Turbines, or rotary engines that create power, have a multitude of uses. They are used in machines that perform work on Earth and are essential components of airplanes. Currently, most turbines are built using metallic based components, and these metal components require cooling to avoid reaching their thermal limits. New, more efficient engine technology requires components that can survive higher temperatures and reduced cooling.
Silicon based ceramic components show great potential for use in advanced, higher efficiency engines, as they are capable of withstanding higher temperatures and weigh less than metal components. However, when unprotected, these silicon based ceramic components react and erode in turbine engine environments due to the presence of water vapor.
New coating processing technology is being pioneered at NASA Glenn's Research Center in Cleveland. The technology is used to protect advanced silicon based ceramic engine components that are being developed for future engines. This coating processing technology will enable more complex and thinner coatings than are currently possible. This is important for coating turbine blades, which need to endure engine environments and stress conditions, while still remaining smooth to avoid the disruption of airflow. This coating processing technology, called Plasma Spray – Physical Vapor Deposition (PS-PVD), has the potential to radically improve the capabilities of ceramic composite turbine components.
"PS-PVD technology is really necessary for the integration of silicon-based ceramic airfoil components into turbine engines. The use of these silicon-based ceramics as engine airfoil components would increase engine operation temperature, which translates into higher efficiencies," says Bryan Harder, the lead for the PS-PVD Facility at Glenn.
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Plasma Spray – Physical Vapor Deposition
It has been known for decades that enveloping metals and other substances, such as silicon based ceramic components, with a ceramic coating can protect them. But there is new, cutting-edge technology that can create ceramic coatings in an extremely precise, uniform fashion-the coatings can be controlled to a thickness of ten microns (a micron is one-millionth of a meter). This technology is made possible by Glenn's Plasma Spray – Physical Vapor Deposition (PS-PVD) Facility.
The Plasma Spray – Physical Vapor Deposition (PS-PVD) Coater was completed at Glenn in 2010. Created in collaboration with Sulzer Metco, the PS-PVD rig is one of only two such facilities in the U.S.A. and one of four in the entire world. The PS-PVD rig, which is currently a research and development facility, uses a state of the art processing method of creating thin ceramic coatings. Planning began for the facility in 2007, and construction began in 2008 (previously constructed infrastructure was reused and is now the base for the new rig).
The rig is nearing completion of its capabilities testing and assessment phase. A team of five, led by Bryan Harder, a materials research engineer, has put the rig through its paces. The rig will soon begin supporting the Supersonic Project within NASA's Aeronautics Research Mission Directorate at Glenn. Eventually, the rig could be of service to many other areas and projects within Glenn, other NASA centers and governmental entities, and private industry partners.
"When you have something that has broad capabilities like this, it really allows us to work with a lot of different areas, which is a great thing," says Bryan Harder.
Super Thin Ceramic Coatings
The Plasma Spray-Physical Vapor Deposition (PS-PVD) rig creates thin, extremely precise ceramic coatings. These coatings are created on metal, ceramic, or other appropriate materials.
"To create these coatings, ceramic powder is injected into a very high power plasma flame under a vacuum. During operation, the plasma is approximately 7 feet long and 3 feet wide. The ceramic material is vaporized within the plasma, and condenses onto the target component," says Bryan Harder.
The coatings can be single or multilayer, and they protect the components from environmental and thermal impact. The extremely high heat and the vacuum within the chamber allow the ceramic coating to be precisely applied, creating durable, long-lasting, effective coatings.
"If you can reduce the thickness, and still provide an effective barrier layer - you can reduce the weight, you can reduce your cost. There are a lot of benefits that come from this technology," Harder says.
Inside the Chamber
Located at Glenn, the Plasma Spray – Physical Vapor Deposition (PS-PVD) is installed in a dedicated room. A large, blimp-shaped chamber is made of stainless steel. The exterior metal, which is welded to a second sheet of stainless steel beneath, has cool water pumped through it to keep the chamber from getting too warm.
Inside the chamber is a steel arm which holds a plate made of a nickel-based superalloy. This plate holds the component that will be coated. Several feet away from this plate is the torch, where the ceramic powder is injected into the plasma. Once the chamber is closed, a system of vacuum pumps and a blower remove air from the chamber, reducing the pressure to one Torr (1/760th of normal atmospheric pressure). Then, helium and argon gases are introduced to the torch. An arc is created between the anode and cathode inside the chamber, ionizing the gases and creating the high temperature plasma.
The plasma, which can grow to seven feet in length, can be observed through one of three portals on the side of the rig. Its steady, fierce, concentrated glow resembles a Lightsaber from the Star Wars movies. Once the vacuum and plasma are stable, the ceramic powder is introduced to the torch. The plasma immediately begins to change colors. Depending on which ceramic powder is introduced, the plasma dramatically erupts into oranges, yellows, aquas, purples and blues.
The gas stream moves at a speed of Mach 2 - a rate of more than 2,000 feet per second. As the ceramic powder and the plasma blast the arm and plate where the component being coated is attached, the plasma appears to envelop the component and splash around it. The plasma, which appeared like a Lightsaber, seems to morph into the effect of the undulating stream of magic that occurs when Harry Potter's wand meets with Lord Voldemort's wand, in the Harry Potter movies.
The entire process is over in about five minutes. The plasma is extinguished and the exhaust system clears the chamber. The pressure is returned to normal atmospheric conditions, and then the chamber can be opened. The newly-coated component glows red hot and must cool down for an hour before it can be handled. The plasma within the chamber can reach a scorching 10,000 degrees Celsius - ten times hotter than a candle flame.
After the sample cools, it will be tested and evaluated to ensure the coating is an effective barrier. And then the sample - be it a small test button or an essential component of a supersonic aircraft - is ready to go. The front, sides and inside of the sample can be coated - a capability never previously available from vapor deposition techniques.
"The PS-PVD allows us to do things that you can't do anywhere else," Harder says.
This newly developed technology could have myriad applications, both within NASA and with potential industry partners. The potential applications are only beginning to be discovered - from membrane technology to fuel cells to ion conductors and beyond.
The rig is a game-changing technology; Glenn is maturing and developing a technology that doesn't exist elsewhere, while making direct contributions to the NASA mission.
"This is new ground," Bryan Harder says. "This was only developed in the last couple of years… and we don't even know the limits of what it [PS-PVD] is capable of."
Tori Woods, SGT Inc.
NASA's Glenn Research Center