Posted by Gordon England (188.8.131.52) on 17:09:45 10/01/06
In Reply to: Re: Cluster oxide in Inco718 posted by Hong
I would go back and spray a test coupon with your original
troublesome parameters. Note the position relative to gun. Section
coating in 3 directions 0, 45 and 90 degrees plus a view looking
down on the coating surface. The metallography of all sections
should allow you to visualise the oxide cluster formation in 3
dimension. If these formations have similar orientation and form to
one-another, then it should be possible to determine the direction
in which the fine material is being projected most prominently.
This should at least give food for thought and hopefully direction
to your coating development.
I would initial consider changing powder port
injector position say by 90 degrees and seeing if this changes the above results. Changing gun air jet orientation and or using crossed jets may also be useful. Please only change one thing at a time though! The spray angle could also be a critical factor and could be worth looking at. Note if you have gun axis lined up perpendicular to substrate the actual spray stream will in fact be angled slightly away from this (target point deviation due to side entry powder injection).
I would first choose the above route initial as changing parameters like nozzles, gas flows, power levels, plasma gas, spray distance etc. will have significant effects on other coating qualities which you probably do not wish to change. Alex, you have noted that increasing particle velocity, reducing particle heating has reduced oxide clusters. This I think is mainly due to lower oxide generation generally, lower particle fluidity (less impact splashing) and possibly making it harder to inject fine fraction of the powder into the plasma plume. Alex, did you make any changes to powder feed carrier gas flow to compensate for increase in plasma velocity? Also, Check that your new parameters are not going to cause excessive nozzle/electrode wear.
Trying different powder suppliers or even different batches of powder has worked for some people with the similar problems. Small changes in the amount of powder fines and changes in the size distribution within a certain powder specification can be significant.
Coating development/trouble shooting is rarely easy or straight forward, just too many interrelated factors to consider. All too easy to lead yourself up a long path to a dead end! On a personal level though important for gaining experience and knowledge to become an expert on the subject.
Please note I have made a small but important edit to the sketch on my previous message.
Best regards Gordon
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