Hirokazu Tahara and Takao Yoshikawa
Division of Mechanical Science, Department of Systems and Human Science, Graduate School of Engineering Science,Osaka University, Toyonaka 560-8531
Spectroscopic measurements were carried out to understand the plasma feature inside and outside a direct-current nitrogen arcjet generator with a supersonic expansion nozzle. In the expansion nozzle, the plasma was in thermodynamical nonequilibrium because the pressure drastically decreased downstream, although the plasma in the throat was expected to be nearly in a temperature-equilibrium condition. The radial profiles of the physical properties showed that there existed a core flow with high vibrational and rotational temperatures and large electron number densities on the center axis, even at the nozzle exit. The vibrational temperature at the nozzle exit ranged from 6000 to 10000 K in input powers of 5-11 kW and the rotational temperature from 500 to 2000 K. In the exhaust flow, the temperatures and the electron density were approaching some values downstream. Nitriding of titanium was conducted using the nitrogen plasma jets under a low pressure environment below 3 kPa. Plasma was accelerated smoothly with an optimally designed supersonic expansion nozzle because a shock diamond was not observed in the plasma flame. Even under a low pressure below 3 kPa, a titanium nitride layer was constructed on the surface of a titanum sample by only 5-min plasma jet irradiation. Although nitride formation decreased with decreasing pressure at the center of the irradiated region, the nitrided area became larger.
(Received April 16, 1998; In Final Form June 24, 1998)
nitrogen plasma jet, supersonic flow, expansion nozzle, low pressure processing environment, plasma spectroscopic measurement, thermodynamical nonequilibrium, plasma nitriding, titanium nitriding
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