I guess I'm just not convinced about the effectiveness of typical ceramic coatings.  From what I can find out they are typically .001 to .002 thick, about 5% of the thickness of the metal.  In order to significantly reduce the heat transfer compared to the metal, they would have to have thermal conductivity in that order compared to steel.  I can't find out what material is used, but some might use zirconia, a very good insulator, which has about 10 to 20% of the conductivity of steel.  But that's not the whole story.  Inside the pipe is a gas at perhaps 1500F moving at a relatively high velocity.  The conductivity to the metal is therefore relatively good, resulting in a metal temperature of probably 1200F.  The air on the outside is at about 180F and it is moving slowly, so the heat transfer from the pipe to the air is "poor."  My guess is that the surface temperature of the ceramic is going to be very close to what the metal would be without the coating.  There just isn't enough thickness for it to be well insulated.  Swain Technologies has a proprietary process that puts .015 thickness on metal and that would probably work.  A big factor in heat transfer is radiation and the best way to reduce that is with a shiny metal surface spaced away from the exhaust pipe, or from the device to be protected.

Maybe ceramic coatings work well, but I'm afraid I'm not convinced.

Gary Casey

ES 157