Color Difference in Ceramic coatings of Plasma Spray
12-21-2012, 12:47 PM,
#1
Color Difference in Ceramic coatings of Plasma Spray
Dear Friends,

Have a look on attached photographs of Chrome Oxide coating done with Plasma Spray System (F- 4 Gun).

1) The color of the coating is not uniform. some areas it is light greenish and in other areas it is dark greenish. Can any body tell why it is happening?

2) By visual inspection, it seems to be there is no proper bonding of layers.

The process parameters used to coat chrome oxide given below:

Current : 500 Amps, Agron Gas --- 38 NLPM, Hydrogen --- 12 NLPM, Carrrier gas Argon --- 2.6 LPM, Disc RPM is 30%

The above phenomena observed in 87Al2O3 +13 TiO2 and 97Al2O3 + 3Al2O3 coating also.

I Request Mr. Gordon to share is experience to solve this issue.

Regards,

SREENIVAS
   
   
   
Reply
12-23-2012, 04:02 PM,
#2
RE: Color Difference in Ceramic coatings of Plasma Spray
Hi SreenIvas

This I think is what I call the "wood grain effect". Your coating is a series of layers. As you move through these layers you encounter changes in in coating structure/composition particularly at interfaces between layers. When finishing you inevitably reveal different parts of these layers. Alumina/titania blends in particular can produce some annoying but spectacularly striking and beautiful effects similar to wood grain.

My first tip and most important one - ceramics are best applied in very thin layers, which may at first sound wrong as you would be more likely to expose more cross-overs between layers, but the finer layers will be more homogeneous in structure and you will have far less stress related faults in the interface between layers. Try spraying at around 5μm (0.0002") coating thickness per pass. The wood grain effect on finishing should reduce, but also the general coating quality will be much better.

After applying tip one - Play with orientation of powder port, cooling air jets and dust extraction relative to spraying direction. Fine dust and crap gets included in the coating, particularly at the interfaces between layers, so reducing this effect will help further.
Reply
01-01-2013, 08:30 PM,
#3
RE: Color Difference in Ceramic coatings of Plasma Spray
I agree with Gordon comment, and I want to add something else, In some cases, in order to have a very quality coating surface, it was necessary to perform all the coating in one pass, especially when the piece is large, and we have to much temperature difference, while we perform the coating, so you can try following as Gordon write, and if you do not have better results, try to do the coating in only one pass, this after you check that gun geometry and parameters are well performed
Best regards
Luigi
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