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.
Major Applications
Other T/S processes
Simple Combustion Process:
In the simple combustion process, energy is generated by the combustion between oxygen and a combustible gas such as propane inside a torch and the powder is fed through a powder port and the resulting coating deposited on to a substrate material. This process has been used to deposit abradable materials such as nickel-graphite and the like. Very high thicknesses of nickel-graphite have been successfully deposited using this process. This has been the forerunner to the now-popular HVOF process. The Metco 6P torch has been used for several decades to accomplish this type of coating. As a sidenote, in depositing nickel-graphite coatings, powder settling becomes a major problem -- powders must be properly blended and kept hot so they will flow easily and form uniform microstructures.
Low Pressure Plasma Spray:
In the LPPS method, coating is accomplished in a vacuum chamber -- this therefore results in oxide levels in low-pressure plasma sprayed coatings to be very low. This has been successfully used in developing thick Co-NiCrAlY coatings, titanium-6Al-4V dental implant coatings as well as some high quality titanium-carbide coatings. Key to watch out for is to also check leak-up rates in the powder feed system, since that can cause leak-contamination in the vacuum chamber.
Simple Wire Gun Process:
Unlike the twin wire-arc process, here there is only one wire involved. Used extensively in aluminizing applications, this has been used for decades successully with a post diffusion heat treatment.
Twin-Wire-Arc process
The twin-wire-arc process of thermal spray is actually a fairly well-established process, wherein two wires are fed through and an electric arc struck between them. This melts the wire and a carrier gas "shoots" the melted particles on-to the substrate material to be coated. It has been used for several decades to re-build worn out surfaces on pump components, etc. Nickel-aluminum wire is the most common. However, there have been several applications where cored wires that comprise of harder core materials such as tungsten carbide particles encapsulated in a wire have been used. The TAFA MX-C series of wires fall under this category. Years ago, there was a wonderful application, wherein the wire arc process was used as a low-cost replacement to developing a hard wear resistant surface on to barrel bars used in the corn oil extraction industry where the previous method was PTA ( Plasma Tungsten Arc ) welding. Again, for sensitive applications, proper control of the wire feed rates, stand-off distance, and current/voltage parameters are needed for superior coating quality.
Plasma Spray Process
In the plasma spray coating process, a plasma plume is created by striking an arc between an anode and a cathode with gas flowing through. The arc rips off electrons from the primary gas flowing through making it highly unstable; its natural tendency is to replace the lost electrons and thus come back to its stable state. This distance can vary quite a bit depending upon the voltage, amperage as well as the properties of the gas used. Generally this is about 4 inches. A coating material in the form of a powder is fed into this plasma plume; the material gets highly energized and reaches a plastic state and reaches the substrate where it bonds to form the coating. This is the succint description of the process.
As one can deduce, if the pressure of the surrounding "atmosphere" is lowered, then the length of the plasma plume increases; this is the case in low-pressure-plasma-spray coating process, where the plasma is generated under reduced pressure conditions. The length of the plasma plume in such cases can be as high as 14 inches. If metallic coating materials are used, then they are prone to oxidation by the surrounding air. The level of oxidation can be controlled either by coating under low-pressure ( sometimes referred to as 'vacuum plasma' ) or under a shroud of inert gas such as argon, referred to as 'shrouded plasma'.
The primary gas used for standard air plasma sprayed coatings is generally nitrogen or agon. In most cases, a secondary gas is introduced into the stream to enhance the energy level. This gas, usually referred to as the secondary gas, is either hydrogen or helium. Hence the gas flow is either a nitrogen-hydrogen or argon-helium combination.
Proper introduction of the powder into the plasma plume is critical to coating quality. Improper introduction of the powder will result in unmelted particles in the coating aggregate. The powder is usually fed by a powder feeder with a carrier gas at a predetermined feed rate that is usually closely controlled. Too high a feedrate will also cause bad coating quality since the particles will not have sufficient energy.
There are several variations to gun/substrate movement -- the common methods being (a) the substrate to be coated is either held stationary and the plasma gun is moved relative to it either manually or attached to a mechansim such as a robot arm; OR (b) the substrate is installed on to a turntable and the plasma gun is moved in an up-down manner. In all instances, proper speeds for the moving equipment is important to ensure uniform high quality deposition. For example, turn-table rpm, traverse speeds (inches/minute), etc are critical. In more critical applications, advanced controls to turn off the powder feeder and the gun if the turntable stops, etc, can be instituted.
For safety aspects of the process, you may want to visit my page http://safetyfirstatwork.blogspot.com/.
Thermal spray what's it
This site is devoted to the understanding of thermal spray coatings technology. We will be addressing several topics in this site that we presume will be useful both to those intending to learn a little bit more about the thermal spray process prior to using it in an application as well as to those professionals in other engineering fields that might be able to solve future application problems using thermal spray technology. It might be of value to refer to the archives for any specific topic of interest. For safety aspects of the process, you may want to visit my page http://safetyfirstatwork.blogspot.com Being essentially a surface modification process, thermal spray coatings consist of applying a variety of coating materials by elevating the thermal energy of the coating materials and sometimes the kinetic energy of the coating materials and bonding them on to the substrate material. Unlike some other methods of coatings deposition, thermal spray techniques allow a wide variety of coating such as metallics, intermetallics, ceramics and cermets to be deposited on to a wide variety of substrate materials such as metals and non-metals.
In our next topic of discussion, we will discuss plasma sprayed coating process as the one method of imparting energy to enable the bonding of coating material to the substrate material.