Thermal Barrier Coating – What Is it?
If you’re looking to purchase or build a power plant, you’ve probably heard about thermal barrier coating for turbines, but what is it? Why is thermal barrier coating important? What is thermal barrier coating made from?
Here’s what you need to know about thermal barrier coating. If you’re buying or building a power plant, contact Allied Power Group (APG) to find out how we can be the partner you need to keep your plant running.
What Is a Thermal Barrier Coating (TBC)?
Thermal barrier coatings (TBC) are ceramic, heat-resistant coatings applied to superalloys to protect them from corrosion, oxidation, and heat, which can increase the operating temperature of the part (such as a gas or steam turbine or a jet engine) by several hundred degrees.
Currently, the most advanced materials for high-temperature applications include nickel-based superalloys with thermal barrier coatings.
TBCs have 4 main layers (if you include the superalloy they cover):
- Metal substrate
- Metallic bond coat
- Thermally grown oxide (TGO)
- Ceramic top coat
The ceramic top coat is most often made from 7 weight percent yttria-stabilized zirconia (7YSZ) due to its erosion resistance, phase stability, and low thermal conductivity. In fact, it exhibits resistance to thermal shock and thermal fatigue up to 1150°C. It’s typically applied using plasma spray or electron beam-physical vapor deposition (EB-PVD).
What Does a TBC Do?
A thermal barrier coating is designed to prevent the melting or degradation of high-temperature inlets, leading to more efficient energy conversion.
What Are Thermal Barrier Coatings Used for?
While thermal barrier coatings are most commonly used for gas turbines, they are also used in a variety of other automotive, aviation, and industrial applications, such as:
- Exhaust manifolds
- Aeroengine parts
- Nozzle guide vanes
- Other components
How Do Thermal Barrier Coatings Work?
TBCs must “stick” to the alloy, have a similar coefficient of thermal expansion, and protect the alloy against both oxidation and heat. To meet these requirements, a ceramic material must be chemically inert, have a low thermal conductivity, and have a high melting point. Additionally, the coating must avoid diffusing with the superalloy.
Thermal barrier coatings are made with 3 layers, each of which performs a different function:
- The top coat – the outermost layer – is a porous oxide with very low conductivity. It can survive stresses caused by changes in thermal expansion and insulates the alloy from high temperatures.
- The bond coat touches the superalloy and serves as a barrier to diffusion and is for oxidation resistance. It’s usually made of many metals, such as NiCoCrAlY.
- A thermally grown oxide (TGO) results from a reaction between the bond coat and air. It helps keep oxygen out as well as keeps the other layers together. The ideal TGO is Al2O3.
All 3 layers work together to protect the superalloy from oxygen and heat and to keep the coating stuck to the superalloy.
Why Are Thermal Barrier Coatings Important?
While TBCs are used in a variety of applications, they are especially important for running power plants. Why?
The hotter you can run a turbine, the less energy is wasted when converting fossil fuels to electricity. Thermal barrier coatings allow you to run turbines hundreds of degrees hotter than you could otherwise, resulting in significantly increased efficiency – meaning more power with less waste.
Although nickel-based superalloys can survive hotter temperatures than traditional steel, the ceramics in TBCs have even higher melting points than the superalloys they cover along with better corrosion and oxidation resistance. By adding thermal barrier coatings to superalloys, turbines can run hotter and more efficiently.
Benefits of Thermal Barrier Coatings
Benefits of TBCs include:
- Protection from extreme temperatures
- Increased lifespan of parts
- Improved thermal conductivity
- Reduced fuel consumption
- Increased engine power
How to Make Thermal Barrier Coatings
Engineers must deposit the top coat and the bond coat, but the TGO will develop naturally.
One of the most common methods of creating a bond coat is Chemical Vapor Deposition (CVD), where the base superalloy is exposed to volatile chemical precursors that are heated and react to form the desired deposit.
Other methods for producing the bond coat include Electron Beam Vapor Deposition (EB-PVD), plasma spray techniques, and electroplating with diffusion-aluminizing.
Top coats are typically created using Electron Beam Vapor Deposition (EB-PVD) or Atmospheric Plasma Spraying (APS).
EB-PVD occurs in a vacuum where an ingot is blasted by electrons, sending vaporized particles to the substrate. With APS, the coating is vaporized in plasma and shot at the superalloy.
Thermal Barrier Coating Materials
Since each layer interacts with the others, it’s crucial to choose the proper materials. Here are a few things you should know about common TBC materials.
The substrate is the material being coated; in this case, the substrate is a superalloy. While there are experimental Co- or Fe-based superalloys, Ni-based superalloys are the most popular. Composed of many different elements (including Ni, Fe, Co, W, Al, Mo, Hf, Re, C, Ti, and others), superalloys are air-cooled through internal hollow channels that don’t interact with the thermal barrier coating materials.
The bond coat is a critical component that helps create the oxidation-resistant TGO. Al2O3 is the most protective oxide, with very low oxygen diffusivity and high strength, so the 2 classes of bond coat alloys are MCrAlY and aluminides (a combination of aluminum and another metal, typically Pt and Ni).
Thermally Grown Oxide (TGO)
The key “ingredient” that allows TBCs to work, thermally grown oxides must act as a diffusion barrier while sticking to both the top coat and the bond coat. TGP forms naturally as the superalloy interacts with oxygen to form an oxide layer.
The most common top coat material is yttria-stabilized zirconia (YSZ) because it’s the toughest ceramic currently available and has a thermal conductivity of 1.2 – 1.8 W/mK.
Make APG Your Power Partner
If you’re running a power plant, you need a partner to help you with repair, maintenance, and other aftermarket solutions. Allied Power Group (APG) could be your perfect partner. Contact us today to find out how.