Abstract Air entrainment in the first 30 mm of a dc Ar/ H 2 plasma jet has been studied by emission spectroscopy. The tests were conducted using 6, 7 and 10-mm diameter nozzles and plasma arc currents of 400 and 600 A. Oxygen, nitrogen, and argon spectral lines were recorded 20 and 30 mm downstream of the nozzle exit during spraying, and the corresponding atom density ratios were estimated based on plasma population temperature and volumetric emission coefficients. The results indicate that at 20 mm air entrainment is mainly due to piston flow for the 10-mm nozzle and both piston flow and engulfment for the 7-mm nozzle. At 30 mm, the engulfment process is found to have 4 to 6 times the impact that it does at 20 mm and is directly linked to the jet velocity. At both locations, the atom density ratios differ from that observed in air due to the time required to dissociate N 2 .

Abstract The gas permeability of plasma sprayed yttria-stabilised zirconia coatings has been measured over a range of temperature, using hydrogen and oxygen gas. The permeability was found to be greater for coatings produced with longer stand-off distances, higher chamber pressures and lower torch powers. Porosity levels have been measured using densitometry and microstructural features have been examined using SEM. A model has been developed for prediction of the permeability from such microstructural features, based on percolation theory. Agreement between predicted and measured permeabilities is good. Ionic conduction through the coatings has also been briefly explored. It is concluded that transport of oxygen through the top coat in thermal barrier coating (TBC) systems, causing oxidation of the bond coat, occurs primarily by gas permeation rather than ionic conduction, at least up to temperatures of about 1000°C and probably up to higher temperatures. Top coat permeabilities appreciably below those measured will be required if the rate of bond coat oxidation is to be reduced by cutting the supply of oxygen to the interface.