Copper Electrode with Tungsten Coating by DC Plasma Spray (2/2)

Both direct current (DC) plasma spraying and radio frequency induction plasma spraying were conducted in the spraying chamber connected to a vacuum system. The soft vacuum/low pressure (10-30KPa) of the spray chamber was used for preventing the oxidation of tungsten particles melted in plasma. Ar was used as secondary plasma gas. The substrate used is a hollow copper cylinder with approximately 20mm in diameter and 400mm in length. The hollow cylinder was mounted on a rotary shaft cooled by high pressure cooling water. Fine tungsten powder used as spray feedstock has the mean particle size of 5μm and coarse tungsten powder has the particle size in the range from 45μm to 75μm.

During DC plasma spraying, tungsten coatings were generated by moving plasma horizontally on the rotary copper substrate. A sphere-shaped chamber with about 1500nm in diameter was used. Tungsten powder was laterally fed into the plasma via two opposite powder injectors. In addition, during radio frequency induction plasma spraying, tungsten coatings were generated by moving the copper substrate horizontally and rotating it simultaneously because plasma torch was fixed to an oscillating unit of power supply system and could not be moved. On the contrary, lateral injection of powders in DC plasma spraying, tungsten powders were axially fed into the center of the plasma with a water-cooled powder injector in induction plasma spraying. It should be pointed out that spray distance was calculated from the tip of water-cooled powder injector to the surface of the copper substrate. Tungsten coatings on the substrate were cooled to room temperature before being taken out from the spray chambers. Coating samples were cut by a low speed rotary diamond saw. After being ground and polished, the samples were examined under scanning electron microscope (SEM). Density of the coating, chiseled from the copper substrate, was measured using the method of water displacement.