What is a thermal barrier coating?

A thin layer of low-conductivity ceramic — mainly yttria-stabilised zirconia (YSZ) — on metal parts such as gas-turbine blades and combustors. Its conductivity of about 1 W/mK is roughly a twentieth of the metal's, so a 0.1–0.5 mm film can lower metal temperature by more than 100 K. A bond coat between ceramic and metal prevents oxidation.

How much does a TBC lower metal temperature?

For a wall between 1500 °C gas (3,000 W/m²K) and 600 °C internal cooling air (2,000 W/m²K), the metal outer surface was 1,172 °C without TBC, 1,118 °C with 0.1 mm, 1,034 °C with 0.3 mm and 974 °C with 0.5 mm. 0.3 mm gives a 138 K reduction, with 217 K across the coating itself.

Does TBC performance change in service?

Yes. Long exposure sinters fine pores in the ceramic, raising conductivity. Going from 1.0 to 1.5 W/mK raised metal temperature under a 0.3 mm TBC from 1,034 °C to 1,072 °C, losing nearly 30% of the benefit. Spallation removes the benefit entirely, so life assessment matters.

Thermal Barrier Coating (TBC) Analysis

Category: Thermal – Industrial Processes | Revised 2026-10-01

CAE visualization for thermal barrier coating theory - technical simulation diagram

Theory: a thin ceramic carries a large temperature drop

Overview

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Can a film of about 0.3 mm on a turbine blade really change temperatures much?

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Yes. The ceramic conducts about 1 W/mK, a twentieth of the blade metal (about 20 W/mK), so 0.3 mm of ceramic resists heat like 6 mm of metal. The blade wall sits between hot gas outside and cooling air inside; heat flows in series gas → coating → metal → coolant, so adding the coating's resistance makes it carry a large share of the temperature drop. Let's compute how much the metal cools using series resistances.

Series thermal resistances

$$ q = \frac{T_g - T_c}{\dfrac{1}{h_g} + \dfrac{t_{TBC}}{k_{TBC}} + \dfrac{t_{BC}}{k_{BC}} + \dfrac{t_m}{k_m} + \dfrac{1}{h_c}},\qquad \Delta T_{TBC} = q\,\frac{t_{TBC}}{k_{TBC}} $$

Five resistances act in series: gas-side convection, TBC, bond coat, metal wall and internal cooling. The drop across the TBC is the heat flux times its resistance.

Coffee Break Trivia

The technology that let jet engines run hotter

TBC research began in the US in the 1950s and 60s for rockets and aircraft engines. In the 1970s yttria-stabilised zirconia proved an excellent material, and by the 1980s it was used on jet-engine turbine blades. Turbine inlet temperatures far exceed what blade metal can withstand, made possible by internal cooling, TBCs and superalloys together. Higher inlet temperature means higher efficiency, so TBCs feed directly into fuel economy.

Worked example: a turbine blade wall

Combustion gas 1,500 °C (3,000 W/m²K), internal cooling air 600 °C (2,000 W/m²K), YSZ (1.0 W/mK), 0.1 mm bond coat (10 W/mK), 2 mm superalloy wall (20 W/mK).

TBC thickness and temperatures

TBC thickness	Heat flux	TBC surface	Metal outer	Metal inner	Drop across TBC
None	0.95 MW/m²	—	1,172 °C	1,077 °C	—
0.1 mm	0.86 MW/m²	1,212 °C	1,118 °C	1,031 °C	86 K
0.3 mm	0.72 MW/m²	1,259 °C	1,034 °C	962 °C	217 K
0.5 mm	0.62 MW/m²	1,292 °C	974 °C	912 °C	312 K

Matching the TBC by other means

Condition	Metal outer temperature
0.3 mm TBC (k 1.0)	1,034 °C
0.3 mm TBC, sintered (k 1.5)	1,072 °C
No TBC, internal cooling ×2.3 (4,540 W/m²K)	1,034 °C

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With a TBC the surface actually gets hotter, yet the metal gets cooler.

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The TBC impedes heat flow, so heat leaves the gas side less easily and the surface rises from 1,182 °C to 1,259 °C — but the ceramic tolerates that. What matters is the metal beneath dropping by 138 K; a few tens of kelvins can multiply a superalloy's creep life. Matching this without a TBC would need 2.3 times the internal heat transfer coefficient, spending more compressor air on cooling and lowering engine efficiency. But sintering that raises conductivity removes nearly 30% of the benefit, and spallation removes it at once, so TBC life must be assessed together with metal margin.

Analysis tips

Obtain gas- and coolant-side coefficients from CFD or correlations as boundary conditions.
Account for TBC conductivity change with service (sintering).
Include thickness scatter from spraying and assess the thinnest spots.
Compute thermal stress from the temperature field to assess spallation risk.
Solve leading/trailing edges and other high-flux regions in 2D or 3D.

Coffee Break Trivia

“Only the spalled spot burned”

During inspection of a power-generation gas turbine, part of a blade leading edge was oxidised and thinned. The TBC there had spalled in service; the bare metal was exposed to temperatures over 100 K higher by calculation, accelerating oxidation and creep. Since then, inspections that detect spallation and cooling margins letting the metal survive for a time after spallation have been built into design.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
As-sprayed conductivity only	Overheating late in life	Assess sintered values
Uniform thickness	Hot thin spots	Include scatter
Omitting bond coat	Oxidation unassessed	Model the layer
Ignoring spallation	Sudden damage	Also assess bare metal
Ignoring radiation	Surface temperature wrong	Include gas radiation

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Thermal Barrier Coating (TBC) Analysis

Theory: a thin ceramic carries a large temperature drop

Overview

Series thermal resistances

The technology that let jet engines run hotter

Worked example: a turbine blade wall

TBC thickness and temperatures

Matching the TBC by other means

Analysis tips

“Only the spalled spot burned”

Common mistakes

Mistakes and fixes

Related Topics

Related fields