Thermal spray coatings are very effective in increasing component life and value. These industrial coatings decreasing machinery down time. They also improve performance in a wide variety of applications.
Thermal spray is a generic term for processes which are designed to rebuild worn material. The process can add different or enhanced properties to an existing surface. It can also correct multiple industrial problems. Other terms used interchangeably with thermal spray are metal spray, flame spray, plasma spray, HVOF spray, plasma coating, and metallizing.
Improving tolerances in complex machines is a great use of the process. It can also be used for manufacturing shaped components layer by layer. Thermal spray can also create thermal barrier coatings to protect against other materials.
Thermal Spray Processes
HVOF, also known as High Velocity Oxygen Fuel, is a mixture of gaseous or liquid fuel and oxygen fed into a combustion chamber. In the chamber, they are ignited and combusted continuously. The resulting hot gas emerges from a converging-diverging nozzle and travels through a straight section. This is where a powder feed stock is injected into the gas stream. The stream of hot gas and powder is directed towards the surface to be coated. The powder partially melts in the stream and deposits upon the substrate. The coating is created through multiple passes. It has low porosity and high bond strength.
In the Plasma spraying process, the material to be deposited is introduced into the plasma jet from a plasma torch. The deposited material may be a powder, liquid, suspension or wire. In the jet, where the temperature approaches 10,000°K, the material is melted and propelled towards a substrate. The molten droplets flatten, rapidly solidify, and form a deposit.
Twin wire arc spray is a form of thermal spray where two consumable metal wires are fed independently into the spray gun. These wires are charged and an arc is generated between them. The heat from this arc melts the incoming wire. It is then routed through an air jet from the gun. This molten feed stock is then deposited onto the substrate. The process is commonly used for heavy metallic coatings.
The combustion thermal spray process is also called a “cold process”. The substrate temperature can be kept low during processing to avoid damage. This prevents changes and distortion to the substrate material. The cold process is relative to the temperature of the substrate material being coated. Combustion coatings are typically not as well bonded as coatings produced with other processes. They can be applied in very thick coatings due to their low coating stress.
Spray & Fuse
The Thermal Spray & Fuse process is a two-step process. A powdered coating material is deposited by using either a combustion gun or plasma spray gun. It is then fused using either a heating torch or a furnace.
Hardfacing is a combustion spray process that uses many different materials. Once sprayed and heated the coating fuses to itself and the substrate. This process can create a coating with a hardness of up to 80 Rc.
The learn more about Thermal Spray and Thermal Coatings, visit Wikipedia.
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