In order to identify means to improve plasma spray consistency, various modifications to the design of a commercial plasma torch nozzle have been investigated. The modifications consist of preparing anode inserts with grooves in the axial direction (spline insert), and introducing a fraction of the plasma gas through a ring of micro-nozzles surrounding the anode nozzle (micro-jet ring). Different designs for each modification have been investigated, and these modifications have also been paired with a modified upstream gas injector. For each of the modified designs, a wide range of characteristics have been measured for the arc, the plasma jet, the in-flight particles and the coating. The results show that most nozzle modifications lead to higher particle velocities and temperatures. The plasma jet is significantly elongated by using some of the modified nozzles, and the cold gas entrainment somewhat reduced. The arc voltage and the luminosity fluctuations are little affected by the nozzle modifications, however, the modification of the gas injector does change the frequencies of the fluctuations. Each of the nozzle modifications can be easily implemented offering an economical way to enhance process reliability.

Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.

Instabilities of plasma spray jets have been a source of inconsistencies in coating properties. These instabilities can be minimized through the use of central injection torches or torches with fixed anode attachment. However, any low density ( < ~0.7) jet is globally unstable to small disturbances. Globally unstable jets are characterized by a short potential core, rapid spreading, and high entrainment, all of which are present in a plasma jet. Plasma jets have ratios of jet density to density of the surrounding gas on the order of 0.01, as well as rather low Reynolds numbers and thick boundary layers. In the present work, the instabilities are investigated through analysis of the disturbance growth in the shear layer between the plasma and the cold surrounding gas. These investigations are using two types of experiments, one consisting of a SG 100 spray torch with several optical diagnostic methods being applied to the shear layer analysis. The other experiment simulates the plasma jet at low temperatures by using a helium core jet exhausting into a sulfur hexafluoride (SF6) environment. The simulated plasma jet (SPJ) has a density ratio of 0.03. The simulated plasma jet (SPJ) allows controlled variation of the boundary layer through different fluid dynamic arrangements. It further allows use of diagnostics such as hot wire anemometry and PIV to clearly characterize the shear layer. Some of the characteristics of the shear layer are presented and comparisons between the plasma jet and the simulated plasma jet, and initial results on controlling the jet instability, are discussed. Abstract only; no full-text paper available.

The distributions of spray particle velocity and temperature were determined with two different instruments, the SpectraViz (Stratonics, Laguna Hills, CA) and the DPV2000 (Tecnar Automation, Montreal, Canada). Experiments were performed with three different powders and a Praxair SG 100 plasma spray torch. The instruments differ by the method they use in imaging the jet ultimately allowing for measurements of velocities and temperatures. The average particle temperature values obtained are very close, indicating that different ways of imaging do not influence the measurement value. The values for the average velocities are in good agreement for two of the powders tested (Al 2 O 3 and CrO 2 ), however, for the YSZ powder a consistent difference in velocity values was found. It appears that for obtaining average values e.g. for process control, both instruments are equivalent.