Ceramic coating on turbine blades thermal stress

Used as the canonical dissimilar-material thermal-mismatch problem. Zirconia chosen because its thermal expansion matches that of metals; bond coat deliberately porous to absorb strain.

When we do dissimilar-material bonding — here's a turbine blade with a ceramic coating on it — the problem in general with metals and ceramics is they vary by about an order of magnitude in thermal expansion coefficient. A ceramic has something on the order of 2 × 10⁻⁶, five times less. So maybe it's 400 or 600 degrees centigrade, you still have some pretty serious problems. And on this blade you've got temperature gradients. If you do the modeling, you've got 500° between here and here. This is attached to the stator, this big massive thing, and the hot gases are hitting right in the center. The thermal model shows a big hot spot in the middle and cool spots down here. How do you keep the ceramic from forming a dried riverbed pattern and flaking off?

Bond-coat / graded-layer architecture for thermal barrier coatings on turbine blades. Used to set up the discussion of porous-surface adhesion and the cost of repair vs. losing an engine.

The substrate could be polycrystalline, unidirectionally solidified, or single crystal. Then you put on a bond coat — a porous, spongy layer. If you listen to my welding lecture on solid-state joining and adhesive bonding, you like a nice porous surface to bond to, to get mechanical interlocking. They do the same thing here, so you grade the layer from single crystal to a porous metal powder that's sintered on, and then maybe several layers up to a vapor-deposited ceramic that gets down in those pores and sticks, so hopefully it doesn't flake off. If it starts flaking off very badly, you're going to lose your blade. But you can lose one or two blades and usually you don't lose the engine. It's an expensive repair, but it's not good to lose an engine.