Thermal Barrier Coating
What a Thermal Barrier Coating Does
Heat moves through metal fast. A thermal barrier coating slows that down. It does not stop the heat. It buys the part time.
That difference is the whole point. In a gas turbine, combustion gases run past 1,500°C (2,730°F). The nickel superalloy underneath starts losing strength long before that. A TBC drops the metal surface temperature by a meaningful margin, and that margin is what keeps blades and vanes running instead of heading for the scrap bin.
A&A Coatings has applied TBC systems for over 70 years. This page covers how they work, where they are used, how they fail, and what to check before you spec one.
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The TBC System: Bond Coat Plus Topcoat
A thermal barrier coating is not one layer. It is two, and each layer has its own job.
The Bond Coat
The bond coat sits right on the metal. It is a thin metallic layer, usually an MCrAlY alloy or a platinum aluminide. The name MCrAlY breaks down into metal, chromium, aluminum, and yttrium.
MCrAlY bond coats do most of the work in the field. The M stands for a metal, usually nickel, cobalt, or both. Nickel-based versions pair well with nickel superalloy substrates because the two metals behave the same way under heat. Cobalt-based versions hold up better against sulfidation, which matters when the fuel has sulfur in it. The chromium and aluminum form protective oxides at temperature. Yttrium helps those oxides cling to the metal instead of flaking off.
Platinum aluminide bond coats show up on high-pressure turbine blades. They go on through a diffusion process that packs platinum and aluminum into the surface. The result is a hard layer that resists oxidation. Platinum aluminide takes thermal cycling well, but repair is difficult. Once it degrades, the blade often gets a full recoat rather than a patch.
So what does the bond coat actually do? Three things. It sticks to the substrate. It gives the ceramic topcoat a surface it can bond to. And it grows a thin oxide layer, called the thermally grown oxide or TGO, that anchors the ceramic in place.
That TGO matters. Without it, the ceramic has nothing to grip. But the TGO also thickens over time. When it gets too thick, it builds stress at the interface, and that stress is one of the main ways a TBC fails.
The Topcoat
The topcoat is the ceramic. The standard material is yttria-stabilized zirconia, or YSZ. Zirconia alone changes crystal structure when it heats and cools, which causes it to crack. Yttria, usually 6 to 8 percent by weight, stabilizes the structure so it stays put through thermal cycling.
YSZ works because of its low thermal conductivity. The value varies with temperature and coating structure, but it runs several times lower than the nickel alloy underneath. That gap is what creates the temperature drop across the coating.
The ceramic also has a relatively high coefficient of thermal expansion for a ceramic, which brings it closer to the metal. It is still lower than the metal, but the bond coat absorbs the remaining mismatch without cracking.
Thickness, Conductivity, and the Trade-Offs
Thicker coatings give more insulation. But thicker coatings also build more stress.
The two main application methods produce different structures.
Air plasma spray (APS) creates a lamellar structure. Molten particles flatten on impact and stack in layers. That structure has high porosity, and the pores are partly connected. The porosity reduces thermal conductivity. But the lamellar structure handles rapid thermal cycling less well than a columnar one.
Electron beam physical vapor deposition (EB-PVD) creates a columnar structure. The ceramic grows in vertical columns with gaps between them. Those gaps let the coating flex without cracking. EB-PVD coatings handle thermal cycling better, but they cost more and take longer to apply.
Low-pressure plasma spray and HVOF are sometimes used for the bond coat, and in some shops for the topcoat as well. HVOF produces a very dense bond coat, which is an advantage for oxidation resistance. The process is less common for the ceramic topcoat itself.
For plasma sprayed YSZ, typical thickness runs 200 to 500 micrometers. For turbine blades, the common target sits around 250 micrometers.
There is a ceiling, though. Thicker ceramic coatings tend to fail sooner under thermal cycling. The stress stored in the thicker layer outweighs the extra insulation benefit. Practical thickness is a balance between heat drop and service life.
Thickness also affects conductivity directly. Thinner coatings can show lower conductivity because of their microstructure. That is counterintuitive, but it is one reason you cannot simply assume thicker equals better insulation. The right thickness depends on the part, the cycle profile, and the fuel.
TBCs and Cooling: How They Work Together
A TBC does not work alone. On a modern turbine blade, it works with internal cooling and film cooling to keep the metal below its limit.
Internal cooling. Blades and vanes are hollow. Compressor air flows through serpentine channels inside the blade, pulling heat out of the metal. The channels are lined with small pins or ribs that increase surface area and turbulence. That internal flow can remove a large share of the heat load before the metal reaches its limit.
Film cooling. Tiny holes drilled through the blade surface push cooling air out onto the surface. The air forms a thin film that shields the metal from the hot gas. Film cooling is why you see rows of small holes on the leading edge and pressure side of turbine blades.
The TBC’s role. The coating reduces the heat that reaches the metal in the first place. With a TBC in place, the cooling air has less heat to remove. That means the engine can run at a higher combustion temperature, which improves efficiency, or use less cooling air, which also improves efficiency. Either way, the TBC pays for itself in fuel savings.
The three systems work together. Take the TBC away and the blade can overheat even with full cooling. Take the cooling away and the TBC alone is not enough. That is why OEMs spec all three as a package.
- Where TBCs Are Used
- Gas Turbines and Aeroengines
The hot section of a gas turbine is the classic TBC application. Turbine blades, vanes, combustor liners, and transition pieces all run at temperatures that would destroy unprotected metal.
In an aeroengine, the high-pressure turbine blade is the hardest-working part. It spins at extreme speed in gas that pushes past 1,400°C. The TBC works alongside internal cooling channels to keep the metal alive.
Industrial gas turbines use the same system. MCrAlY as the bond coat, YSZ as the topcoat, applied by low-pressure plasma spray, HVOF, or APS depending on the part and the OEM specification.
Diesel Engines
Diesel pistons are usually aluminum alloy. Aluminum is light and conducts heat well, but it loses strength at high temperatures. A TBC on the piston crown lowers the surface temperature, which lets the engine run at higher combustion temperatures without melting the piston.
The challenge with diesel is the environment. Diesel exhaust contains sulfur and other corrodants that can attack the ceramic. For thick ceramic layers in diesel service, surface modifiers may be needed to block the ingress of corrodants.
Other Applications
TBCs also show up in industrial furnaces, rocket engine components, and power generation turbines. Anywhere metal faces sustained high heat, a TBC is worth considering.
How TBCs Fail
A TBC system fails when the ceramic topcoat separates from the bond coat. That separation is called spallation. The causes vary, and they often interact.
Thermally Grown Oxide (TGO) Growth
The bond coat oxidizes during service, growing the TGO layer. Some TGO is needed. Too much is fatal.
As the TGO thickens, it creates compressive stress at the interface. When that stress exceeds the bond strength, cracks form and spread. Eventually, a section of the ceramic lifts off. This is a common failure mode in clean fuel service.
CMAS Attack
CMAS stands for calcium-magnesium-alumino-silicate. It is the technical name for sand, dust, and ash that gets pulled into an engine. At high temperatures, CMAS melts and soaks into the porous ceramic.
Once inside, CMAS fills the pores and changes the coating’s mechanical properties. On cooling, the infiltrated layer cracks and takes the ceramic with it. CMAS-driven delamination is a particular problem for engines operating in dusty environments.
CMAS melts in a specific temperature range. Below that range, deposits stay solid and shed harmlessly. Within the range, they stick and cause damage. Above it, they can soak in deeply and cause rapid failure.
Erosion and Foreign Object Damage
High-velocity particles erode the ceramic. Sand, volcanic ash, and runway debris all take their toll. Erosion thins the coating, which reduces insulation and eventually exposes the bond coat.
Foreign object damage, or FOD, is a single impact event. A large particle hitting the coating can chip or crack it, creating a site for spallation to begin.
Thermal Cycling
Every time the engine heats up and cools down, the TBC system goes through a stress cycle. The metal expands more than the ceramic. The bond coat absorbs some of that mismatch, but not all of it.
Over thousands of cycles, cracks start at the TGO interface and grow. The coating eventually spalls. This is why thicker coatings tend to fail sooner under cycling. More ceramic means more stored elastic energy, and more energy released when a crack forms.
A Worked Example: A High-Pressure Turbine Blade
Here is how a TBC system comes together on a blade, start to finish.
Step 1: Substrate prep. The blade arrives as a cast nickel superalloy, already hollow with internal cooling passages. The surface is cleaned and grit blasted to remove oxides and give the bond coat something to grip.
Step 2: Bond coat. MCrAlY is applied by low-pressure plasma spray or HVOF to a thickness of 100 to 150 micrometers. The blade goes into a vacuum chamber for the low-pressure process, which prevents oxidation during application and produces a dense, well-bonded layer.
Step 3: Topcoat. YSZ is applied on top of the bond coat. For aeroengine blades, EB-PVD is common because the columnar structure handles thermal cycling. For industrial turbines, APS is more common. Topcoat thickness lands around 250 micrometers for aero, thicker for industrial.
Step 4: Heat treatment. The coated blade goes through a diffusion heat treatment. This step strengthens the bond between the bond coat and the substrate, and it starts the formation of the TGO.
Step 5: Inspection. Thickness is checked with an ultrasonic gauge. Bond strength is verified on a coupon from the same batch. Microstructure is confirmed by cross-section under a microscope. Any defect that would create a stress riser is flagged.
Step 6: In service. The blade goes into the engine. Every flight cycle puts the TBC through a stress cycle. Inspectors track spallation and erosion at scheduled intervals. When the coating degrades past a threshold, the blade is either repaired or replaced.
That is the life of a TBC system. Every step matters. Skip one and the coating fails early.
Spec and Quality: What to Check
When you spec a TBC system, these are the parameters that matter.
Coating Thickness
Thickness is specified for both the bond coat and the topcoat. Typical bond coat thickness runs 100 to 150 micrometers. Topcoat thickness depends on the application and the cycling environment. For aeroengine blades, 250 micrometers is common. For industrial gas turbines, thicker coatings may be used.
Measure thickness with a micrometer on a coupon or with an ultrasonic gauge on the part. Destructive testing on a sample from the same batch confirms the metallurgical thickness.
Bond Strength
Bond strength is measured by pulling a coated plug from the substrate, or by a bend test. The value depends on the bond coat, the substrate, and the application method. A&A Coatings runs bond strength checks as part of our in-house lab work.
Porosity and Microstructure
Porosity affects both thermal conductivity and mechanical behavior. Too little porosity means higher conductivity and less strain tolerance. Too much means lower bond strength and faster erosion.
Microstructure analysis by scanning electron microscopy confirms the lamellar or columnar structure, the pore distribution, and the TGO thickness.
Thermal Cycling and Oxidation Resistance
Furnace cyclic testing and burner rig testing simulate service conditions. A coated coupon goes through repeated heat and cool cycles, and inspectors track when spalling begins. These tests are expensive and time-consuming, but they are the only way to predict service life with confidence.
Thermal Conductivity
Thermal conductivity is measured by laser flash or similar methods. The value depends on the composition, the porosity, and the microstructure. Lower conductivity means better insulation, but it often comes with a trade-off in mechanical properties.
Inspection and Repair in Service
TBCs do not last forever. Inspection and repair programs extend the life of coated parts.
Borescope inspection. Engines are inspected with a borescope between service intervals. Inspectors look for spallation, cracking, and CMAS deposits. Any of those trigger a closer look.
Coating thickness measurement. Portable ultrasonic gauges measure remaining coating thickness on installed parts. When thickness drops below a threshold, the part is scheduled for repair or replacement.
Repair options. When the TBC degrades but the substrate is sound, the coating can be stripped and reapplied. Stripping uses chemical or mechanical methods to remove the ceramic and bond coat without damaging the base metal. The part is then recoated and heat treated as if it were new.
When repair is not possible. If the substrate has been damaged by overheating, oxidation, or cracking, the part cannot be salvaged. That is why inspection intervals matter. Catching TBC degradation early keeps the substrate alive.
Quality Certifications
A&A Coatings holds AS9100 and ISO 9001 certifications and meets NADCAP requirements for coatings. Our in-house lab checks thickness, bond strength, hardness, and microstructure before parts ship. Customer specs welcome, with certification paperwork supplied on request.
Related Pages
For the material spec on the ceramic topcoat, see our page on Plasma Zirconia, Titania, Yttria Composite.
For the pure YSZ thermal barrier material, see Plasma YSZ.
For aerospace and rotorcraft applications, see our Aerospace and Rotorcraft industry page.
Start Your Quote
Call 888-725-0150 or send your part details through our quote form. Tell us the operating temperature, the cycle profile, and what the part does. We will recommend a TBC system and tell you what it costs.
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