Thin and Dense Ceramic Coatings by Plasma Spraying at Very Low Pressure
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Georg Mauer, Robert Vaßen, and Detlev Sto¨ver (Submitted March 31, 2009; in revised form July 28, 2009) The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses 1 m in length and >0.2 m in diameter depending on the parameters. Accordingly, the profiles of specific enthalpies and temperature change only slightly over a large range of the axial spray distance and are also broadened in the radial direction. Furthermore, the specific enthalpy and temperature on the jet axis are enhanced. The plasma velocities are also distinctly
This article is an invited paper selected from presentations at the 2009 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Expanding Thermal Spray Performance to New Markets and Applications: Proceedings of the 2009 International Thermal Spray Conference, Las Vegas, Nevada, USA, May 4-7, 2009, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2009. Georg Mauer, Robert Vaßen, and Detlev Sto¨ver, Forschungszentrum Ju¨lich GmbH, Institut fu¨r Energieforschung IEF-1, 52425 Ju¨lich, Germany. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
dependant on the pressure. The lower the pressure, the higher the velocity level becomes and the velocity is almost constant over the axial distance. Similarly, the radial velocity profiles are broadened as pressure is decreased.
2. Literature Review Enthalpy probe measurements clearly show that at lower ambient pressure the plasma jet becomes supersonic. This means that the plasma gas can exit the nozzle at a pressure which is different from the chamber pressure. As the flow is faster than the pressure waves traveling in the plasma at the local speed of sound, no information on the chamber pressure is communicated inside the nozzle (Ref 2). If the exit pressure of the jet is lower than the chamber pressure it is over-expanded showing typical luminous oblique shock waves originating from the edge of the nozzle and forming subsequent compression and expansion cells indicating that the jet local static pressure is brought to the chamber pressure level. However, in the VLPPS process the ambient pressure is si
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