Pore size in carbides

[Jeff]

All,
I have been spraying CRC and WC coatings with APS. The specification that I must meet requires a maximum pore size of 20 microns. It also constrains me to APS and powder type. I am having trouble meeting the pore size requirement. The over all porosity looks good, but the random large pore fails the sample. We have worked on the metallographic preparation technique and have seen minimal improvement. I'm using SG100 gun. What variables are the most effective to change to decrease this pore size, and what way to change them?
Thanks,
Jeff

[Gordon]

Hi Jeff

to the Surface Engineering Forum.

I think your problem could be caused by:
* "pluck-out" during metallographic preparation
* stray material entering the plasma stream
* lumpy or contaminated powder
* powder injection port fouling

I think you are best not to adjust plasma parameters as it seems your coatings are fine apart from the random large pore. Adjusting parameters really is the last thing to consider and make sure equipment is in good order (serviced and calibrated) before going down this route.

Look again at your metallographic preparation technique, little point changing anything else if the problem lies here and the coatings actually have no problem . If you are very sure metallography is telling you the truth, look at your spray process, see if you have any stray airborne material floating around the plasma spray stream, check powder ports for fouling, check powder quality and powder feeder.

Hope that helps and good luck.

[Jeff]

Gordon Wrote:Hi Jeff

to the Surface Engineering Forum.

I think your problem could be caused by:
* "pluck-out" during metallographic preparation
* stray material entering the plasma stream
* lumpy or contaminated powder
* powder injection port fouling

I think you are best not to adjust plasma parameters as it seems your coatings are fine apart from the random large pore. Adjusting parameters really is the last thing to consider and make sure equipment is in good order (serviced and calibrated) before going down this route.

Look again at your metallographic preparation technique, little point changing anything else if the problem lies here and the coatings actually have no problem . If you are very sure metallography is telling you the truth, look at your spray process, see if you have any stray airborne material floating around the plasma spray stream, check powder ports for fouling, check powder quality and powder feeder.

Hope that helps and good luck.

Gordon, Thanks for the quick response. To give a little more background. We have had our samples prepared and evaluated by an outside lab and they confirmed our internal pore problem. We tried different powders and powder lots. We are looking into the stray material entering the stream and the fouling of the powder ports. I was given the suggestion to increase the volts. I saw a positive result, but am getting close to where I can't raise them any more. Will increasing the amps give the same results as increasing the volts? After all the KW will increase whether it is the amps or the volts increased.
Thanks,
Jeff

[Gordon]

Hi Jeff

Increasing current/Amps will enable you to increase heating of the particles as may reducing the primary gas flow or increasing the secondary gas flow. My concern would be that you may start to overheat the primary carbide constituent, oxidising/taking into solution with matrix material and ending up with a coating low in primary carbide and changing a normally ductile matrix to a brittle one. Tungsten carbide/cobalt materials are particularly sensitive to changes in plasma spray conditions. Ideally, only the cobalt matrix material should melt/soften leaving the tungsten carbide intact, so producing a coating very similar in constituent phases as the original powder.

To be honest the fact that you are using an "outside lab" would not necessarily increase my confidence in that they are reporting a true result . Experience has taught me not to always to trust in metallographic reports, particularly those outside my control.

[Jeff]

Gordon Wrote:Hi Jeff

Increasing current/Amps will enable you to increase heating of the particles as may reducing the primary gas flow or increasing the secondary gas flow. My concern would be that you may start to overheat the primary carbide constituent, oxidising/taking into solution with matrix material and ending up with a coating low in primary carbide and changing a normally ductile matrix to a brittle one. Tungsten carbide/cobalt materials are particularly sensitive to changes in plasma spray conditions. Ideally, only the cobalt matrix material should melt/soften leaving the tungsten carbide intact, so producing a coating very similar in constituent phases as the original powder.

To be honest the fact that you are using an "outside lab" would not necessarily increase my confidence in that they are reporting a true result . Experience has taught me not to always to trust in metallographic reports, particularly those outside my control.

Hello Gordon,
Good thoughts on the carbides over heating and going into the matrix solution. I believe we have sprayed some development samples that have showed this. With regards to the lab, I'm always skeptical with this being our internal lab or outside services. I will keep working to resolve the issue. Thanks again for all the input.
-Jeff

[Gordon]

Hi Jeff

Your welcome and please let us know how you get on .

Just like to stress for every bodies benefit including my own . that it is all too easy when under pressure to resolve a problem like this, to be lead on a "wild goose chase" or "up the garden path" optimising spray parameters, when in truth the original parameters are fine. Coating evaluation/interpretation or equipment condition/calibration/use are usually to blame, so thoroughly investigate these first.