Materials > Coatings
The last line of protection and defence against the extreme environment inside a jet engine is coatings. It may seem a bit silly to think a layer of something only a millimeter or less is going to have a big impact on a component, but in truth these coatings play a very important and varied role.
Thermal Barrier Coatings (TBCs)
These coatings are designed specifically to protect a component from high temperatures. In the case of a jet engine, a component has heat coming in from the surface in contact with the hot air. It also has interpass cooling coming from the centre. The problem is that although our part is cooled from the centre, at the surface it is hot because of the air passing over it. Either we limit the temperature of the air making the engine less efficient, or we need to protect the component. TBCs means we can cover a component in a material with really good high temperature capability, that may not have good enough mechanical properties for the whole component to be made out of. Actually TBCs mean we can have a component operating in temperatures above their melting point!
Ni, 20% Cr, 20% Co, 2.4-6% Mo, 0.6% Ti
Nimonic C263 is a nickel based superalloy that has γ’ phase precipitates. Being a superalloy C263 has good high temperature capability up to 800°C. It is used in the manufacture of the the combustor chamber. This is because in the combustor temperatures are very high and so the materials we use are usually quite brittle. By covering these components in C263 we given them a coating with more ductile mechanical properties so that when damage happens instead of the component breaking the C263 gets damaged.
Ni, 20-24% Cr, 0-5% Co, 1-3% Mo, 13-15% W, 0.2-0.5% Al, 0-3% Fe, 0.5% Si, 0.3-1% Mn, 0.05-0.15% C
We talked about Haynes 230 in the nickel alloy section. It is a solid solution strengthened nickel superalloy that can be used in temperatures up to 1150°C. Given it’s high temperature capability, this material doesn’t have as good a mechanical properties as other nickel superalloys. So using this material as a coating over other materials means we can deal with the high temperatures seen in the engine on the surface and still have a component beneath which can deal with the stresses.
In a jet engine we need the parts to fit closely together so the air goes through the blades rather than around them. This can cause a problem as the blades need to be made out of strong materials, but as the material experiences a life a heat and stress is undergoes creep. This means the component slowly stretches and at some point blades and casing may run into each other resulting in a big ball of fire, bits flying everywhere, and possibly a plane falling out of the sky. So we need to make the blades smaller than the casing, but we need to make sure the air can’t go around the blades. To do this we use abradable coatings. As the component creeps or stretches the coating rubs away giving us the air seal we need and preventing the danger of bits running into each other. It also means we know how much a component has changed by looking at the coating and so can help tell us when we need to repair or replace it.
Al, S, B, N
AlSi-hBN is basically a special composite used as a coating. The AlSi is a metal matrix. The phase hBN acts as a particle. This particle is made of hexagonal boron nitride (hBN) and acts like a solid lubricant. This is important as we want the coating to rub away, but also not create any friction or resist the movement of the blade. Similar coatings to this are NiCrAl-Bentonite-BN and NiCrAl-Bentonite where Bentonite is a clay/ceramic (as we know good in high temperatures).
Although peening isn’t a coating, it is something we do to the surface of a component to make it perform better. Shot peening is a process where we take a component then fire lots of little ball bearings at the surface creating tiny dents. I know it does sound a little strange but there is a very good reason we do this. By making little dents in the surface of a material we are creating stresses within the material which can change how it reacts to certain things. If you think about chewing a piece of gum, when you start it is soft but as you chew it gets harder and tougher. By peening what we are doing is trying to get a hard/tough layer on the outside of a component able to withstand damage, whilst keeping the inside nice and flexible so it can deal with the stresses from spinning really fast.