Colour TBC
06-29-2007, 02:09 PM,
#1
Colour TBC
Hi all,

My question is about the colour of TBC. If I understand correctly, this has to do with (reaction) stoichiometry.

When Zr0(2) is formed (using plasma spraying for example), not all Zr reacts with oxygen, resulting in non-stoichiometric solid, for example ZrO(1,95), a softgray coloured TBC. When heating the TBC in an oven (for about an hour at 600 degrees Celsius), the rest of the Zr atoms will react with oxygen forming stoichiometric ZrO(2), a white coloured TBC.

When heating the TBC longer and/or at higher temperatures the TBC will become yellow-white coloured. This is strange to me as I thought it's not possible to form non-stoichiometric ZrO(2,05), as all Zr is bonded...

Can anybody explain this to me? I probably didn't describe my question really clear, but I assume you will understand what I mean.

Thanks in advance.

Best regards,

Rogier
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06-29-2007, 04:09 PM,
#2
RE: Colour TBC
Hi Rogier

An interesting question, one which is probably well beyond me answering properly. It is very complex, particularly as we are not just talking about zirconia, as thermal barrier coatings (TBC) usually contain additional stabilising compounds like yttria, calcia etc.. Physical properties like density and degree of particle splatting, unmelted particle content etc. probably have an effect as well as chemical oxidation states. Probably need to delve into quantum physics Rolleyes to explain colour. Typically, a grey coloured coating indicates cool process conditions while a orange/brown colouration indicates hot process conditions at the other extreme. Antique white I think is the desired colour Rolleyes

Another interesting one is titanium dioxide. White powder typically used as a white pigment in paint. Thermal spray this material and almost magically you get a black or blue/black coating Happy0193
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07-02-2007, 10:36 AM,
#3
RE: Colour TBC
Gordon Wrote:Hi Rogier

An interesting question, one which is probably well beyond me answering properly. It is very complex, particularly as we are not just talking about zirconia, as thermal barrier coatings (TBC) usually contain additional stabilising compounds like yttria, calcia etc.. Physical properties like density and degree of particle splatting, unmelted particle content etc. probably have an effect as well as chemical oxidation states. Probably need to delve into quantum physics Rolleyes to explain colour. Typically, a grey coloured coating indicates cool process conditions while a orange/brown colouration indicates hot process conditions at the other extreme. Antique white I think is the desired colour Rolleyes

Another interesting one is titanium dioxide. White powder typically used as a white pigment in paint. Thermal spray this material and almost magically you get a black or blue/black coating Happy0193
Hi Gordon,

I already thought it wouldn't be easy to answer this question. It seems both zirconium dioxide and titanium dioxide are special cases. It's not exactly the study course I follow (mechanical engineering), but these are still very interesting matters. Especially because it seems there is still a lot to discover and develop when it comes to thermal spray.

When somebody is able to answer (a part of) the question above... please do so.

Best regards,

Rogier
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07-04-2007, 04:33 AM,
#4
RE: Colour TBC
Gordon / Rogier,

I face the same issue of discoloration from yellow white to greyish white on Zro2-YtO2 Composite.

Under normal circumstances, when hard ware (nozzle, electrode, and the rest of the parameter is established) I will get a yelowish white TBC coating..which is ideal and good.

However, when nozzles or electrodes start to wear, voltage drop, to compensate the voltage, we adjust and increase slightly on the 2ndary gas(H2), this is where the greyish white coating will be formed. I have observed many times like this.

Can you maybe link this process control to the explanation why increasing secondary gas would result in non-stoichiometric solid(grey color)? When increasing the H2, the gas enthalphy has change to greater, wouldnt it be the ionization of plasma gases and the forming of the "plasma" better/hotter?

Regards,
ALexangel1226
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07-06-2007, 03:13 AM,
#5
RE: Colour TBC
Hi Alex/Rogier

Quote:I face the same issue of discoloration from yellow white to greyish white on Zro2-YtO2 Composite.

Under normal circumstances, when hard ware (nozzle, electrode, and the rest of the parameter is established) I will get a yelowish white TBC coating..which is ideal and good.

However, when nozzles or electrodes start to wear, voltage drop, to compensate the voltage, we adjust and increase slightly on the 2ndary gas(H2), this is where the greyish white coating will be formed. I have observed many times like this.

Can you maybe link this process control to the explanation why increasing secondary gas would result in non-stoichiometric solid(grey color)? When increasing the H2, the gas enthalphy has change to greater, wouldnt it be the ionization of plasma gases and the forming of the "plasma" better/hotter?
Great observation, but we need to be careful when considering what is actually causing this effect. I don't think raising the hydrogen ratio in the plasma alone (all other things being equal apart from voltage) would have this effect, in fact I think probably the reverse. Increasing hydrogen flow in your case is a compromise, to bring the voltage back to "normal" (as in new nozzle/electrode) for your worn nozzle/electrode. This does set the power level the same, but unfortunately as I'm sure you know, power levels mean very little when not considering all the other factors. My guess would be that the effect is more likely to be caused by increased turbulence in the worn nozzle, in fact it is probably this turbulence that reduces the voltage in the first place. My own preference is not to change secondary hydrogen flow (let voltage be the variable). If the voltage is going outside of the usual range, it normally indicates the nozzle or electrode are worn and it is really best practise to change hardware at this point. Increasing hydrogen to normalise the power level does not necessarily put the plasma conditions back to normal (your issue I think illustrates this). Sure, extending the nozzle life is an important issue, but this needs to be offset with coating quality control and increased risk of plasma gun damage.

I have observed and it is well documented that the colour is effected by substrate/coating temperature during spraying, though I'm sure this is not the only factor.

Alex, have you identified any different metallographic features that correspond to the various colours? When you get a grey coloured coating, do they pass or fail your quality tests?

Changes in colour are certainly an indicator of changes in process, but at what degree do they become significant in coating quality and fitness for service?
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07-06-2007, 05:22 AM, (This post was last modified: 07-15-2007, 06:20 PM by Gordon.)
#6
RE: Colour TBC
Gordon,

Before reading your reply I was thinking I have left out the metallorgical evaluation result review in my posting. Toungue Well, the metallorgical examination does not exemplify notable difference. I have tested that and verified on the microstructure requirement, the porosity, is meeting the limits of 5~25%, and unmelt is ok. So my hypotesis of discoloration induces TBC metallurgical defect is not justified.

Another experience to share on sparing Al2O3 ceramic powder of 99% purity, the coating gives a pure white finishing using plasma process(F4 gun), however, the color changes also happen in this coating and turn slight yellowish and it is very obvious when arrange the part side by side.

So, I guess, more or less the same coating family of Ceramic (oxide based) coating has this general discoloration phenomenon....and the root cause is still vaque, but there is some clue in your research and hope we can get it!!

Regards,
Alexangel1226
Reply
07-15-2007, 06:13 PM,
#7
RE: Colour TBC
Hi Alex/Rogier

Alex, can't say I've ever notice any correlation between microstructure and colour alone, not that I've really made a specific effort to look for one, so it's possible there might be one. All I know is a colour change reflects a change in process conditions and should raise concerns for coating quality.
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07-29-2007, 06:01 AM, (This post was last modified: 07-29-2007, 06:03 PM by Gordon.)
#8
[split] Blistering on Zirconia coating
Hi Gordon,

TiO is dark color, TiO2 is white color

http://www.webelements.com/webelements/c...63677.html


Moved here from https://www.gordonengland.co.uk/sef/blis...t-233.html
Reply
07-29-2007, 05:59 PM,
#9
RE: [split] Blistering on Zirconia coating
powerman Wrote:Hi Gordon,

TiO is dark color, TiO2 is white color

https://www.webelements.com/webelements/...63677.html

Yes, but I don't think the colour change in thermal spray coatings is due to oxide reduction TiO2 to TiO or even Ti2O3, Ti3O5 or possible intermediate compositions, though it does seem to fit with the black, blue, violet colours we sometimes see. I think it is more likely due to crystallographic phase changes like rutile to anatase or other metastable or amorphous phases.
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