Posted by Gordon England (62.252.0.12) 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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