The effect of particle size and morphology on the in-flight behavior of particles during high-velocity oxyfuel thermal s
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I. INTRODUCTION
HIGH-VELOCITY oxyfuel (HVOF) thermal spraying is one of the most significant developments in the thermalspray industry since the development of the original plasma spray technique. It is being used in an increasing variety of coating applications. Metallic, ceramic, and composite coatings are frequently applied to substrates to improve wear resistance, abrasion resistance, thermal and electrical barriers, and corrosion protection.[1] More recently, HVOF thermal spraying has been successfully used as a means of producing nanocrystalline coatings.[2–7] The extremely brief exposure of the precursor nanocrystalline particles to the high temperatures of the HVOF process appears to preserve the nanocyrstalline structure in most of the particles deposited on the substrate. The HVOF is characterized by high particle velocities and relatively low thermal energy when compared to plasma spraying. The HVOF uses an internal combustion jet fuel (propylene, acetylene, propane, and hydrogen gases) to produce a gas temperature greater than 3029 K[8] and to generate a supersonic or hypersonic gas velocity of approximately 2000 m/s, more than 5 times the speed of sound, through a convergent-divergent nozzle. The powder particles are injected into the gas jet, and simultaneously heated, and propelled toward the substrate. With the relatively low temperature of the flame gas associated with the HVOF system, as compared to plasma spraying (about D. CHENG, formerly Graduate Student, Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, is Research Engineer, Pace Enterprises, Los Angeles, CA 90013. Q. XU, Research Engineer, and E.J. LAVERNIA, Professor and Chair, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697. G. TRAPAGA, formerly Principal Research Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is Professor, Laboratorio de Investigacion en Materiales del CINVESTAV-IPM, Unidad Queretaro, Fracc. Real de Juriquilla, C.P. 76230, Qro., Mexico. Manuscript submitted March 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
15,000 K),[9] the particles are made highly plastic by convective heat transfer, and superheating or vaporization of individual particles is prevented.[10] Furthermore, low particle temperatures experienced during the deposition of carbide coatings lead to less carbide depletion than plasma-sprayed coatings. In effect, the advantages of HVOF over conventional plasma spraying include higher coating bond strength, higher deposition rate, higher hardness, lower oxide content, and improved wear resistance due to a homogeneous distribution of particles.[11,12] From the fluid dynamics point of view, the system is very complex and involves two-phase (gas and liquid or solid particles) flow with turbulence, heat transfer, chemical reaction, and supersonic/subsonic flow transitions. In an engineering applicatio
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