Posted by Gordon England on April 25, 2003 at 12:09:28:
In Reply to: Stripes in plasma sprayed cobalt alloys posted by Benjamin Russell on April 25, 2003 at 08:19:28:
: I am working with some plasma sprayed CoCrNiW coatings and am puzzled by some of the oxide formations i am seeing. Typically these are stripes all in the same direction and with consitent angles. Also i have seen some V shaped oxide clusters, generally originating at a point in the substrate and extending though the whole thickness of the coating. One theory is that turbulence effects resulting from defects (eg unmelted particles) cause vortecies in which oxidised fines accumilate and form these structure. I would appriciate any liturature on the subject and any other suggested reasons for this phenomenom.
Hi Benjamin
Can you descibe the spray set-up (with speeds and feeds) and work piece.
Oxide clusters - Are these effecting the as-sprayed surface texture of the coating producing small nodules or warts? If you grind and polish the surface of the coating, can you see oxide clusters forming a horse shoe shape roughly 1 mm diameter (best view at about X10). Does the cross-section through oxide clusters resemble that seen in this photo of Plasma Sprayed NiCr coating:
http://www.gordonengland.co.uk/pmg6.htm
Ignoring coating lamination effect and bearing in mind that the oxide clusters appear different depending on sectioning direction. You need to visualize this effect in 3D as the 2D view from cross-section can be confusing. This photo is not the best illustration of this effect, but it is all I have at hand at the moment.
Generally, I think your theory is on the right track. Give us some more detail and I will try to explain this phenomenom.
Regards Gordon
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