Powder fluidity and injector clogging (DJ 2700 HVOF gun).

[pulaunias]

Dear members,

I am new to this forum and also a starter in HVOF spray. I'll appreciate it if you can shine me some light on some practical aspects of thermal spray.

I am using a SulzerMetco 2700 HVOF gun, which is an axial feeding system. Recently, for some reason, our powder wasn't well sprayed out but instead got clogged in the "powder injector", which then blocked carrier stream. It appeared that the flame back-expanded and got the injector burned.

I suspected that there might be a few reasons behind this damage. I roughly know that the powder (made by atomization) has a mean size of 20 microns or so, but do not know its distribution. A friend experienced in plasma spray (radial feeding) told me that I should narrow the particle size range by sieving.

I am wondering that, did any of you members experience similar problems? Do you feel it really important to control the powder size range in your practice? If somebody has expereince with 2700 torch, how would you recommend to control powder fluidity?

Thanks dear guys.

Pulaunias

[Gordon]

Hi Pulaunias

to the Surface Engineering forum.

What is the powder and powder feeder you are using?

First, with well calibrated/serviced equipment using powder products and parameters designed for use with it, these problems should be uncommon.

The powder quality (particle size distribution, shape, composition, flow ability etc) is a critical factor. If your powder is an unknown quantity as far as spraying with your equipment, these issues will need addressing. The problems you are experiencing may also not be related to powder but equipment condition or parameters set.

[pulaunias]

Thanks Gordon.

I was spraying a calcium phosphate ceramic powder, at feed rate of 20 g/min. To be frank, I was spraying this powder for a trial because I knew it was made by atomization and has a spehrical morphology, so I expected an excellent fluidity. I am not sure if that powder was specifically designed for thermal spray use or not. I tried this because I initially experienced fluidity problem with an alumina powder (intermittent feeding-clogging), which I knew was hexagonal in morphology and thus guessed that this mophology caused bad fluidity.

Do yo think it wise to go ahead with the real commercial thermal spray powder that I expect to spray, which is WC/Co, instead of fooling around with cheaper substitutes.

Thanks again!

[Stephen Booth]

Interesting, do you have parameters from the Manufacturer for the Calcium Phosphate and the Aluminia powders.

I have not heard of these powders being used with the DJ 2700?

[LEN WOOD]

Calcium Phosphate and Alumina powders are hardly cheaper substitutes for use in place of WC/Co.

Forgive me if I'm missing the point of the exercise but whatever you're goal is I think that you're missing the specificity of material to function (of coating).

By all means try the economical alternatives to what you describe as the 'real commercial thermal spray powder' out there. You would be amazed at the origins of some of the main vendors' materials. You could save yourself the high cost of flash packaging and rebranding albeit maybe without the certifications!

Besides, I think that the equipment vendor should be supportive whatever you try to process - that is if they are able?

Regards

[pulaunias]

Stephen and Len, thanks for your input.

Stephen, Prof. Kuo at the National Univ of Singapore reported HVOF spray of calcium phosphate powder, intended for biomedical implant applications. I cann recall at this moment which equipment they used, but I remember (if correctly) they used hydrogen as the fuel. Since we are academics here, we would like to try powders that outise typical candidates recommended by equipment suppliers. But, now seems obviously, we are paying for our lessons.


Len, thanks for your point. As I said, as academic people, we need to try new possibilities (and sometimes with more expensive powders than cheaper, well-commercialized ones), but we also must be able to work on existing powders/coatings to generate income in a sustainable fashion (as we know HVOF is quite a money-consuming monster) to support our research. I think we will start with "main vendors' materials". By the way, I am not exacly sure on what you mean by "amazed at the origins....".

[karunanidhi]

The type of fuel used is an important factor in the deposition of coating material. LPG or propane can not spray ceramic materials

[Gordon]

Hi Pulaunias

DJ2702 propylene extended air cap, for spraying ceramics using the Diamond Jet gun.

It is important though to get a powder that will feed in a stable manner first and to have a suitable particle size to form a good coating.

Good luck with your experimenting, costly but necessary for learning and possible development of new coatings.