Coating production by thermal plasma spray is dependent on the residence time of particles in the plasma jet produced by the gas flow inside a plasma torch. To ensure a high fraction of well-molten particles to be accelerated towards the substrate a long reproducible residence time is needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. While single electrode plasma torches need an unstable attachment of the anodic arc root in order to avoid excess erosion, which also causes an unstable plasma jet, multi-electrode torches allow operation with fixed anode attachments by subdividing the anode current by the number of electrodes used, and thereby thus reducing the power input for each separated arc root. Once the steady anode attachment has been obtained the produced plasma jet exhibits a steady characteristic, but also looses rotational symmetry. The separation can be achieved by using either multi anode or multi cathode geometry with appropriate electrical control. Both version have been produced with 3 electrodes each resulting in two systems known as the Delta Gun (3 anodes) and Triplex (3 cathodes).

Recent advances in the development of new plasma torches rely on the use of multiple electrodes to improve plasma jet stability. Examples are the Triplex, employing three cathodes and a single anode, and the Delta, which uses three anodes with a single common cathode. To characterize the plasma jet produced by these plasma torches, initial experiments have been performed using tomography in the visible range. Due to the improved stability of the plasma jet in the multielectrode configuration it is possible to carry out tomography by means of a regular charge-coupled device (CCD) camera, which is rotated around the plasma jet axis. The two-dimensional information obtained by the CCD camera is subsequently processed to produce an image of the three-dimensional emission distribution. The tomographic analysis is mated with a simple but effective simulation tool, which can be used as a basis for parameter-dependent multielectrode plasma torch design.

No abstract

Present standard d.c. plasma torches are characterized by the combination of a single rod-shaped cathode and a conical one-piece anode. Mixtures of inert gases and molecular gases are used for plasma spraying of high melting powders. By adding the molecular gases high enthalpy plasma jets are generated, but simultaneously the well-known art root fluctuations cause an inhomogeneous treatment of the injected powder. The innovative plasma torch system TRIPLEX is characterized by a long nozzle and by three parallel cathodes. The nozzle consists of several rings all electrically insulated except for the last one which operates as an anode. By increasing the number of rings the three arts between the cathode tips and the anode ring can be lengthened. This leads to a higher art voltage and therefore to a higher enthalpy in the plasma jet. No molecular gases have to be used and are root fluctuations are prevented. The result is a uniform treatment of the injected powder. Using optical and electrical diagnostics the TRIPLEX torch ist compared to a typical conventional plasma torch. High spray rates and deposition efficiencies show the good performance of TRIPLEX.

No abstract