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Powder / ceramic coatings to fill surface voids?
by
subsonic
on 04 Feb, 2008 10:15
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I was doing a bit of research. I was looking at the before and after photo's of powder coated items. The before photo's showed a fairly rough surface, cast iron from a mold as an example. The after photo's showed a much smoother surface. Much of the surface roughness was gone. I assume this is because the powder coat filled in the voids when it hit it's flow stage.
Does the same thing happen with ceramic coatings? Will they flow over the surface and leave a smoother finish? Do ceramic coatings even have a flow stage when they are baked on?
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#1
by
935racer
on 04 Feb, 2008 10:30
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Powder coatings are much thicker than ceramic coatings and yes when they flow they fill in small voids and scratches in things like castings an rough surfaced items. Ceramic coating is usually applied wet like a pain in a very thin layer, in fact you can coat a bolt and nut and still thread them together. Some people will apply the ceramic coating thicker than others and for some items you may want to spray a thicker layer than others. There isn't a flow stage with ceramic coatings, they are dried at room temperature before they even hit the oven.
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#2
by
subsonic
on 04 Feb, 2008 11:12
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That helps clear up some things. One of the reasons I was asking, was that in my turbine housing, the surface the exhaust gas flows over is rough as hell. I don't know if it from years of use or just the way it came out of the casting. I thought it would have a nice smooth surface, almost like a mild polish, to help the flow. Not so.
It looks like it would be very difficult to get some kind of flapper wheel into the scroll section to smooth it out. I was hoping that the CC would flow like the powder coating and smooth it out.
Can CC go over a high temp powder coating in something like a turbine application, or will it run into adhesion issues? The more I think about it, I would be worried about the high egt's cooking through the CC and causing the powder coating to come apart. The highest rated high temp Powder coating was this:
Hotcoat Powder Silver Hi Temp 8 oz - 50 lbs
At Last! High temperature powder for header and exhaust manifold applications! We're pleased to present this range of 4 High Temperature Exhaust System coatings similar in color to our popular liquid high temp coatings. These high silicone content powders withstand temperatures up to 1000ºF and offer superior durability when compared to brush and spray-on coatings.
Curing:
Bake coated surface at 450ºF for 10 minutes or 400ºF for 20 minutes after powder transitions from a dry powder look to a wet look. Cured surface will withstand up to 1000ºF with peaks up to 1200 degrees F.
I know that under full throttle operations our egt's can spike much higher than 1200F. Would CC be able to carry the difference in heat?
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#3
by
935racer
on 04 Feb, 2008 11:27
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The high temp powder coating don't work on turbine housings or exhaust manifolds, we tried.
We port turbine housings all the time, there are special mandrels used to get into the turbine housing, but you still can't get past the line of sight of the turbine inlet.
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#4
by
subsonic
on 04 Feb, 2008 11:30
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Yeah, Right past the bend is where the surface starts to get pretty rough.
What do you do? Just leave it as is?
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#5
by
935racer
on 04 Feb, 2008 11:50
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Yeah, Right past the bend is where the surface starts to get pretty rough.
What do you do? Just leave it as is?
Yep once I can't see it, it is just left, not much you can do, I have inquired about extrude honing to a few places but they tell me they cannot guarantee the profile will not be affected.
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#6
by
subsonic
on 04 Feb, 2008 12:13
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Damn! That was the next option I was going to ask you about. :x
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#7
by
935racer
on 04 Feb, 2008 13:31
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Damn! That was the next option I was going to ask you about. :x
I could tell!
I'd love to find someone who felt comfortable guaranteeing the condition of turbine profile, I should really just look into extrude honing and understand how it works, maybe we can figure something out.
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#8
by
subsonic
on 04 Feb, 2008 14:02
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That would be nice. I am guessing that that if the nasty roughness in the scroll (or whole turbine for that matter) could be cleaned up, there would be a significant increase in flow velocity with a coresponding drop in emp.
Faster spool, less heat soak? Would CC stick to metal that has been extrude honed? Would it be too smooth?