Relation Between the Arc-Root Fluctuations, the Cold Boundary Layer Thickness and the Particle Thermal Treatment
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E. Nogue`s, P. Fauchais, M. Vardelle, and P. Granger (Submitted March 12, 2007; in revised form August 12, 2007) In plasma spraying, the arc-root fluctuations, modifying the length and characteristics of the plasma jet, have an important influence on particle thermal treatment. These voltage fluctuations are strongly linked to the thickness of the cold boundary layer (CBL), surrounding the arc column. This thickness depends on the plasma spray parameters (composition and plasma forming gas mass flow rate, arc current, etc.) and the plasma torch design (anode-nozzle internal diameter and shape, etc.). In order to determine the influence of these different spray parameters on the CBL properties and voltage fluctuations, experiments were performed with two different plasma torches from Sulzer Metco. The first one is a PTF4 torch with a cylindrical anode-nozzle, working with Ar-H2 plasma gas mixtures and the second one is a 3MB torch with either a conical or a cylindrical anode-nozzle, working with N2-H2 plasma gas mixtures. Moreover, arc voltage fluctuations influence on particle thermal treatment was studied through the measurements of transient temperature and velocity of particles, issued from an yttria partially stabilized zirconia powder with a size distribution between 5 and 25 lm.
Keywords
arc voltage fluctuation, particle thermal treatment, plasma spray, spray parameter influence
1. Introduction In d.c. plasma spray process, the arc is generated between two electrodes (Ref 1). The arc column emanating from the cathode tip does not directly hit the anode, because they have the same axis. Thus the arc attaches at the anode wall through a connecting column (Fig. 1), crossing the cold boundary layer (CBL) surrounding the arc column. The arc column corresponds to the area where the electrical conductivity is high enough to let the current go through, which, for conventional plasma forming gases, correspond to temperatures over 7500-8000 K. The connecting column strikes at the anode wall, where the CBL,
This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007. E. Nogue`s, P. Fauchais, and M. Vardelle, SPCTS Laboratory, Limoges, Haute-Vienne, France; E. Nogue`s and P. Granger, BOC Edwards Company, Corbeil-Essonnes, Essonnes, France; and E. Nogue`s, 123, Avenue Albert Thomas, 87060 Limoges Cedex, France. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
between the arc column and the anode wall, has been sufficiently heated (Ref 2). This connecting column is submitted to two forces: the Drag force issued from the gas flow in the CBL and elec~ where~j is the current density and
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