Major Applications

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Thermal sprayed Coatings have numerous applications to cover in one paragraph or one page. This article might provide an insight to many engineers in other fields to potentially use thermalsprayedcoatings in their applications to solve problems; it might also clue in to current thermal spray applications engineers to bring in new business to their companies.
Thermal Barrier Coatings:Thermal barrier coatings or TBCs as they are shortly called form a barrier as their name indicates to the flow and transfer of heat. This is used widely in aerospace applications in burner cans, combustion chamber cowls and domes, nozzles, etc. Generally, the combination is NiCrAlY as bond-coat and yttria-stabilized-zirconia (YSZ) as the top coat. Several variations are used including magnesium zirconate as the top coat or varied levels of yttria in the YSZ. There have also been graded coatings where a transition middle layer is used between the bond coat and the top coat. Bond coat thicknesses are usually .004"-.008" and top coat thicknesses are usually .010"-.014" in TBC systems. TBCs have also been used in power generation transition pieces where the coatings are deposited in the internal surfaces to redirect the heat to the inside and prevent the transfer to the outer casing. Industrial furnace components have been coated with TBCs as well as some high end components for space travel applications.
Abradable Coatings:
Abradable coatings as the name implies are for instances where the coating is designed to be cut into such as the case of aeroengine hot section abradables -- CoNiCrAly coatings formed by the LPPS process or lately with the HVOF process are used in hot sections. Used for clearance control, these coatings can be deposited to high thicknesses. Plasma sprayed nickel-graphite has been used for decades in certain applications. Aluminum-polyester coatings deposited by the plasma spray process is another alternative -- an example is by the use of powders such as Metco 601NS. In jet engine compressor cases, aluminum-silicon has been used very successfully. The porosity levels as well as porosity distribution in the latter two are very critical to abradability. Metallurgical lab evaluation of these coatings is extremely sensitive to pull-outs during polishing and special attention needs to be paid.
Abrasive Seal Coatings:
Aluminum oxide is the most common material used in abrasive seal coatings used in the aerospace industry. Knife edge seals have been coated with alumina-titania coatings for decades for this application. Usually, the combination is a nickel-aluminum bond coat of about .002"-.004" followed by a .008"-.012" top coat of alumina-titania. Key to coating knife edge seals is the uniform deposition of the coatings -- robotic application provides the best quality. These knife edge seal coatings can be stripped and re-coated -- we will be addressing stripping techniques in a different article.
Sealed coatings:
Many thermal spray applications involve first depositing the thermalsprayed coating and then infiltrating it with special sealants. For example, in a paper converting application, you can simply coat using Nickel-Chrome-Aluminum ( such as Metco 443 NS ) and then infiltrate using a Ultra-violet cured silicone for release properties ( UV coating ). The plasma coating holds the otherwise weak UV silicone coating. There are other sealants that can be infiltrated also. This is heavily used in the paper converting and printing press applications.

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1 comment:

Joe said...

Thanks for taking the time to write that. Good info to know.
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