This paper discusses the double-arc phenomenon that occurs in plasma arc spraying and its prevention through the use of a bi-anode torch. It also presents the results of a study that compares voltage-current characteristics of the arc for different plasma gases and arc root attachment points. It is shown that the arc has different electrical characteristics when changing from the first to the second anode, which is explained using a simplified arc model.

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20µm were used to deposit coatings. Spraying properties such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency.

A special bi-anode plasma torch has been developed for elongating the arc length and obtaining the arc with a fixed length. The relations between the arc voltage and the arc current were investigated for different gas compositions, and the effect of the gas flow rate on the arc voltage was also studied for the pure Ar plasma. The results show that for the pure Ar plasma, the arc voltage had a dropping characteristic for an arc with instable length and a rising characteristic arc voltage for an arc with fixed length, but for Ar-H 2 plasma or Ar-N 2 plasma, the arc voltage always had dropping characteristic. Increasing the gas flow rate will enhance the arc voltage both for an instable and fixed length arc.

In this paper, a low-power plasma spraying system is used to deposit layers of tungsten carbide on carbon steel substrates. Numerous coating samples were produced and examined in order to determine how plasma gas composition, voltage, current, and other parameters affect the hardness and porosity of WC coatings and how injected particles interact with nozzle surfaces. Test results indicate that there is a tendency for less particle adherence to the nozzle at higher voltages, lower currents, and certain plasma gas compositions. Paper includes a German-language abstract.