Has anyone benefited from using ITC213 Ceramic coating on their graphite or metal nozzles? It seems like a good idea to me.
ITC213 is especially formulated to protect metals and graphite, including those exposed to molten metals. It helps prevent stainless steel and steel parts from erosion, oxidation and fatigue from prolonged use.
I've never tried it, but I would question how much good it would do. I'm not as proficient at EX as some, but I have some nozzles that have 30+ firings. I would have to say that I have retired nozzles because they broke, but never because they eroded enough to become unusable. Actually, the problem is the opposite - the metals and oxides form on the exit cone and throat, and have to be drilled back out to the original diameter. Erosion is not a problem for me. I usually run propellants with 3-5% Aluminum. Other metals might be a different story, of course.
The other question I would have is what the surface finish is like. Graphite nozzles that are polished very smooth are easier to clean because you can chip off the slag with a knife, sometimes in one big chunk. If the graphite gets rough, the slag sticks better and is a royal pain to pick off without gouging the graphite. Often, I just leave it and it gets burned away and replaced by a new layer after the next firing. I would think that this ceramic would be fairly rough and maybe a little porous, and would form a very tenacious bond with the slag, making it impossible to remove without also removing the ceramic.
The biggest reason to use ceramic is for heat insulation. Graphite conducts heat really well. A thin coating of ceramic won't do much for the heat soak. If the whole nozzle was ceramic, that would be ideal for heat, but I have doubts as to the durability of such a nozzle.
Thermal barrier coating systems are notoriously hard to keep on a surface due to mismatch in coefficients of thermal expansion, hence the use of a Ni-Cr or Pt bond coating underneath Yt-stabilized zirconia TBC. It is usually the bond coat that is the weak link. My impression is these coatings have not been used in liquid rocket engines because:
1) high chamber wall heat flux requires a very smooth, very thin layer of TBC to avoid overheating of the surface
2) surface roughness (like from overheating the TBC) increases the local heat transfer coefficient and heat flux
Heat transfer is both a local and global issue - you can burn up because your overall quantity of coolant was too low, or because it was just too low locally and you experience a cascade of failures.
I'm no expert and I haven't used ITC213, but I have used ITC100 to coat the inside of forges. But, if I am not mistaken, ITC213 is essentially "polished" onto the surface... as thin as possible. So I don't think it is rough or porous, it is a protective thin film. Someday I'll give it a try, but the stuff is fairly expensive.