A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics, analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, different degree of these effects was observed for copper and tungsten.

Impact of liquid injection on atmospheric plasma has been measured by an enthalpy probe and visualized by shadowgraphy. Liquid was injected by two nozzles. A solid stream nozzle with 0.3 mm diameter and an atomizing nozzle with a spray angle of 30°. For constant liquid flow rate and varied plasma current an optimal injection parameter has been obtained. Influence of different flow rates on plasma characteristics has been examined. Full profiles along each axis show differences for different injection techniques in enthalpy, temperature, velocity and composition.

High heat load on the surface of electrodes in DC arc plasma torches results in strong wear of electrodes. This takes place during the start up and shutting down procedure as well as during long-term running of the torch and results in arc voltage reduction. Such changes in voltage lead to a modification of the plasma jet properties and effect adversely reproducibility of deposits, which are undesirable for industrial applications. The present work offers a new approach to compensate cathode wear in DC arc gas stabilized plasma torches by using moveable anode. With this design of torch, the distance between the cathode tip and anode can be varied. Voltage fluctuation analysis show the possibility of a reasonable arc modification by approaching the nozzle to the cathode caused by a change of the flow field inside the arc chamber and the arc dynamics. Smaller cathode tip-nozzle distances result in higher arc voltage with reduced fluctuations accompanied by a positive effect on the plasma jet properties. Thus, the negative influence of cathode wear on generated plasma properties can be suppressed and the lifetime of the cathode can be extended by simple approaching the nozzle.

The present paper examines a hybrid DC arc thermal plasma torch with gas-water stabilisation and its application for thermal spraying. The torch was worked out based on the water-stabilised plasma torch WSP, main features of which are high temperature of the generated plasma and high powder through-put. In the hybrid torch the cathode part is modified in the way like in gas torches, which not only provides proper stabilisation of the arc in this region and cathode protection from contact with stabilising water but also allows to vary properties of the generated plasma. Change of the secondary gas flow rate, which is usually argon, results in strong changes of the plasma gas density, while the energy balance of the torch remains almost unchanged. This in turn leads to modification of the plasma jet properties. The paper describes effect of the plasma torch parameters on behaviour of the generated plasma jet and injected powder particles. The results show how particle velocities follow changes of the plasma jet properties according to the plasma torch operation parameters. Both increase of arc current and argon flow rate provided increase of particle velocities.

Thermal plasma jets interact intensively with the surrounding atmosphere. This interaction leads to strong changes of the plasma jet properties affecting the resulting products. Modification of the nozzle parameters and conditions at the exit of the torch helps to vary and better control the process of plasma jet and ambient air interaction. In the present study, the DC arc plasma torch was equipped with a modified anode nozzle (M2.5) and a surrounding shroud at the jet exit. The process of air entrainment was investigated when shroud gas was supplied producing a protecting envelope to reduce the air entrainment. Schlieren photography and the enthalpy probe with the mass spectrometer were applied to study the influence on plasma jet behavior. The effect of shroud nozzle geometry as well as the effect of the shroud gas flow rate was investigated. Likewise, influence of gas shrouding on the resulting coatings was studied.

Plasma jets for thermal spraying are strongly affected in a negative sense by the interaction with the relatively static surrounding atmosphere, particularly at atmospheric spray conditions. Turbulences at the jet fringes arise resulting in entrained cold gas, in slowing and cooling down of the jet and in causing eventually its disintegration. All means suppressing or delaying this phenomenon, called cold gas entrainment, help to improve the interaction of plasma and spray material and hence lead to better product quality and higher deposition efficiency of the process. To observe the cold gas entrainment, to investigate the thermal and kinetic properties of DC plasma jets at different operating conditions and to study the effect of plasma source and powder injection modifications a diagnostic equipment with Schlieren optics, enthalpy probe and mass spectrometry was installed. By modification of the internal and external anode nozzle contours and also by application of a shroud nozzle around the plasma jet exit encouraging results with reduced penetration of cold ambient air into the jet could be obtained.

This paper presents selected research results of the DFG founded project group, consisting of four institutes focusing on diagnostic methods in thermal coating processes. The aim of this group is to characterize the Atmospheric Plasma Spraying (APS) process by means of diagnostic methods so that – based on the requirement profile of the coating – appropriate adjusting of the process parameters can be realized. For this purpose, different diagnostic tools like Particle Shape Imaging, Laser Doppler Anemometry, Schlieren Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. Most of the results presented in this article are limited to the area close to the substrate which is difficult to handle with diagnostic methods. Additionally, new achievements concerning nozzle design and system enhancements are introduced.