With the goal to improve the plasma spraying technique a considerable number of plasma gun types using different physical principles have been developed in the past. In this paper for conventional DC discharge plasma torches widely used in industry, some aspects of operation as arc fluctuations and non-symmetric plasma jets are discussed. Different construction principles with single or multiple electrodes as well as with one-piece or cascaded nozzles are compared. Finally a new torch concept is presented.

In the thermal spray industry there remains a constant demand for more stable, controllable and economical processing devices. Most thermal spray applicators apply a wide range of materials especially those who are allied with the turbine markets. Each material coating specification requires a relatively specific range of velocity and temperature transferred to the powder particle to achieve the required material properties on the part. The plasma gun has become a dominant process tool in the turbine industry due to the wide range of parameters that are achievable with the basic tool. In addition, there are today a large number of individual plasma guns that are each known to provide excellent results with some, but not all coating requirements. In many cases the optimum gun running condition can never be achieved due to the interdependence of arc behavior and gas conditions. The turbine industry is currently largely populated with guns that are based on the technology as it was developed in the 1960’s. These guns are characterized by poor voltage stability with a large quasi periodic oscillation in the 3-5 kHz range as described by Bisson [1], with poor performance and life under more extreme operating conditions. A key element of any plasma gun is the nozzle geometry. The cascaded gun types as typified by the Triplex offer a unique opportunity to study and document a wide range of operational parameters, especially at extreme operating conditions.

For the effective development and optimization of plasma torches and plasma processes a knowledge of the spatial distribution of plasma properties within the generated plasma jets is useful. Information about some of these properties also in case of arbitrary distribution can be achieved by computer tomography (CT) using radiation emitted by the plasma jet. The stationarity of the plasma jet is a necessary condition for the CT technique described in this paper. By CT in principle the radiation from a small cross-sectional disk of the plasma jet is recorded successively under different directions perpendicular to the torch axis. From the recorded data the spatial distribution of radiation emissivity within the jet is calculated using an algebraic recursive algorithm. The CT was applied for the investigation of a TRIPLEX II plasma jet giving the following results: The jet is characterized by a very high degree of stationarity and exhibits a definite triple symmetry, which can be described by three partial plasma jets. Measuring their positions the influence of the arc current on the TRIPLEX plasma jet rotation was determined.

This paper provides an overview of two particle imaging techniques that have proven useful in the development and implementation of thermal spraying processes. It covers the basics of each method, including the hardware setup, the interpretation of output data, and example applications. Paper includes a German-language abstract.

The innovative diagnostic system, PFI, records the radiation of the plasma jet as well as of the luminous particle flux. A desktop computer immediately converts these brightness distributions to a set of ellipses. The simple set-up of the system and the fast algorithm enables the utilization of the system in serial production. A statistical design of experiments (DOE) was applied for various process parameters to correlate the determined characteristic ellipses to the process parameters. Measurements of the process by DPV2000 and properties of the resultant coating were used to validate the established correlations. The results demonstrate the suitability of the diagnostic method PFI for quality control and quality assurance in serial production.

Conventional DC plasma torch designs lead to a circular cross-section of the emanating plasma jet. Consequently in surface treatment applications the plasma jet hits the substrate within a limited circular working area. Large scale workpieces therefore have to be scanned resulting in a time-consuming procedure. The innovative DC plasma torch system LARGE is characterized by the arrangement of the anode and the cathode opposite to each other on a common axis with a variable distance. The central body of the torch between the electrodes is divided into electrically insulated cascade plates. The plasma gas is injected perpendicular to the torch axis. Passing through the arc, the gas is transferred to the plasma state and leaves the torch laterally through a slit as a plasma jet with extended stripe width. The plasma torch LARGE is investigated by electrical, optical and enthalpy probe diagnostics. Shrouding the electrodes with an inert gas and feeding reactive gas mixtures as main plasma gas allow the torch to be used for plasma chemical reactions, too. Preliminary applications focus on preheating, surface modification of paper and plastic materials as well as on sterilization of nutrition packaging. The capability of plasma enhanced CVD is examined experimentally.

This paper presents a low-cost in situ diagnostic method that monitors and controls thermal spray processes using a CCD camera and a PC. The method, called particle flux imaging (PFI), records light emitted by thermal spray particles and the hot propellent carrier-medium in which they are conveyed. Brightness distributions corresponding to temperature and density profiles are represented by sets of ellipses that are compared in real time to a reference image. An image analysis algorithm adjusts relevant spray parameters based on the comparisons, maintaining a constant and unchanged spraying process.

In the thermal spraying process the quality of the produced coating is determined by the state of the particles before they impact on the substrate[l]. For the spray particle diagnostics a new method is offered by the development of the Particle-Shape-Imaging (PSI) technique. This method is intended for the analysis of size and shape of individual particles within the plasma jet. The method is based on telemicroscopic imaging of the particle shades. Similar to the Laser-Doppler-Anemometry a cw laser beam is split into two beams of equal intensity, which are superimposed in the focal plane of a Long-Distance-Microscope. The detection system consists of a CCD camera with a Micro-Channel-Plate intensifier allowing exposure times of few nanoseconds. When a particle passes the measuring volume exactly in the focal plane, the two laser beams generate individual shades, which congruently superimpose on the CCD Chip in the image plane of the telemicroscope. If a particle passes the measuring volume not exactly in the focal plane, the two generated shades are separated in the image plane. By this effect the position of the particle relatively to the focal plane can be measured. From the area and the contours of the shades, particles can be classified regarding size and form. Corresponding distributions of the particles within the plasma jet as well as changes of the particle form in the melting process can be determined.

Plasma spraying is a well established process for producing ceramic and metallic coatings for many technical applications. The quality of the coatings and the efficiency of the process depend on the powder and on the operating parameters as well as on the plasma torch properties. Whereas many efforts have been made on creating novel powders and on optimizing the operational parameters, the principle, however, of DC plasma torches has remained unchanged for many years. This was the reason for developing a novel DC plasma torch system. This paper presents the construction and operating principle of this innovative torch system. Its high performance is demonstrated in selected ceramic coatings. Paper includes a German-language abstract.

Dc plasma torches typically use a mixture of inert and molecular gases when spraying high melting powders. The addition of molecular gases increases the enthalpy of plasma jets, but it also produces arc root fluctuations that can cause variations in injected powders. This paper describes an innovative plasma torch system characterized by a long nozzle and three parallel cathodes. The nozzle consists of several electrically insulated rings and a ring-shaped anode. By adding more rings, the arc gap and voltage can be increased along with the enthalpy of the plasma jet. The results of various tests, comparing the spray rates and deposition efficiencies of new and conventional torches, are presented in the paper.