Using traditional methods of thermal spraying (flame spraying, plasma spraying and high-speed flame spraying) impossible to obtain a nano-structured coating, even in the case spraying of suspensions. Closest to the nano-structured coatings suitable new technology from Sulzer Metco based on vacuum plasma spraying from a big distance. In this technique, the material is not melted but evaporate and deposited on the substrate from the vapor phase. In this sense, this technology is related to PVD, CVD- processes, which are regularly used for obtain of nano-structured coatings.

Dear Vadim,

Thank you. If I've understood well, the coating to be formed using my feedstock via PS,FS & HVOF won't possess any nano-structural properties at all unlike a feedstock consisting of agglomerated nanoparticles.

Hi Nadiir,

Even if you use the agglomerated powder in which all particles have nano dimensions, for example, commercial powder of YSZ, in the coating no nano particles are exist. This occurs because the flame melts the entire agglomerate and form a large drop, which provides the coating layer with a size of tens of micrometers. You can not get melt drops with a size of a few nanometers , because these drops immediately connected into larger ones. The only chance - to evaporate the material, and besieged it from the vapor phase in a vacuum.

Hi Dr. Vadim,

Thanks again for your prompt reply. I got your point here. There will be no nanostructured coating if the particles are completely melted, right?

I've attached an article which refers to nano-structured coatings in which the concept of partially molten micro-sized powder agglomerated from nanosized particles is pointed out. Of course, the coating will not exhibit a nano characteristics completely but at least partially which makes up a bimodal structure.

This is where my initial question originates.

I'd like to have your opinion on this matter, if you don't mind.

Thanks a lot.

Best Regards,

Nadiir

Hi Nadiir,

I read this article but, frankly, did not see anything interesting in it for practice. Spraying suspensions creates a lot of difficulties, and often is worse than conventional spraying. From the point of view of the theory of nano-structured materials should be better micro-structured, but it is valid for solid bodies without pores and cracks. Yet the real ceramic coatings have cracks and pores, because the size of the particles in them does not matter.

The suspension plasma (or HVOF) sprayed coatings have often two-zones microstructure (see attached paper). Some particles having trajectory close to the jet axis get molten then agglomerated and may form large lamellas after impact. On the other hand, the fine particles which remain in the periphery of the jet keep the initial size as in suspension.

Another important point to be mentioned  is such that the crystal grains even in large powder sprayed coatings are generally smaller than 100 nm i.e. the coating are often nanocrystalline.

Finally, the small spray distance results in high heat input and sintering of mentioned previously small particles (see another paper)

Thank you Dr. Vadim. I appreciate your contribution of knowledge in this matter. Suggestions noted with thanks. However, right now I might not get a hold of a VPS or CVD.

Dr. Lech, thank you for sharing these informative articles. Before I go though them thoroughly, I'd like to double confirm something.

In your opinion, the possibility to obtain a two-zones coating is higher if the suspension spray technique is used compared to conventional plasma spray. So, do you think if I suspension spray a material with properties which I described in my query, would I get a coating which may retain some nano-characteristics of the material?

My primary aim is that I want to keep some nano-properties of the powder in the coating, i.e, somehow some feedstock particles shouldn't be completely molten.

Would really appreciate for any comments.

Thanks a lot.

Nadiir

The answer of your question is that the microstructures of suspension sprayed coatings and the ones sprayed using agglomerated fines grains are slightly different. Please have a look on Fig. 9 of attached paper.

As the of suspension plasma spraying is the subject of my research I have worked since about 2004, you can find some papers in my profile (see contributions).

Dear Dr. Lech,

Many thanks once again for your constant support. I'll go through these recommended articles of yours.

But if I cannot arrange for a suspension spray for the time being, would it be worth trying to conventionally plasma spray a nanostructured material at micron-size, but agglomerated with a secondary ceramic, in the quest of having a coating consisting some nano characteristics derived from that nanostructured material.

Is there any hope in achieving the above?

Your opinion will be greatly appreciated.

Best Regards,

Nadiir

You will have slightly different microstructure than that of suspension plasma sprayed coatings. The question is rather, what is the specification of your coatings and how this specification can be met with spraying of agglomerated powders.

My experience and a few papers from the 1990/2000-ties (see an example) tell that the agglomerated powders are harder to melt and the resulting coatings are much more porous that other.

Dear Dr. Lech,

Thank you once again for your kind consideration into this matter.

Indeed, I got your point here. I believe that I'll have to give it a try and see. I am still developing my spray-dried particles but it is a pleasure to have your guidance for my next stage.

I'll check out the articles as well.

My desired specifications of my coating is that the latter be of low porosity and part of its microstructure should consist of similar microstructure of my feedstock material, which means that some particles haven't melted completely.

This is what somehow I want to achieve, preventing full melting.

Merci!

Nadiir

Hello,

let's say we rule out the deposition of vaporized particles and suspension spraying. An option to get the nano-structure form the agglomerated and sintered powder into the  coating microstructure, is to embed  semi-molten or unmolten (but plasticized) particles into your coating. However, this can reduce the mechanical strength of the coating. In most thermal spray coatings, you have unmolten particles anyway to some extent. Using HVOF instead of APS  can help to achieve better mechanical properties of the coating (e.g. see paper below). So my recommendation would be: HVOF-spraying of fine agglomerated and sintered feedstock powder.

best regards

Richard

Hi Richard,

Nice of you for having shared your opinion here. I've noted it. However, my ceramic cannot be sintered because I believe that the temperature will be too high. I'll try to obtain relatively high density particles via the spray drying.

Could you please re-attach the article?

With thanks,

Nadiir

The nanostructured powder particles are required to be molten partially to keep bimodal structure after the solidification of surface coatings.. The ranges of thickness depend on the material used for ceramic it is about 80 micron meter.

Dear A. Ouerdane,

Thank you for your answer.

Therefore, even though a feedstock powder is not made up of nano-agglomerated micron-sized particles BUT rather is composed of micron-sized particles having a nanostructured network, a bimodal structure can be obtained as long as some powder particles are not fully melted.

sorry, this was the paper

http://link.springer.com/article/10.1361/105996305X59413

Thanks Richard!

In our case, we have sprayed nano structured agglomerates, coming from high energy ball milling. We have identified in general a superficial zone of the sprayed particles/splashes which is more or less molten and an internal part, often more than 50% of the volume, keeping its initial nanostructure form, but with nano sintering .

Dear Dr. Michail,

Thank you for sharing your key deductions. Do you have any publications on the stated work?

Best Regards,

Nadiir

yes you will find some in my publications list in Research Gate. Please have a look and if there is interest I will try to send you the full texts

Alright Doc. I'll do that. Thank you!