Posted by Hong Wei Wang on September 16, 2003 at 13:57:36:

In Reply to: Re: coating titanium oxide TiO2 posted by Gordon England on September 16, 2003 at 00:43:31:

: : 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

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