Insulation for metal in hot gas
Parts in the hottest section of a gas turbine, such as combustor liners and turbine blades, are exposed to gas that can be hotter than the metal itself can tolerate. Designers cope in two ways: they cool the part from the inside with air, and they coat the outside with a ceramic that insulates it. The ceramic layer is called a thermal barrier coating, or TBC.
Ceramics suit this job because many have low thermal conductivity, hold up at high temperature and resist oxidation. A coating that is a fraction of a millimeter thick can produce a meaningful temperature drop between the hot gas and the metal underneath when the part is also cooled. Its effect is to reduce metal temperature, extend component life or allow hotter gas, which can help efficiency. It does not replace internal cooling; the two work together.
The layered system
A TBC is a system rather than one coating:
- Substrate. The metal part, often a nickel-based superalloy.
- Bond coat. A metallic layer, commonly of an MCrAlY type alloy (a metal base with chromium, aluminum and yttrium) or an aluminide. It improves adhesion, and it supplies aluminum to form a slow-growing protective oxide.
- Thermally grown oxide. A thin layer of aluminum oxide forms between bond coat and ceramic during service. It protects the metal from further oxidation but grows over time and is a main factor in coating failure.
- Ceramic top coat. Usually zirconia stabilized with yttria. This choice is related to the phase behavior discussed in the article on zirconia toughening: the stabilizer keeps the high-temperature crystal form from changing disruptively during heating and cooling. Zirconia is chosen for its low thermal conductivity and because its thermal expansion is relatively close to that of metals compared with most ceramics.
How the coatings are applied
Plasma spray. Ceramic powder is melted in a plasma jet and the droplets are flung at the surface, where they flatten and freeze into overlapping splats. The result is a layered structure with fine gaps and pores that reduce thermal conductivity and help absorb strain. It is a form of thermal spray, relatively economical and well suited to large parts such as combustors.
Electron beam physical vapor deposition. Ceramic is evaporated in a vacuum chamber and condenses on the heated part, forming a columnar structure of narrow columns that stand up from the surface. The gaps between columns allow the coating to flex as the metal expands and contracts, giving better strain tolerance, though the coating is usually a better conductor of heat than plasma-sprayed layers and the equipment is more costly. It is a variation on physical vapor deposition and is common for high-pressure turbine blades, where smooth surfaces and durability matter.
Both approaches depend on careful surface preparation. Coating adhesion to the bond coat is critical and relies on cleaning and controlled roughness.
Why the coating is the weak link
The ceramic and the metal expand at different rates and run at very different temperatures, so each heating and cooling cycle stresses the interface. Sintering of the coating at high temperature gradually closes the gaps that gave it flexibility, making it stiffer and less strain tolerant. Meanwhile the oxide layer thickens. Eventually cracks form near the interface and join, and a piece of coating flakes off, an event called spallation. The metal is then exposed to extreme temperatures, which can lead to rapid damage.
Another threat is molten deposits. Airborne sand, dust or ash entering an engine can melt on hot surfaces into a glassy mixture often described as CMAS (calcium, magnesium, aluminum and silicon oxides). The melt penetrates the coating and, once it cools, makes the layer brittle. Volcanic ash and desert operation are known concerns.
Residual stress left from the coating process, thermal cycling and the number of start-stop cycles all affect life, so coating life is often described in terms of cycles as well as hours.
Evaluation and inspection
Coated parts are examined by microscopy on cross sections to check thickness, porosity and oxide growth, and thermal conductivity and cycling tests are run on samples. Nondestructive methods, including thermography and other techniques, are used to look for hidden delamination. Engines are inspected at intervals, and coatings are often stripped and reapplied during overhaul.
Related uses and alternatives
TBCs are used mainly in gas turbines for aircraft and power generation, and in some diesel engine components such as piston crowns. Research continues on materials with lower conductivity and better resistance to molten deposits, and on coatings for ceramic matrix composites, where environmental barrier coatings serve a related protective role. Because designs differ and test data are proprietary, general statements about lifetime or temperature benefit should be read as indicative only.