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Meet Our New Colleagues This column presents selected currently graduating Ph.D. students in the thermal spray field from around the world. Students planning to graduate in the area of thermal spray within next 3-6 months are encouraged to submit a short description (1-2 pages, preferably as Word document) of the projects they performed during their studies to Jan Ilavsky, JTST Associate Editor, address: Argonne National Laboratory, Advanced Photon Source, 9700 S. Cass Ave., Argonne, IL, 60439; e-mail: [email protected]. gov. After limited review and corrections and with agreement of the studentÕs thesis advisor, selected submissions will be published in the upcoming issues of JTST.
Arc Voltage Fluctuations: Comparison Between Argon and Nitrogen Plasma Jets -------------------------Elise Nogue`s, Ph.D. student, SPCTS (Science of Ceramic Processes and Surface Treatment Laboratory), University of Limoges, France; e-mail: elise.nogues@ bocedwards.com -------------------------Background and Motivation In the plasma spray process, the particle thermal treatment is strongly linked to the arc root fluctuations. These fluctuations are due to the effect of two Elise Nogue`s forces on the connecting arc column: the drag force issued from the gas flow in the cold boundary layer (CBL) and the electromagnetic self-induced forces, resulting in its continuous fluctuations along the anode wall. The connecting column lengthens and when the arc voltage, increasing with it, reaches the breakdown voltage, a new arc root is created with lower voltage drop (Ref 1). This movement, depending on the CBL thickness, induces a periodic variation in the arc voltage, modifying the enthalpy input to the gas, the plasma jet velocity and temperature, its length, and width. These arc fluctuations are function of the plasma torch geometry (anode shape and internal diameter), the plasma spray
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parameters (mass flow rate, secondary gas percentage, and arc current), and the nature of primary plasma gas (argon or nitrogen) (Ref 2-4). During the process, the plasma momentum continuously varies following the voltage fluctuations, at a few kHz, but the momentum of particles injected remains constant. Therefore, the particle trajectories fluctuate and correlatively their temperatures and velocities (Ref 5). The measurements of Bisson and Moreau performed on alumina particles with a size distribution between 32 and 45 lm for an Ar-H2 plasma (35 SLPM/10 SLPM), working at 550 A have shown that, due to the arc root fluctuations, at the same location, particle temperatures vary between 2400 and 2900 K, while their velocities range between 260 and 430 m/s (Ref 6, 7). BOC Edwards Society uses nitrogen as primary gas to achieve zirconia coatings, although argon is usually employed. One of the objectives of this project is to compare the properties of the two different plasma jets (argon or nitrogen with hydrogen) in terms of arc voltage fluctuations and CBL thickness. The aim is to determine the effects on the particle thermal treatment and
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