"Strange" aspect of WC 17Co coating after machining

[pabloarriola]

Hi,

I have been reading the forum for several years, it has been very useful for me to learn about thermal spraying processes.

Now, I'm facing a problem and I have no clue on what it's happening, maybe you could give me some idea.

We are spraying a part made from an aluminium alloy with a WC17Co superfine grade coating by air plasma spray, thickness aprox. 0.30 - 0.35 mm (.012-.014 inch). The coating is finished by dry turning with CBN.

Up to now, we have been obtaining a smooth surface finish after machining (see attached picture).

However, since a few weeks ago, the appearance of the coating has been getting worse after machining, the last one the worst of them. The surface seems rougher than usual, in all the sprayed area (see atached picture 2)
It is as if the turning tip is not finding enough material, -but it did, comparing the original as-sprayed dimension and the finished one there is around 0.15mm (.006 inch) of difference. Or as if the coating is more porous.
However the metallography seems correct (see attached picture 3)

Any idea on what is going on? Apparently, there has not been any change in the thermal spraying nor machining process.
Powder feed rate is done before all sprayings and it's correct.
Spraying parameters are monitored and ok.

So, probably something has changed but I'm not sure what...


Thanks for your help, best regards,

Pablo Arriola
   
   
   

[LEN WOOD]

When spraying tight or enclosed bores any deviations in angle, distance, extent of travel or cooling differences can affect the coating integrity. Entrapment of cooled/spent overspray dust (turbulence - once again) will result in porosity and lesser quality. If you were able to take an accurate comparable micro from this location you would see the difference.

Something must have changed since you were getting right.

[ServiceTech]

I agree with Len. The coating stucture you have posted looks suspect as well as it is nearly impossible to obtain that kind of density with APS on WC 17Co and it appears the coating could be smeared. I don't think the coating is as high quality as the photo depicts.

I would re-examine polishing methods by the lab. Also, place some test samples in the spray area and you will see a difference providing the lab technique is sound.

Also, these kind of coatings must be ground to achieve good finishes, not single point machined.

[sreenuvundela]

Hi

I agree with Len & Service Tech.

The microstructure is not looking like Plasma Sprayed One and Turning operation is not all advisable on High Hardness Coatings such as Tungsten Carbide Coating.

Even though, Coating is well done, during machining also internal surface cracks may initiate and that may create problems.

Regards,

SREENIVAS

[derek]

When this problem happens, first in my mind is cooling air, do you have a cooling air for APS process? Have you change the pressure or the cooling angle?

[pabloarriola]

Hi,

Thanks for your replies and comments. I will try to reply all in one post.

We are using external cooling air to refrigerate the part, we have check that it is not interfering with the spray stream.

We are changing the finish process to grinding, hopefuly this will improve the surface finish.

We have checked the masking, and will try to improve it as now it is "closing" the exit of heat and overspray dust at the bottom of the part.

We will put a metallographic sample in the spray area, right now these are located outside, to compare.


Best regards,
Pablo

[henryperez123]

Can the coating be done in vacuum, and if yes will the finishing received will be better in vacuum then in the environment.

[LEN WOOD]

Can the coating be done in vacuum, and if yes will the finishing received will be better in vacuum then in the environment.

INDEED, but shouldn't you be more concerned about Venetians and Verticals than Vacuum Plasma Spray.
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