How to decrease the bigger voids of Ni-18.5Cr-6Al coating?

[Justin]

Dear all,

I spray a Ni-18.5Cr-6Al coating by 9MB gun.
I wanna know how to decrease the bigger voids of Ni-18.5Cr-6Al coating.

Could you please provide some suggestions with me?

By the way, please refer to the worst metallographic picture(200X) as below.
(link dead)

[Gordon]

Hi Justin

I take it that the powder is a composite and not an alloy.

No magical suggestion I'm afraid other than doing a bit of coating development work. Be careful though that the metallography is not leading you up the garden path with grinding/polishing artefacts.

[Justin]

Hi Gordon

Thanks for your reply.

The powder is aluminum clad nickel chromium alloyed powder.

Excuse me.
I'm not sure that I understand what you mentioned.

Do you mean that grinding/polishing may result in the appearance of the bigger voids?

Is it useless to decrease the bigger voids by adjusting the thermal spray parameters?

Hi Justin

I take it that the powder is a composite and not an alloy.

No magical suggestion I'm afraid other than doing a bit of coating development work. Be careful though that the metallography is not leading you up the garden path with grinding/polishing artefacts.

[Gordon]

Hi Justin

Excuse me.
I'm not sure that I understand what you mentioned.

Do you mean that grinding/polishing may result in the appearance of the bigger voids?

Sorry, what I'm trying to say, is that preparation for metallography can cause pull-outs, which can make porosity look worse than it actually is. On the other hand smearing can have the opposite effect. So make sure you are confident in your metallographic results before making parameter changes.

Is it useless to decrease the bigger voids by adjusting the thermal spray parameters?

No, I think most coatings could be improved through good coating development work. It just takes time and effort which can be costly. It is difficult to judge the quality of your coating without first hand knowledge and experience with your specific powder and process, but if this is the worst case, I don't think it is too bad.