Modeling of oxidation of molybdenum particles during plasma spray deposition
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I. INTRODUCTION
MOST metal coatings formed by an atmospheric plasma spray entrain oxides that can significantly affect the properties of coatings. The presence of oxide in metal coatings is generally detrimental for corrosion applications, with very few exceptions in which improvement of certain properties could be achieved. The degradation of coating performance by metal oxides can be exhibited in many aspects: for example, the coating layer becomes brittle and the resistance to corrosive environments is weakened. Enhancement of coating performance can also be found in, for example, the wear resistance and the hardness of the coating. The influence of the metal oxide on the property and performance of coatings depends on the sort of oxide formed, the concentration of the oxide, and the distribution of the oxide. Therefore, an understanding of where, how, and how much oxide is formed during the whole spray-deposition process is essential to the control of oxide formation and, eventually, the quality of the sprayed coatings. In various spray systems, the oxidation path that a sprayed metal particle experiences might be slightly different. As an example, a d.c. plasma system with the injection of powders from outside the plasma torch, e.g., a Metco 9MB system, is considered here. Along the trajectory of a flying particle, four regions can be identified, roughly divided according to the oxygen concentration of the surrounding gas and the temperature of the flying particle (Figure 1): (1) the cold region near the powder feed-port, where the particle is still cold and the surrounding gas is essentially ambient air, (2) the core region, where the plasma gas is not yet contaminated by entrained ambient gas and the particle temperature Y.P. WAN, formerly Research Scientist with the Center for Thermal Spray Research and Process Modeling Laboratory, State University of New York at Stony Brook, is Applications Engineer, GT Equipment Technologies, Inc., Nashua, NH 03063. J.R. FINCKE, Scientific Fellow, is with the Idaho National Engineering and Environment Laboratory, Idaho Falls, ID 83415-2211. X.Y. JIANG, formerly Ph.D. Candidate with the Center for Thermal Spray Research and Process Modeling Laboratory, State University of New York at Stony Brook, is Engineer, Caterpillar Corp., Peoria, IL 61614. S. SAMPATH, Associate Professor, and V. PRASAD and H. HERMAN, Professors, are with the Center for Thermal Spray Research and Process Modeling Laboratory, State University of New York at Stony Brook, Stony Brook, NY 11794-2275. Manuscript submitted October 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
reaches or even exceeds its melting point, (3) the free jet region, where the plasma jet is well mixed with ambient air and the particle is a molten droplet, and (4) the surface region, where the droplet deposits on a surface and the gas jet impinges on the surface. The oxidation mechanism and oxidation rate in each region depend on the particle temperature and the concentration of surrounding oxidizer, including both oxygen (O2)
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