Dear Gordon,
I have seen about the relevant forming of oxided cluster that you said. But I do not understand very much. Can you detailed explanation once?

Thanks,

best regards,

William

Hi William

I think oxide clusters are mainly formed in coatings by the following mechanisms:

• When material from nozzle or powder port fouling (powder build-up) is released into spray stream.
• Inclusion of dust and materials into spray stream from the surrounding environment.
• By mechanism descibed below.
  

Quote:Posted by Gordon England on 19:14:13 08/01/06

In Reply to: Re: Cluster oxide in Inco718 posted by Hong Wang

I will try to explain my theory on how these oxide cluster formations occur (see rough sketch). The effect is certainly not confined to Inco 718 alloy coatings and I believe it may effect many different thermal spray coatings. Also, the problem is not confined just to the plasma spray process, but HVOF and possibly others.
I think the problem stems from fine material being ejected sideways (parallel to substrate surface)from the spray stream at or around the point of primary particle impact with the substrate. These ejected particles then form secondary deposits on protrusions on the coating/substrate surface. As the coating builds up these protrusions become magnified due the additional secondary deposits, resulting in a tube like column (horseshoe shaped as viewed from coating surface) rising up through the coating, usually at an angle of about 60-70 degrees from the substrate surface plane. It is important to note that these structures are three dimensional and will appear different when cross-sectioned from different angles.
These column like structures usually consist mainly of oxide phases and sometimes excessive porosity.
The fine material that produce the secondary deposits probably originates from two sources:
1. Fines in the original spray powder, which over heat and oxidise.
2. Splash from impacting particles with substrate/coating.
I have observed this problem with plasma sprayed 80/20 NiCr, CoCrNi alloys, chromium carbide/NiCr,
and possibly a similar effect with chromium oxide.
Problems with HVOF coatings include chromium carbide/NiCr, Inconel 625, Hasteloy C.
Predominantly, the effect occurs on flat substrate surfaces and large external diameters, smaller diameters seem less effected. It is probably reasonable to assume internal diameters would be effected the most, but I have no observations to confirm this.
The as-spray coating texture is usually effected when the problem is severe. Wart or pimple like protrusions being evident. When the surface is ground, the surface may show dark spots about 1mm in diameter. when view closer, a horse shoe shape may be seen.
I would be interest to hear of any alternative theories, or cases which involve coatings not containing chromium.
Regards Gordon

Hi Gordon,
Thank for your reply.
I am spraying T-400 Coating by plasma system. I tried to get rid of some interference factors that you mention, but still produce oxided clusters.
I don't know that there is emergence which can influence the oxided clusters.
Is the air jet(Gun) pressure, Powder Port Position, Powder feeder or other ?

Can you give me some suggestions?

Our equipment is Metco 9MC system.
Parameter as follow:
Nozzly: 733/ 732
Gas: Ar/H2

Thank for your assistance.

Best regards,

William[/php]

Hi William

As you are probably aware, this is not an easy problem to overcome.

Making changes like powder port orientation, carrier gas flow and cooling air jet pressure and direction can influence the oxide cluster problem. I know sometimes that changing to a different powder supplier can help (slight differences in particle size distribution and reduction in fines). Setting parameters to give a slightly cooler spray may help with reducing oxidation and particle splash, but may increase porosity/unmelts. Making changes to the environment local to your coating, so that dust and overspray are effectively extracted away from the area. This becomes a very important aspect if you are spraying internal diameters and enclose areas where good non-turbulent extraction/ventilation is difficult.

Also, check that you are not suffering from any powder fouling on the tip of the powder port or the face of the nozzle.

Hope that helps and good luck.

William,

Gordon helped me alot in understanding of the formation of cluster oxide, the attachment cut-up was actually to my query previously in the old forum. And I did suffered a great time trying to tweak the paramater for the coating.

To share with youa lil of my experience. I even carry out a DOE on 6 important parameter, so it is 6 factorial...imagine the number of sample to be tested. Yet i did not do all, as I am almost eliminating the cluster oxide frrom across the samples, to 20+ bad area (Frame of View, FOV), to 10+, to less than 10..5, 2.. and eventually to non-rated oxide clusters.

If you are having overall good metallurgrapic condition, and tensile, hardness, the well distributed oxide level, and your unmelt particles is ok, I do not advice you to have aggressive cahnge on the primary/secondary gas flow, your distance shd also be maintained.

2 major parameter that give me significant improvement is the feedrate(which have effect on the oxidation rate), and also carrier gas. These 2 parameter compensate each other to get to the outcome of noncluster oxide. With the rest of the parameters un-with minimal alteration.

Increase your carrier gas would means your powder is ejected into the frame with a higher velocity. This would then help to "blow off" the dust inclusion on the secondary flame when overlapping to buid up surface happen.

Also as gordon said, the environment must be less agitated or try to reduce the potential turbulent flow that may happen due to position of airjet, external air jet, ensure your exshaust system is strong and efficient of removing particle formt he surrounding...etc.

Hope this info can help you a bit. Form the sample you spray, you can roughly gauege if the oxide level is high or there will have cluster or not. If you see potrusion, samll ones, lumpy like surface, the custer will most probably exist. If you manage to get smooth and lighter grey color surface on T800, then the oxide and cluster would be less. Ofcourse , the metallurgraphy test will give you the detail reports.

Afterall, whta u need is passion and be patient until you slowly improve and succed. Good luck

Regards,
Alex