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Thermal spraying

Thermal spraying

Plasma spraying

Cross-section of a spray gun during plasma spraying

In technical terms, plasma spraying is the most advanced spraying method.

It works on the following principle: A plasma gas consisting of argon, argon/hydrogen, argon/helium or nitrogen/hydrogen is fed through an electric arc. The electric arc is produced between a tungsten electrode (cathode) and a copper nozzle (anode), both of which are water-cooled.

Considerable heat is generated within the electric arc and the gas is broken down. In this state, the gas holds large quantities of energy at a high temperature – up to 20,000 degrees C. The hot expanding gas exits the nozzle at high speed (approx. 3000 m/s) in the form of a plasma jet.

The powder is added to the jet from a separate powder feed via a powder nozzle. The powder particles melt.

Excellent coating quality is achieved, and this method allows spraying of material with a very high melting point, such as chromium carbide, tungsten carbide and ceramics.

Powder flame spraying

Cross-section of a spray gun during powder flame spraying

From a container on the spray or a separate powder feed, powder is dosed into a carrier gas and conveyed to the nozzle, where it is melted and impelled by the gas jet against the work piece Powder spraying has a considerably greater range of variation than wire arc spraying, as it can also be used to spray feedstock which cannot be produced in wire form. In addition to metal powders, various oxides and carbides can be sprayed, as well as metal-plastic mixes. Powder spraying is a cold coating method (maximum of 150 degrees C) except where so-called liquid powder is sprayed. Coats of this type are sintered at 1000 degrees C after spraying, making them spread out and solder to the parent material.

Wire flame spraying

Cross-section of a spray gun during wire flame spraying

Specially manufactured metal wires are used as feedstock: stainless steel, chromium steel, bronze, molybdenum, zinc, aluminium, etc. The wire is fed through the spray by a controllable air or electrically powered feed system. At the nozzle, the wire melts in a jet of oxygen/fuel gas. A powerful stream of compressed air is added around the jet, which splits the molten metal into fine particles and propels them at high speed against the work piece. The air stream cools both the drops and coating. This means the work piece is not heated to any appreciable extent, generally below 150 degrees C.

Electric arc spraying

Cross-section of a spray gun during electric arc spraying

In electric arc spraying, two metal wires are fed through the spray. Each of the wires passes through its own electrode and becomes conductive. The electrodes are positioned so that the wires meet, generating an electric arc which melts the tips of the wires. From a nozzle between the wires, a powerful stream of air splits the molten material into fine particles and propels them against the surface which is to be coated. With its big spraying capacity and low operating costs, electric arc spraying is mainly suited for large items.

High velocity oxygen fuel spraying (HVOF)

Cross-section of a spray gun during high velocity oxygen fuel spraying (HVOF)

High velocity oxygen fuel spraying (HVOF) is a further development of flame spraying and detonation spraying.

A fuel, usually LPG, hydrogen or paraffin, is mixed with oxygen and ignited in a combustion chamber. The combustion gases expand and are emitted at a velocity greater than the speed of sound through a nozzle in which powder is supplied with a carrier gas. The temperature is a maximum of 3000 C, and the hot particles achieve a very high impact speed with a capacity of 2–11 kg/hour. The coating has a very high bond strength.

Sintering