Need some help with Metco 51NS

[Eric Tolliver]

I have this issue with Metco 51NS every now and then where the spray quality just changes sometimes. It creates this layer of porosity and very small unmelted particles. The attached image is at 200X note the spray between the bondcoat and the upper area of the top coat.

What would cause something like this?

(link dead)

[Gordon]

Hi Eric

To me this looks like staining or etching. What is your metallography preparation technique?

[loriolo]

I agree with gordon, it seems that the problem is in the test preparation technic, for a tipical 200x metallographic test, I can not see the granes of the powder in the coating.