Posted by Gordon England on October 06, 2003 at 13:24:42:

In Reply to: thermal spray of nanoTiO2 posted by Leo on September 30, 2003 at 12:02:12:

: : : : : Dear sir,
: : : : : I'm doing a research project on coating TiO2 by thermal spray process. Will there be any useful information? I'm expecting to find someway spray nano-TiO2 powder on substrate without phase change.
: : : : : Thanks.

: : : : : regards,
: : : : : Leo

: : : : Hi Leo

: : : : For clarification, nano sized powders are normally too fine for thermal spraying (micro sized powders are used). To spray nano materials they need to be agglomerated into micro sized particles.

: : : : With ceramics like titanium dioxide it is necessary during spraying to get the particles near or above their melting point, hence phase changes (anatase to rutile). With nano TiO2 agglomerated particles it is possible to form coatings containing some original nano crystal phase as unmelted particles, but it would be very difficult to form a coating consisting purely of unmelted particles. A lower melting point binder with nano-TiO2 may be worth consideration.

: : : : The following links may be of interest:

: : : : http://www.altairnano.com/main_content/DevProj_TSGP.html

: : : : http://www.scientific.net/0-87849-923-7/288.htm

: : : : Regards Gordon

: : : _______________
: : : Just a thought:

: : : Yes, to start, formulating a heavily loaded colloidal or quasi-colloidal solution of the original nano-particles dispersed in a medium (water- or organic-typed)is crucial. spray-drying will produce the mentioned, agglomerated "sized-up" particles suitable for spray.

: : : Since the resultant agglomerated particles containing multiple nanoparticles also contains the organic binder(s), it is also crucial to adjust the spray conditiions such that the binders will not break down too soon (resulting in the premature loss, hence lowered yield, of the disintegrated nano) or too late (resulting in too much binders in the coating).

: : : Also, since the agglomerated particles are usually hollow, it might be difficult to have good coating adhesion, due to a difficulty in them being accelerated sufficiently. It might be helpful to change the spray method - a change from the general plasma spray for oxide ceramics (e.g. titania) to the unusual HVOF - an added bonus being that the lower flame temperature of the latter won't breakdown the binder phase prematurally.

: : : Spraying "nano" might be a fashionable thing, e.g for increasing the mechanical properties of coating itself. When in complex situations such as some types of wear, it might not be a good idea.

: : : Regards,
: : : Hong-Wei Wang
: : : Greenhey Engineering Services, UK

: : :
: : Thanks for your kindly suggestions sir. I have just recieved more detail of my project. I have to spray nano-TiO2 by flame spray process. Actually the problem is that I can;t find the nano TiO2 phase diagram and how to vary the flame temperature of the flame gun.
: : Regard to your suggestions, a kind of powder mixture would give better results. I'll try this method. And what do you think if I put the sample in low temp (below phase changing temperature) oven to increase the adhensive strength of the powders?

: : regards,
: : Leo
: : MSE, CUHK student

Hi Leo

I have not seen a nano TiO2 phase diagram. I believe nano TiO2 phase is usually anatase which is thermally unstable and will change to rutile phase on heating. The formation of nano particles and the thermal spray process tend to produce conditions so far away from equilibrium conditions that I think a equilibrium phase diagrams would probably be of little use.

What thermal spray process and equipment are you planning on using? If you are using oxygen/fuel combustion process, the flame temperature will be dependent on fuel type, gas flow balance and combustion pressure. Most thermal spray processes use flame temperatures well above the melting points of the material sprayed with plasma spray being an extreme case. Parameter selection is usually looked at from the point of view of particle dwell time in the flame and obviously considers many more factors than just flame temperature (process is far from equilibrium and is not like furnace heating).

Heating your powder composite to sinter or to increase strength is normal, but will depend on the binder phase used.

Hope this helps.

Regards Gordon

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