Machining Thermal Spray Coating

[clay87]

We are having difficulty machining a thermal spray coating buildup. We have never had a problem with this coating but in this application it is flaking off - it appears to be in layers. The coating is aluminum-bronze arc-wire thermal spray (hand-held).

We have machined this same coating numerous times with no issues. The only difference in this application and previous applications is that we are spraying into a sort-of corner (a boss fit).

Anybody run into this?

 

[unclesyd]

i would give my equipment maker and wire supplier a call with your problem with all the details. The only problem with this that it is hard to find them sometimes as everyone has bought out everybody.

We had had troubles with thermal spray coatings peeling off but it's generally from the base metal not interlayer.

We use essentially the same process to reclaim vertical pump shafts but we found that to replicate the results it had to be done mechanically, on a lathe with gun on the cross feed. This keeps the standoff distance set, which takes out one very big variable. Try to setup where you keep the finish dimension out of the bond lines

If possible use a similar setup and restore your parameters, one at time, that were successful. We found that the standoff distance was extremely critical to keep the material from oxidizing. The oxide doesn't stick very well.
While you are troubleshooting the process and if possible check all your meters and gauges.

If you can get to a microscope (50x or above) take a look at a piece of the peeled off material and look for the oxides.

 

[unclesyd]

If you have access to a grinder capable of working with your component you might want to grind it to size. There are some coatings that just don't respond to machining very well.
A lot of consideration have to met if this part can be ground.

 

[clay87]

Thanks unclesyd.

We have talked to our supplier who also recommended a turntable. I don't think it is a batch issue since we qualify our spray (bend and peel test) prior to every application.

We have decided - for now - to complete our current task with a machinable ceramic coating (Belzona 1111). For future thermal spray jobs, we have fabricated a turntable to use with our hand-held.

I'll also keep in mind grinding, which we should be able to set up. This could be time-consuming though.

 

[coatingengineer]

Great advice by unclesyd. Also, can you add a radius to that corner? Thermal spray adhesion, you've discovered, is poor on sharp edges.

William Gunnar

http://www.IndustrialCoatingsWorld.com