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Thermal spraying is the general term used to describe a variety of coating processes. Common processes described simply as "thermal spraying" include plasma spraying, HVOF spraying, arc spraying and flame/combustion spraying. These processes have also been generally called "metallising" or "metal spraying." While differing in method, all the listed processes are employed to add a coating to a substrate that, when combined, results in a "system" that is better suited to the engineering application than just the substrate alone.
Examples of substrate alone versus combined system properties include:
Substrate |
Coating |
System properties |
Cast iron |
Stainless steel |
Low cost casting with good corrosion resistance |
Nickel based alloy |
Yittria base ceramic |
Strong with excellent heat handling |
High strength steel |
Tungsten carbide chrome |
Strong with excellent wear & corrosion resistance |
Each thermal spray process uses two powerful characteristics of energy—heat and mechanical force—but in differing ways. In order to achieve a predictable and repeatable result, the process energy must be controlled to convert the coating raw material to a molten or plasticized state and accelerate it toward the substrate so that the coating particles can adhere to the surface, cool, and build up a coating.
The application of the design dictates the coating properties, and the coating properties oftentimes dictate the thermal spray method.
For example:
Light-duty corrosion protection on low-cost components generally can be sprayed with the economical process of electric arc spray using wire as not only the feed stock but also as the source of the electrical potential for melting. In this application, zinc or aluminum based materials in wire form are fed toward an intersecting point with each of the two feed wires having an externally applied electrical potential. The resulting arc causes the feed wires to melt. Since the potential energy in this process is converted mainly to heat and light, compressed air is directed at the intersection/arc location to atomize and "blow" the molten material toward the substrate. Subsequent passes of the torch, either by manual means or automation, build up the coating to the desired thickness.
A porous ceramic coating applied to a high melting point/low heat corrosion component is an important application for the hot section of aircraft engines and industrial gas turbines. In this application powder, with a very high melting point, is used as the raw material. In order to melt this type of powder, the gun must have a very high heating capability. The intense heat generated in the plasma plume is a good choice to properly melt these powders. The plasma process uses very high electrical potential to disassociate ions from gases. As the gases reclaim their lost ions a tremendous amount of energy is released, mainly in the form of heat, which melts the powder. The process also releases enough kinetic energy to accelerate the molten material toward the substrate. Subsequent passes of the torch, either by manual means or automation, build up the coating to the desired thickness.
Airplane landing gear piston surfaces require a very hard, smooth, and fatigue resistant coating over the high strength steel substrate. The HVOF process combines fuel and oxygen to create rapidly expanding combustion gases inside a specially designed combustion nozzle. This process converts much of its energy into mechanical force which accelerates the powder particles through the flame very quickly. The powder moves so quickly through the flame that the particles in many applications do not fully melt. The plasticized particle's collision with the substrate at very high velocity supplies the final heat needed to complete the melting process of the powder. This allows the coating to be built up on the substrate in a very dense and uniform manner. Subsequent passes of the torch, either by manual means or automation, build up the coating to the desired thickness.
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