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          Plasma Spray Process

Metalizarea cu Plasma

                                   Schematic Diagram of the Plasma Spray Process

Schematic representation of the plasma jet metallization process.

The plasma metallization process is based on the spraying of a material (ceramic, metal powder, etc.) melted in the plasma jet on a support to obtain a coating. The procedure was invented in 1920 by H. Gerdien in Germany. Commercial systems appeared after 1950. The material in powder form is injected into the plasma jet which has a very high temperature of 10,000-16,000 degrees C and over 3000 m/s. In the jet, the powder is heated quickly and accelerated to speeds over 600 m/s.

The powder that has reached the softening point is projected onto the substrate where it suddenly cools forming the coating. This correctly performed process is called “Cold Process” because the temperature of the substrate material can be kept low during the process, avoiding deformations, metallurgical changes, or distortions in the substrate.

The plasma metallization gun is composed of a copper anode and a tungsten cathode, both of which are cooled by water. The plasma gas flows around the cathode and through the anode, which has the shape of a nozzle.

The plasma is initiated by a discharge under high voltage (>10,000 V), leading to the local ionization of the plasma gas which becomes a good electrical conductor for a direct current arc that forms between the cathode and the anode. When passing through the electric arc, the gas dissociates and ionizes, forming plasma.

Upon exiting the nozzle, the ions recombine, giving up the absorbed energy in a very short time, which leads to the formation of a very hot plasma jet. The powder from a dispenser is injected into this jet.

The powder is immediately heated to the softening point and accelerated over a spraying distance of 25-150 mm at 450-650 m/s. The plasma metallization process is most often used in normal atmospheric conditions under the name APS.

Metallization is also practiced in vacuum chambers where a low-pressure shielding gas was introduced, the process being called VPS or LPPS. Plasma jet metallization has the advantage that it can use materials with a very high melting point such as refractories or ceramics. Combustion processes (AC-HVAF, HVOF) cannot deposit these types of materials.

Thermal Barier Coating ZrO2-8%Y2O3

                               Thermal Barrier Coating ZrO2-8%Y2O3 on a turbine blade


                  Plasma Spray Coating Photomicrographs ZrO2-8Y2O3


         Hardening of the ball valve by metallization with ceramic powder, over 1.100 HV

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