Porosity in D gun coating

[DavidH]

We have been having trouble with high pressure(15,000 psi) seal integrity in a new valve which has an enhanced D gun coating of Carbide with a nickel and chrome binder on the shaft. We took some mold impressions of the sealing surface and inspected them under magnification and were surprised at the amount of what appears to be porosity. The coating is supposed to have around a .1% porosity level and there was a sealant applied prior to the grinding and honing process. Can anyone of the coating experts out there provide feedback on the amount of porosity present in the image and whether it is higher than normal for this type of coating and if there may be an issue with coating process, sealing process or materials involved.
Thanks,
David

[Vadim Verlotski]

We have been having trouble with high pressure(15,000 psi) seal integrity in a new valve which has an enhanced D gun coating of Carbide with a nickel and chrome binder on the shaft. We took some mold impressions of the sealing surface and inspected them under magnification and were surprised at the amount of what appears to be porosity. The coating is supposed to have around a .1% porosity level and there was a sealant applied prior to the grinding and honing process. Can anyone of the coating experts out there provide feedback on the amount of porosity present in the image and whether it is higher than normal for this type of coating and if there may be an issue with coating process, sealing process or materials involved.
Thanks,
David

It doesn't look like porosity, but like outbursts. Such outbreaks can arise for several reasons:
1. Weak layer cohesion due to incorrect spray parameters
2. Too much content of oxides
3. Too hard grinding

If you are lucky and the cause lies with wrong grinding, the layer surface could become better by gentle grinding.
The problem with bad cohesion is more serious. In order to see if the layer is of sufficient quality, you have to make a microscopy of the cross section.