Pore clusters in Cr-C HVOF coating

[DanyGans]

Anybody know or can assume what can be cause for big pores in Cr-C coating. Plaese see attaced photo.

I'm using DJ2600 Gun for coating AMDRY367 on 2" ring outside diameter.
Spraying angle is perpendicular to surface.
Coating thickness/pass is 0.01mm.
Powder preheated before spraying.
The most interesting fact is that the these pore clusters sometimes does not appear or disappear by additional spraying. My be the pores are not seen in any cross section.........

I assume some few reasons:
1. Barrel loading
2. Clusters formed in hopper due to self weight compression or unstability of hopper stirrer.

Thanks,
Dany

[Met.Eng.]

Hi Dany,

I think u have Nozzle Cloging problem. I never use DJ but I have same problems especially with CrC-NiCr. You have to set powder feed Rate lower than WC parameters and decrease carrier gas flow rate. The second point , you can set the spray parameter fast and cool. So that The NiCr particles will be less heat in the flame. (You know that there is two stoichiometric combustion parameters in HVOF . Fast&cool parameters and hot&slow parameters. ) And last point is check the barrel inside surface roughness. May be you have to choose coarse powder with your spray parameters. And may be buy new barrel

Regards

[DanyGans]

Hi,
Many thanks for your recommendations.
More two following questions:
1. I also have a problem with bond strength test for this powder. 8.5ksi where the requirement is 10ksi. Do you think the barrel clogging or high feed rate could be the reason for low coating cohesion? The thickness is about 0.1mm (0.01mm/pass). Specimens were well preheated before.
2. How do you check barrel inside surface roughness? I'm familiar only with visual inspection.

Thank again for essential information.

[loriolo]

Hi Dany
Could you please tell which powder feeder do you use with DJ 2600?, I presume you use an DJP or 9MPDJ, both can cause this kind of problem, and this can produce barrel Clogging problems, I agree with Met.eng recommendations.

[Met.Eng.]

Hi again,

The bonding problem is too specific and hard to say what the problem is. There are lats of point you have to check. Preparation Blasting (24 F correct angle and set true psi pressure), Surface speed and traverse speed is important too. Lower travers speed and surface speed is much denser than higher one. Actually you can set 1 m/s surface speed is enough, and 2 mm to 4 mm travers scan is good. But sometimes the coating comes overheating this parameters you have to check when operation. The other point you can preheat your specimen with oxyacetylene torch (set rich acetylene) Gas quality, system calibration can be affected. But the 8500 psi is not too bad with dj2600. Your gun is air cooled right? If it's true 8500 is maybe good . I think the 10000 + psi bonding is possible with as liquid fuel. But i think your problem is not bonding. The dj gun is very sensitive gun, you have to use correct powder size,

if i remember correctly amdry powder uses with plasma gun, You must use diamalloy 3004 powder with dj.

Regards

[DanyGans]

Hello Gentlemens,
Thank you again for information!!!!!! All things are very logic and I hope I'll succeed to reproduce it.....
Few words about my powder feeder. His is brand new TWIN-120-AH with special HVOF powder hopper intended for higher pressure than APS hopper.
Please also review my process:
H2:520NLPM; O2:140NLPM; Sh.Gas:300NLPM; CarrierGas(Ar):13NLPM; Sp.Dis:200mm; FeedRate:35gr/min
Specimens were well preheated and sprayed 0.006mm/pass. Total thickness 0.13mm. No cooling was applied during spraying. Spraying by passes (not cycles).