, 2008,
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The application of ceramic die coatings on tool steel dies in the casting industry has been common practice for many decades. The main function of these coatings is to provide a thermal barrier to prevent premature solidification during die filling, and protect the tool steel die from the effects of molten metal during casting with aluminium alloys. Although these coatings provide good insulation they are fragile and require on-going in-situ maintenance by machine operators. These inherent poor qualities makes the die casting process difficult to control and to maintain cast product quality because the solidification pattern and porosity changes and leads to increased cast product rejects. To overcome the limitations a novel die coat has been developed for the light metal casting industry utilising thermal spraying of co-deposited MgZrO 2 and polymer particles. The coating is then thermally treated to reveal a fine network of porosity that has been found by heat transfer coefficient testing to enhance the thermal properties and overall coating durability during casting. This paper describes the porosity control system which was used to tailor the heat transfer coefficient of co-deposited MgZrO 2 and polymer coatings and compare them with the heat transfer coefficient of commercially available die coats. The inherent porosity and the overall coating thickness were found to have a large effect on the heat transfer coefficient. Results of industrial trials are also presented and show that co-deposited MgZrO 2 and polymer coatings provide considerable improvements to productivity and enhanced coating life in Gravity and Low Pressure Die casting of aluminium alloys.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 235-240, June 2–4, 2008,
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This paper presents an investigation of the influence of the spray angle on thermally sprayed coatings. Spray beads were manufactured with different spray angles between 90° and 20° by means of atmospheric plasma spraying (APS) on heat-treated mild steel (1.0503). WC-12Co and Cr 3 C 2 -10(Ni20Cr) powders were employed as feedstock materials. The spray beads were characterized by a Gaussian fit. This opens the opportunity to analyse the influence of the spray angle on coating properties. Furthermore, metallographic studies of the surface roughness, the porosity, the hardness and the morphology were carried out and the deposition efficiency as well as the tensile strength were measured. The thermally sprayed coatings show a clear dependence on the spray angle. A decrease in spray angle changes the thickness, the width and the form of the spray beads. The coatings become rougher and their quality decreases. In addition, the spray process becomes inefficient as the deposition efficiency declines. Especially, below 30° for Cr 3 C 2 -10(Ni20Cr) and below 50° for WC-12Co a significant change in microstructure was observed.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 241-243, June 2–4, 2008,
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The use of gas mixtures containing hydrocarbons for plasma generation results in higher plasma enthalpy because, molecular gases must dissociate before ionization which requires larger energy input. The torch developed at CACT which operates with CO 2 +CH 4 gas mixtures was used for coating deposition with input power in a range of 30 to 45 kW. This study was focused on the effect of CO 2 +CH 4 mixtures on the particle parameters during spraying of Ni alloy powder. Results of gas composition analysis at various distances from the nozzle exit are presented. The particle in-flight conditions, coating microstructure, and deposition efficiency also were analyzed.
Proceedings Papers
Effective Parameters in Axial Injection Suspension Plasma Spray Process of Alumina Zirconia Ceramics
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 244-249, June 2–4, 2008,
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Suspension Plasma Spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using extra small particles as compared to conventional thermal spraying. Suspension spraying involves, atomization, solvent evaporation and melts consolidation, which can cause substantial complexity in the system. Using feedstock mixtures for composite coatings, such as alumina and zirconia, intricacy of the system increases even more. There is consequently a need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/ 8 wt% yttria stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated by using Taguchi design of experiment (DOE). The microstructural features include microcracks, porosities and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e. temperature and velocity were also measured. The role of the porosity in this multi-component structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 250-253, June 2–4, 2008,
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Solution precursor plasma spraying has been used to deposit ceramic coatings with submicron/nanocrystalline structures. Previous studies revealed that the deposition mechanism in the solution precursor plasma spraying differs from that in the conventional plasma spraying. To increase the understanding of the deposition mechanism in the solution precursor plasma spraying, a numerical model is used to predict the particle conditions on the substrate. Five types of particle conditions, melted particles; small sintered particles; dry agglomerates; wet agglomerates; and wet droplet are assumed based on the computed temperature distribution of the particles. An analysis of the deposition mechanism in the solution precursor plasma spraying is performed. Experiment results s are also collected to verify the numerical prediction and the analysis of the deposition mechanisms.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 506-511, June 2–4, 2008,
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Recent works have been devoted to achieve dense and thin (<15 µm) zirconia coatings using a relatively new process, Suspension Plasma Spraying (SPS). Nevertheless, the parameters controlling the microstructure of the deposit are not yet clearly identified, particularly for the injection of suspension. Hence, the liquid penetration into the plasma has been observed with a fast shutter (10 -5 s) camera coupled with a laser flash and triggered by a defined instantaneous voltage level of the plasma torch. This paper is focused on the treatment of the suspension jet or drops according to the suspension properties (with the viscosity, particles load, injection velocity…) and depending on the different spray parameters such as the plasma forming gas mixture composition and the plasma torch design (either PTF4 or home made torch). These works have permitted the obtention of zirconia coatings with low thicknesses (~10 µm) and dense structure (~4% of porosity).
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 512-516, June 2–4, 2008,
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The use of liquid precursors in plasma spraying makes it possible to produce coatings with more refined microstructures than in conventional plasma spraying. Depending on the injection device, the liquid feedstock is injected into the plasma jet in the form of liquid jet or droplets. The instabilities on the liquid-gas interface cause the mechanical break-up of liquids into drops that are subjected to further break-up until the droplets reach a stable state or evaporate. The process break-up may strongly influence the size, trajectories and, therefore, treatment of the droplets in the plasma medium. This study deals with the experimental observation of liquid break-up under plasma spray conditions when using a conventional DC plasma torch with radial injection by means of a pneumatic injection system that can deliver either liquid stream or blobs.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 517-522, June 2–4, 2008,
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The solution precursor plasma spray (SPPS) process, in which a solution precursor of the desired resultant material is fed into a plasma jet by atomizing gas or high pressure, was developed in the 1990’s and has been studied extensively since then. Recently, it has been shown that the SPPS process is suitable for deposition of porous electrodes for solid oxide fuel cells (SOFC). High efficiency SOFC requires electrodes with 30%-40% porosity. Because of the complexity of the SPPS process and the large number of processing parameters, it is difficult to investigate the effect of each parameter on the two important properties, i.e. coating porosity and deposition efficiency, separately. Design of experiments can use a small number of experimental runs to analyze the effect of each processing parameter on the properties of the fabricated product, after which the processing parameter combinations for fabricating a target product can be found. In this project, a small central composite design, a second order statistical model, was used to analyze and optimize the SPPS process for Ni-YSZ anode deposition. The processing parameters investigated include: 1) Hydrogen flow rate, which determines arc voltage, 2) Current , 3) Solute flow rate, 4) Solution concentration, 5) Distance between nozzle and gun, and 6) Stand off distance. The effects of the selected processing parameters were analyzed, and the resultant model used to select a combination of processing parameters which produced a coating with the desired characteristics.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 523-528, June 2–4, 2008,
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High heat load on the surface of electrodes in DC arc plasma torches results in strong wear of electrodes. This takes place during the start up and shutting down procedure as well as during long-term running of the torch and results in arc voltage reduction. Such changes in voltage lead to a modification of the plasma jet properties and effect adversely reproducibility of deposits, which are undesirable for industrial applications. The present work offers a new approach to compensate cathode wear in DC arc gas stabilized plasma torches by using moveable anode. With this design of torch, the distance between the cathode tip and anode can be varied. Voltage fluctuation analysis show the possibility of a reasonable arc modification by approaching the nozzle to the cathode caused by a change of the flow field inside the arc chamber and the arc dynamics. Smaller cathode tip-nozzle distances result in higher arc voltage with reduced fluctuations accompanied by a positive effect on the plasma jet properties. Thus, the negative influence of cathode wear on generated plasma properties can be suppressed and the lifetime of the cathode can be extended by simple approaching the nozzle.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 529-534, June 2–4, 2008,
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Experiments for estimation of influence of microplasma parameters in ZrO 2 – coating properties were performed with using of fractional-factorial design of the experiments. Regression equations for determination of the parameters significance and a selection of conditions for a deposition of coatings with specified properties were obtained. Possibility of deposition of ZrO 2 - Y 2 O 3 coating with tetragonal phase content 90 wt.% and more without cubic phase content by microplasma spraying was shown. Conditions of microplasma ZrO 2 -coating spraying with high content of tetragonal phase (93-95%) and porosity (1,3-1,8%) were established. Measurement of microplasma sprayed ZrO 2 -coating spots has established that the spots have an elliptical shape. Size of major and lateral axis (and also sprayed hill altitude) depends on spray parameters, mainly on amperage and spray distance. Microplasma sprayed ZrO 2 -coating can be applied for repair of small local defects on thermo barrier coatings and a deposition of zirconia electrolyte layer in solid oxide fuel cell.